diff --git a/.luacheckrc b/.luacheckrc
index 71b0f872c..3bf2c2c41 100644
--- a/.luacheckrc
+++ b/.luacheckrc
@@ -17,7 +17,8 @@ ignore = {
 -- are largely unsupported.
 include_files = {
     'src/main/resources/assets/computercraft/lua/rom',
-    'src/main/resources/assets/computercraft/lua/bios.lua'
+    'src/main/resources/assets/computercraft/lua/bios.lua',
+    'src/test/resources/test-rom',
 }
 
 files['src/main/resources/assets/computercraft/lua/bios.lua'] = {
diff --git a/.travis.yml b/.travis.yml
index 327098845..0e4be822e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,4 +11,4 @@ cache:
     - $HOME/.gradle/wrapper/s
 
 jdk:
-    - oraclejdk8
+    - openjdk8
diff --git a/README.md b/README.md
index 283cd4591..f353fe778 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,5 @@
 # ![CC: Tweaked](logo.png)
-[![Current build status](https://travis-ci.org/SquidDev-CC/CC-Tweaked.svg?branch=master)](https://travis-ci.org/SquidDev-CC/CC-Tweaked "Current build status") [![Download CC: Tweaked on CurseForge](https://cf.way2muchnoise.eu/title/cc-tweaked.svg)](https://minecraft.curseforge.com/projects/cc-tweaked "Download CC: Tweaked on CurseForge")
+[![Current build status](https://travis-ci.org/SquidDev-CC/CC-Tweaked.svg?branch=master)](https://travis-ci.org/SquidDev-CC/CC-Tweaked "Current build status") [![Download CC: Tweaked on CurseForge](http://cf.way2muchnoise.eu/title/cc-tweaked.svg)](https://minecraft.curseforge.com/projects/cc-tweaked "Download CC: Tweaked on CurseForge")
 
 CC: Tweaked is a fork of [ComputerCraft](https://github.com/dan200/ComputerCraft), adding programmable computers,
 turtles and more to Minecraft.
diff --git a/build.gradle b/build.gradle
index b31811486..94f7d11c3 100644
--- a/build.gradle
+++ b/build.gradle
@@ -4,7 +4,7 @@ buildscript {
         mavenCentral()
         maven {
             name = "forge"
-            url = "http://files.minecraftforge.net/maven"
+            url = "https://files.minecraftforge.net/maven"
         }
     }
     dependencies {
@@ -67,7 +67,7 @@ minecraft {
 repositories {
     maven {
         name "JEI"
-        url  "http://dvs1.progwml6.com/files/maven"
+        url "https://dvs1.progwml6.com/files/maven"
     }
     maven {
         name "SquidDev"
@@ -79,7 +79,7 @@ repositories {
     }
     maven {
         name "Amadornes"
-        url "http://maven.amadornes.com/"
+        url "https://maven.amadornes.com/"
     }
 }
 
@@ -102,8 +102,8 @@ dependencies {
 
     shade 'org.squiddev:Cobalt:0.5.0-SNAPSHOT'
 
-    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.1.0'
-    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.1.0'
+    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.4.2'
+    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.4.2'
 
     deployerJars "org.apache.maven.wagon:wagon-ssh:3.0.0"
 }
@@ -117,6 +117,8 @@ sourceSets {
     }
 }
 
+// Compile tasks
+
 javadoc {
     include "dan200/computercraft/api/**/*.java"
 }
@@ -141,6 +143,14 @@ jar {
     from configurations.shade.collect { it.isDirectory() ? it : zipTree(it) }
 }
 
+[compileJava, compileTestJava].forEach {
+    it.configure {
+        options.compilerArgs << "-Xlint" << "-Xlint:-processing" << "-Werror"
+    }
+}
+
+
+
 import java.nio.charset.StandardCharsets
 import java.nio.file.*
 import java.util.zip.*
@@ -276,7 +286,14 @@ task compressJson(dependsOn: jar) {
 
 assemble.dependsOn compressJson
 
-/* Check tasks */
+// Check tasks
+
+test {
+    useJUnitPlatform()
+    testLogging {
+        events "skipped", "failed"
+    }
+}
 
 license {
     mapping("java", "SLASHSTAR_STYLE")
@@ -300,6 +317,13 @@ license {
     }
 }
 
+gradle.projectsEvaluated {
+    tasks.withType(LicenseFormat) {
+        outputs.upToDateWhen { false }
+    }
+}
+
+
 task licenseAPI(type: LicenseCheck);
 task licenseFormatAPI(type: LicenseFormat);
 [licenseAPI, licenseFormatAPI].forEach {
@@ -310,7 +334,7 @@ task licenseFormatAPI(type: LicenseFormat);
     }
 }
 
-/* Upload tasks */
+// Upload tasks
 
 task checkRelease {
     group "upload"
@@ -441,23 +465,3 @@ task uploadAll(dependsOn: uploadTasks) {
     group "upload"
     description "Uploads to all repositories (Maven, Curse, GitHub release)"
 }
-
-test {
-    useJUnitPlatform()
-    testLogging {
-        events "passed", "skipped", "failed"
-    }
-}
-
-gradle.projectsEvaluated {
-    reobfJar.dependsOn proguardMove
-
-    tasks.withType(JavaCompile) {
-        options.compilerArgs << "-Xlint" << "-Xlint:-processing" // Causes Forge build to fail << "-Werror"
-    }
-
-    tasks.withType(LicenseFormat) {
-        outputs.upToDateWhen { false }
-    }
-}
-
diff --git a/gradle.properties b/gradle.properties
index c0f4eb432..44330812b 100644
--- a/gradle.properties
+++ b/gradle.properties
@@ -1,5 +1,5 @@
 # Mod properties
-mod_version=1.83.1
+mod_version=1.84.0
 
 # Minecraft properties
 mc_version=1.14.4
diff --git a/src/main/java/dan200/computercraft/api/turtle/ITurtleAccess.java b/src/main/java/dan200/computercraft/api/turtle/ITurtleAccess.java
index 7d9b80721..9b1563329 100644
--- a/src/main/java/dan200/computercraft/api/turtle/ITurtleAccess.java
+++ b/src/main/java/dan200/computercraft/api/turtle/ITurtleAccess.java
@@ -144,7 +144,9 @@ public interface ITurtleAccess
     GameProfile getOwningPlayer();
 
     /**
-     * Get the inventory of this turtle
+     * Get the inventory of this turtle.
+     *
+     * Note: this inventory should only be accessed and modified on the server thread.
      *
      * @return This turtle's inventory
      * @see #getItemHandler()
@@ -155,6 +157,8 @@ public interface ITurtleAccess
     /**
      * Get the inventory of this turtle as an {@link IItemHandlerModifiable}.
      *
+     * Note: this inventory should only be accessed and modified on the server thread.
+     *
      * @return This turtle's inventory
      * @see #getInventory()
      * @see IItemHandlerModifiable
diff --git a/src/main/java/dan200/computercraft/api/turtle/ITurtleUpgrade.java b/src/main/java/dan200/computercraft/api/turtle/ITurtleUpgrade.java
index e36f9230a..1f4b96154 100644
--- a/src/main/java/dan200/computercraft/api/turtle/ITurtleUpgrade.java
+++ b/src/main/java/dan200/computercraft/api/turtle/ITurtleUpgrade.java
@@ -98,8 +98,8 @@ public interface ITurtleUpgrade
      * Will only be called for Tool turtle. Called when turtle.dig() or turtle.attack() is called
      * by the turtle, and the tool is required to do some work.
      *
-     * Conforming implementations should fire {@link BlockEvent.BreakEvent} and {@link TurtleBlockEvent.Dig}for digging,
-     * {@link AttackEntityEvent} and {@link TurtleAttackEvent} for attacking.
+     * Conforming implementations should fire {@link BlockEvent.BreakEvent} and {@link TurtleBlockEvent.Dig} for
+     * digging, {@link AttackEntityEvent} and {@link TurtleAttackEvent} for attacking.
      *
      * @param turtle    Access to the turtle that the tool resides on.
      * @param side      Which side of the turtle (left or right) the tool resides on.
diff --git a/src/main/java/dan200/computercraft/shared/peripheral/diskdrive/DiskDrivePeripheral.java b/src/main/java/dan200/computercraft/shared/peripheral/diskdrive/DiskDrivePeripheral.java
index 855486cbd..aadb9f15d 100644
--- a/src/main/java/dan200/computercraft/shared/peripheral/diskdrive/DiskDrivePeripheral.java
+++ b/src/main/java/dan200/computercraft/shared/peripheral/diskdrive/DiskDrivePeripheral.java
@@ -11,6 +11,7 @@ import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.media.IMedia;
 import dan200.computercraft.api.peripheral.IComputerAccess;
 import dan200.computercraft.api.peripheral.IPeripheral;
+import dan200.computercraft.shared.MediaProviders;
 import dan200.computercraft.shared.media.items.ItemDisk;
 import dan200.computercraft.shared.util.StringUtil;
 import net.minecraft.item.ItemStack;
@@ -19,11 +20,11 @@ import javax.annotation.Nonnull;
 
 import static dan200.computercraft.core.apis.ArgumentHelper.optString;
 
-public class DiskDrivePeripheral implements IPeripheral
+class DiskDrivePeripheral implements IPeripheral
 {
     private final TileDiskDrive m_diskDrive;
 
-    public DiskDrivePeripheral( TileDiskDrive diskDrive )
+    DiskDrivePeripheral( TileDiskDrive diskDrive )
     {
         m_diskDrive = diskDrive;
     }
@@ -55,7 +56,7 @@ public class DiskDrivePeripheral implements IPeripheral
     }
 
     @Override
-    public Object[] callMethod( @Nonnull IComputerAccess computer, @Nonnull ILuaContext context, int method, @Nonnull Object[] arguments ) throws LuaException
+    public Object[] callMethod( @Nonnull IComputerAccess computer, @Nonnull ILuaContext context, int method, @Nonnull Object[] arguments ) throws LuaException, InterruptedException
     {
         switch( method )
         {
@@ -63,21 +64,26 @@ public class DiskDrivePeripheral implements IPeripheral
                 return new Object[] { !m_diskDrive.getDiskStack().isEmpty() };
             case 1: // getDiskLabel
             {
-                IMedia media = m_diskDrive.getDiskMedia();
-                return media == null ? null : new Object[] { media.getLabel( m_diskDrive.getDiskStack() ) };
+                ItemStack stack = m_diskDrive.getDiskStack();
+                IMedia media = MediaProviders.get( stack );
+                return media == null ? null : new Object[] { media.getLabel( stack ) };
             }
             case 2: // setDiskLabel
             {
                 String label = optString( arguments, 0, null );
 
-                IMedia media = m_diskDrive.getDiskMedia();
-                if( media == null ) return null;
+                return context.executeMainThreadTask( () -> {
+                    ItemStack stack = m_diskDrive.getDiskStack();
+                    IMedia media = MediaProviders.get( stack );
+                    if( media == null ) return null;
 
-                ItemStack disk = m_diskDrive.getDiskStack();
-                label = StringUtil.normaliseLabel( label );
-                if( !media.setLabel( disk, label ) ) throw new LuaException( "Disk label cannot be changed" );
-                m_diskDrive.setDiskStack( disk );
-                return null;
+                    if( !media.setLabel( stack, StringUtil.normaliseLabel( label ) ) )
+                    {
+                        throw new LuaException( "Disk label cannot be changed" );
+                    }
+                    m_diskDrive.setDiskStack( stack );
+                    return null;
+                } );
             }
             case 3: // hasData
                 return new Object[] { m_diskDrive.getDiskMountPath( computer ) != null };
@@ -86,14 +92,16 @@ public class DiskDrivePeripheral implements IPeripheral
             case 5:
             {
                 // hasAudio
-                IMedia media = m_diskDrive.getDiskMedia();
-                return new Object[] { media != null && media.getAudio( m_diskDrive.getDiskStack() ) != null };
+                ItemStack stack = m_diskDrive.getDiskStack();
+                IMedia media = MediaProviders.get( stack );
+                return new Object[] { media != null && media.getAudio( stack ) != null };
             }
             case 6:
             {
                 // getAudioTitle
-                IMedia media = m_diskDrive.getDiskMedia();
-                return new Object[] { media != null ? media.getAudioTitle( m_diskDrive.getDiskStack() ) : false };
+                ItemStack stack = m_diskDrive.getDiskStack();
+                IMedia media = MediaProviders.get( stack );
+                return new Object[] { media != null ? media.getAudioTitle( stack ) : false };
             }
             case 7: // playAudio
                 m_diskDrive.playDiskAudio();
@@ -129,8 +137,7 @@ public class DiskDrivePeripheral implements IPeripheral
     @Override
     public boolean equals( IPeripheral other )
     {
-        if( this == other ) return true;
-        return other instanceof DiskDrivePeripheral && ((DiskDrivePeripheral) other).m_diskDrive == m_diskDrive;
+        return this == other || other instanceof DiskDrivePeripheral && ((DiskDrivePeripheral) other).m_diskDrive == m_diskDrive;
     }
 
     @Nonnull
diff --git a/src/main/java/dan200/computercraft/shared/peripheral/diskdrive/TileDiskDrive.java b/src/main/java/dan200/computercraft/shared/peripheral/diskdrive/TileDiskDrive.java
index d424fbde4..5f6c35203 100644
--- a/src/main/java/dan200/computercraft/shared/peripheral/diskdrive/TileDiskDrive.java
+++ b/src/main/java/dan200/computercraft/shared/peripheral/diskdrive/TileDiskDrive.java
@@ -320,35 +320,31 @@ public final class TileDiskDrive extends TileGeneric implements DefaultInventory
     }
 
     @Nonnull
-    public ItemStack getDiskStack()
+    ItemStack getDiskStack()
     {
         return getStackInSlot( 0 );
     }
 
-    public void setDiskStack( @Nonnull ItemStack stack )
+    void setDiskStack( @Nonnull ItemStack stack )
     {
         setInventorySlotContents( 0, stack );
     }
 
-    public IMedia getDiskMedia()
+    private IMedia getDiskMedia()
     {
         return MediaProviders.get( getDiskStack() );
     }
 
-    public String getDiskMountPath( IComputerAccess computer )
+    String getDiskMountPath( IComputerAccess computer )
     {
         synchronized( this )
         {
-            if( m_computers.containsKey( computer ) )
-            {
-                MountInfo info = m_computers.get( computer );
-                return info.mountPath;
-            }
+            MountInfo info = m_computers.get( computer );
+            return info != null ? info.mountPath : null;
         }
-        return null;
     }
 
-    public void mount( IComputerAccess computer )
+    void mount( IComputerAccess computer )
     {
         synchronized( this )
         {
@@ -357,7 +353,7 @@ public final class TileDiskDrive extends TileGeneric implements DefaultInventory
         }
     }
 
-    public void unmount( IComputerAccess computer )
+    void unmount( IComputerAccess computer )
     {
         synchronized( this )
         {
@@ -366,7 +362,7 @@ public final class TileDiskDrive extends TileGeneric implements DefaultInventory
         }
     }
 
-    public void playDiskAudio()
+    void playDiskAudio()
     {
         synchronized( this )
         {
@@ -379,7 +375,7 @@ public final class TileDiskDrive extends TileGeneric implements DefaultInventory
         }
     }
 
-    public void stopDiskAudio()
+    void stopDiskAudio()
     {
         synchronized( this )
         {
@@ -388,7 +384,7 @@ public final class TileDiskDrive extends TileGeneric implements DefaultInventory
         }
     }
 
-    public void ejectDisk()
+    void ejectDisk()
     {
         synchronized( this )
         {
diff --git a/src/main/java/dan200/computercraft/shared/peripheral/printer/PrinterPeripheral.java b/src/main/java/dan200/computercraft/shared/peripheral/printer/PrinterPeripheral.java
index 0a786b1e2..91ea62895 100644
--- a/src/main/java/dan200/computercraft/shared/peripheral/printer/PrinterPeripheral.java
+++ b/src/main/java/dan200/computercraft/shared/peripheral/printer/PrinterPeripheral.java
@@ -11,6 +11,7 @@ import dan200.computercraft.api.lua.LuaException;
 import dan200.computercraft.api.peripheral.IComputerAccess;
 import dan200.computercraft.api.peripheral.IPeripheral;
 import dan200.computercraft.core.terminal.Terminal;
+import dan200.computercraft.shared.util.StringUtil;
 
 import javax.annotation.Nonnull;
 
@@ -51,8 +52,13 @@ public class PrinterPeripheral implements IPeripheral
     }
 
     @Override
-    public Object[] callMethod( @Nonnull IComputerAccess computer, @Nonnull ILuaContext context, int method, @Nonnull Object[] args ) throws LuaException
+    public Object[] callMethod( @Nonnull IComputerAccess computer, @Nonnull ILuaContext context, int method, @Nonnull Object[] args ) throws LuaException, InterruptedException
     {
+        // FIXME: There's a theoretical race condition here between getCurrentPage and then using the page. Ideally
+        //  we'd lock on the page, consume it, and unlock.
+
+        // FIXME: None of our page modification functions actually mark the tile as dirty, so the page may not be
+        //  persisted correctly.
         switch( method )
         {
             case 0: // write
@@ -89,10 +95,13 @@ public class PrinterPeripheral implements IPeripheral
                 return new Object[] { width, height };
             }
             case 4: // newPage
-                return new Object[] { m_printer.startNewPage() };
+                return context.executeMainThreadTask( () -> new Object[] { m_printer.startNewPage() } );
             case 5: // endPage
                 getCurrentPage();
-                return new Object[] { m_printer.endCurrentPage() };
+                return context.executeMainThreadTask( () -> {
+                    getCurrentPage();
+                    return new Object[] { m_printer.endCurrentPage() };
+                } );
             case 6: // getInkLevel
                 return new Object[] { m_printer.getInkLevel() };
             case 7:
@@ -100,7 +109,7 @@ public class PrinterPeripheral implements IPeripheral
                 // setPageTitle
                 String title = optString( args, 0, "" );
                 getCurrentPage();
-                m_printer.setPageTitle( title );
+                m_printer.setPageTitle( StringUtil.normaliseLabel( title ) );
                 return null;
             }
             case 8: // getPaperLevel
@@ -123,13 +132,11 @@ public class PrinterPeripheral implements IPeripheral
         return m_printer;
     }
 
+    @Nonnull
     private Terminal getCurrentPage() throws LuaException
     {
         Terminal currentPage = m_printer.getCurrentPage();
-        if( currentPage == null )
-        {
-            throw new LuaException( "Page not started" );
-        }
+        if( currentPage == null ) throw new LuaException( "Page not started" );
         return currentPage;
     }
 }
diff --git a/src/main/java/dan200/computercraft/shared/peripheral/printer/TilePrinter.java b/src/main/java/dan200/computercraft/shared/peripheral/printer/TilePrinter.java
index b525dfbfd..4c6f422ce 100644
--- a/src/main/java/dan200/computercraft/shared/peripheral/printer/TilePrinter.java
+++ b/src/main/java/dan200/computercraft/shared/peripheral/printer/TilePrinter.java
@@ -120,10 +120,7 @@ public final class TilePrinter extends TileGeneric implements DefaultSidedInvent
         }
 
         // Read inventory
-        synchronized( m_inventory )
-        {
-            ItemStackHelper.loadAllItems( nbt, m_inventory );
-        }
+        ItemStackHelper.loadAllItems( nbt, m_inventory );
     }
 
     @Nonnull
@@ -141,15 +138,12 @@ public final class TilePrinter extends TileGeneric implements DefaultSidedInvent
         }
 
         // Write inventory
-        synchronized( m_inventory )
-        {
-            ItemStackHelper.saveAllItems( nbt, m_inventory );
-        }
+        ItemStackHelper.saveAllItems( nbt, m_inventory );
 
         return super.write( nbt );
     }
 
-    public boolean isPrinting()
+    boolean isPrinting()
     {
         return m_printing;
     }
@@ -173,73 +167,59 @@ public final class TilePrinter extends TileGeneric implements DefaultSidedInvent
 
     @Nonnull
     @Override
-    public ItemStack getStackInSlot( int i )
+    public ItemStack getStackInSlot( int slot )
     {
-        return m_inventory.get( i );
+        return m_inventory.get( slot );
     }
 
     @Nonnull
     @Override
-    public ItemStack removeStackFromSlot( int i )
+    public ItemStack removeStackFromSlot( int slot )
     {
-        synchronized( m_inventory )
-        {
-            ItemStack result = m_inventory.get( i );
-            m_inventory.set( i, ItemStack.EMPTY );
-            markDirty();
-            updateBlockState();
-            return result;
-        }
+        ItemStack result = m_inventory.get( slot );
+        m_inventory.set( slot, ItemStack.EMPTY );
+        markDirty();
+        updateBlockState();
+        return result;
     }
 
     @Nonnull
     @Override
-    public ItemStack decrStackSize( int i, int j )
+    public ItemStack decrStackSize( int slot, int count )
     {
-        synchronized( m_inventory )
+        ItemStack stack = m_inventory.get( slot );
+        if( stack.isEmpty() ) return ItemStack.EMPTY;
+
+        if( stack.getCount() <= count )
         {
-            if( m_inventory.get( i ).isEmpty() ) return ItemStack.EMPTY;
-
-            if( m_inventory.get( i ).getCount() <= j )
-            {
-                ItemStack itemstack = m_inventory.get( i );
-                m_inventory.set( i, ItemStack.EMPTY );
-                markDirty();
-                updateBlockState();
-                return itemstack;
-            }
-
-            ItemStack part = m_inventory.get( i ).split( j );
-            if( m_inventory.get( i ).isEmpty() )
-            {
-                m_inventory.set( i, ItemStack.EMPTY );
-                updateBlockState();
-            }
-            markDirty();
-            return part;
+            setInventorySlotContents( slot, ItemStack.EMPTY );
+            return stack;
         }
+
+        ItemStack part = stack.split( count );
+        if( m_inventory.get( slot ).isEmpty() )
+        {
+            m_inventory.set( slot, ItemStack.EMPTY );
+            updateBlockState();
+        }
+        markDirty();
+        return part;
     }
 
     @Override
-    public void setInventorySlotContents( int i, @Nonnull ItemStack stack )
+    public void setInventorySlotContents( int slot, @Nonnull ItemStack stack )
     {
-        synchronized( m_inventory )
-        {
-            m_inventory.set( i, stack );
-            markDirty();
-            updateBlockState();
-        }
+        m_inventory.set( slot, stack );
+        markDirty();
+        updateBlockState();
     }
 
     @Override
     public void clear()
     {
-        synchronized( m_inventory )
-        {
-            for( int i = 0; i < m_inventory.size(); i++ ) m_inventory.set( i, ItemStack.EMPTY );
-            markDirty();
-            updateBlockState();
-        }
+        for( int i = 0; i < m_inventory.size(); i++ ) m_inventory.set( i, ItemStack.EMPTY );
+        markDirty();
+        updateBlockState();
     }
 
     @Override
@@ -290,14 +270,18 @@ public final class TilePrinter extends TileGeneric implements DefaultSidedInvent
         return new PrinterPeripheral( this );
     }
 
-    public Terminal getCurrentPage()
+    @Nullable
+    Terminal getCurrentPage()
     {
-        return m_printing ? m_page : null;
+        synchronized( m_page )
+        {
+            return m_printing ? m_page : null;
+        }
     }
 
-    public boolean startNewPage()
+    boolean startNewPage()
     {
-        synchronized( m_inventory )
+        synchronized( m_page )
         {
             if( !canInputPage() ) return false;
             if( m_printing && !outputPage() ) return false;
@@ -305,49 +289,36 @@ public final class TilePrinter extends TileGeneric implements DefaultSidedInvent
         }
     }
 
-    public boolean endCurrentPage()
+    boolean endCurrentPage()
     {
-        synchronized( m_inventory )
+        synchronized( m_page )
         {
-            if( m_printing && outputPage() )
-            {
-                return true;
-            }
-        }
-        return false;
-    }
-
-    public int getInkLevel()
-    {
-        synchronized( m_inventory )
-        {
-            ItemStack inkStack = m_inventory.get( 0 );
-            return isInk( inkStack ) ? inkStack.getCount() : 0;
+            return m_printing && outputPage();
         }
     }
 
-    public int getPaperLevel()
+    int getInkLevel()
+    {
+        ItemStack inkStack = m_inventory.get( 0 );
+        return isInk( inkStack ) ? inkStack.getCount() : 0;
+    }
+
+    int getPaperLevel()
     {
         int count = 0;
-        synchronized( m_inventory )
+        for( int i = 1; i < 7; i++ )
         {
-            for( int i = 1; i < 7; i++ )
-            {
-                ItemStack paperStack = m_inventory.get( i );
-                if( !paperStack.isEmpty() && isPaper( paperStack ) )
-                {
-                    count += paperStack.getCount();
-                }
-            }
+            ItemStack paperStack = m_inventory.get( i );
+            if( isPaper( paperStack ) ) count += paperStack.getCount();
         }
         return count;
     }
 
-    public void setPageTitle( String title )
+    void setPageTitle( String title )
     {
-        if( m_printing )
+        synchronized( m_page )
         {
-            m_pageTitle = title;
+            if( m_printing ) m_pageTitle = title;
         }
     }
 
@@ -365,116 +336,100 @@ public final class TilePrinter extends TileGeneric implements DefaultSidedInvent
 
     private boolean canInputPage()
     {
-        synchronized( m_inventory )
-        {
-            ItemStack inkStack = m_inventory.get( 0 );
-            return !inkStack.isEmpty() && isInk( inkStack ) && getPaperLevel() > 0;
-        }
+        ItemStack inkStack = m_inventory.get( 0 );
+        return !inkStack.isEmpty() && isInk( inkStack ) && getPaperLevel() > 0;
     }
 
     private boolean inputPage()
     {
-        synchronized( m_inventory )
+        ItemStack inkStack = m_inventory.get( 0 );
+        if( !isInk( inkStack ) ) return false;
+
+        for( int i = 1; i < 7; i++ )
         {
-            ItemStack inkStack = m_inventory.get( 0 );
-            if( !isInk( inkStack ) ) return false;
+            ItemStack paperStack = m_inventory.get( i );
+            if( paperStack.isEmpty() || !isPaper( paperStack ) ) continue;
 
-            for( int i = 1; i < 7; i++ )
+            // Setup the new page
+            DyeColor dye = ColourUtils.getStackColour( inkStack );
+            m_page.setTextColour( dye != null ? dye.getId() : 15 );
+
+            m_page.clear();
+            if( paperStack.getItem() instanceof ItemPrintout )
             {
-                ItemStack paperStack = m_inventory.get( i );
-                if( !paperStack.isEmpty() && isPaper( paperStack ) )
+                m_pageTitle = ItemPrintout.getTitle( paperStack );
+                String[] text = ItemPrintout.getText( paperStack );
+                String[] textColour = ItemPrintout.getColours( paperStack );
+                for( int y = 0; y < m_page.getHeight(); y++ )
                 {
-                    // Setup the new page
-                    DyeColor dye = ColourUtils.getStackColour( inkStack );
-                    m_page.setTextColour( dye != null ? dye.getId() : 15 );
-
-                    m_page.clear();
-                    if( paperStack.getItem() instanceof ItemPrintout )
-                    {
-                        m_pageTitle = ItemPrintout.getTitle( paperStack );
-                        String[] text = ItemPrintout.getText( paperStack );
-                        String[] textColour = ItemPrintout.getColours( paperStack );
-                        for( int y = 0; y < m_page.getHeight(); y++ )
-                        {
-                            m_page.setLine( y, text[y], textColour[y], "" );
-                        }
-                    }
-                    else
-                    {
-                        m_pageTitle = "";
-                    }
-                    m_page.setCursorPos( 0, 0 );
-
-                    // Decrement ink
-                    inkStack.shrink( 1 );
-                    if( inkStack.isEmpty() ) m_inventory.set( 0, ItemStack.EMPTY );
-
-                    // Decrement paper
-                    paperStack.shrink( 1 );
-                    if( paperStack.isEmpty() )
-                    {
-                        m_inventory.set( i, ItemStack.EMPTY );
-                        updateBlockState();
-                    }
-
-                    markDirty();
-                    m_printing = true;
-                    return true;
+                    m_page.setLine( y, text[y], textColour[y], "" );
                 }
             }
-            return false;
+            else
+            {
+                m_pageTitle = "";
+            }
+            m_page.setCursorPos( 0, 0 );
+
+            // Decrement ink
+            inkStack.shrink( 1 );
+            if( inkStack.isEmpty() ) m_inventory.set( 0, ItemStack.EMPTY );
+
+            // Decrement paper
+            paperStack.shrink( 1 );
+            if( paperStack.isEmpty() )
+            {
+                m_inventory.set( i, ItemStack.EMPTY );
+                updateBlockState();
+            }
+
+            markDirty();
+            m_printing = true;
+            return true;
         }
+        return false;
     }
 
     private boolean outputPage()
     {
-        synchronized( m_page )
+        int height = m_page.getHeight();
+        String[] lines = new String[height];
+        String[] colours = new String[height];
+        for( int i = 0; i < height; i++ )
         {
-            int height = m_page.getHeight();
-            String[] lines = new String[height];
-            String[] colours = new String[height];
-            for( int i = 0; i < height; i++ )
-            {
-                lines[i] = m_page.getLine( i ).toString();
-                colours[i] = m_page.getTextColourLine( i ).toString();
-            }
-
-            ItemStack stack = ItemPrintout.createSingleFromTitleAndText( m_pageTitle, lines, colours );
-            synchronized( m_inventory )
-            {
-                for( int slot : BOTTOM_SLOTS )
-                {
-                    if( m_inventory.get( slot ).isEmpty() )
-                    {
-                        setInventorySlotContents( slot, stack );
-                        m_printing = false;
-                        return true;
-                    }
-                }
-            }
-            return false;
+            lines[i] = m_page.getLine( i ).toString();
+            colours[i] = m_page.getTextColourLine( i ).toString();
         }
+
+        ItemStack stack = ItemPrintout.createSingleFromTitleAndText( m_pageTitle, lines, colours );
+        for( int slot : BOTTOM_SLOTS )
+        {
+            if( m_inventory.get( slot ).isEmpty() )
+            {
+                setInventorySlotContents( slot, stack );
+                m_printing = false;
+                return true;
+            }
+        }
+        return false;
     }
 
     private void ejectContents()
     {
-        synchronized( m_inventory )
+        for( int i = 0; i < 13; i++ )
         {
-            for( int i = 0; i < 13; i++ )
+            ItemStack stack = m_inventory.get( i );
+            if( !stack.isEmpty() )
             {
-                ItemStack stack = m_inventory.get( i );
-                if( !stack.isEmpty() )
-                {
-                    // Remove the stack from the inventory
-                    setInventorySlotContents( i, ItemStack.EMPTY );
+                // Remove the stack from the inventory
+                setInventorySlotContents( i, ItemStack.EMPTY );
 
-                    // Spawn the item in the world
-                    BlockPos pos = getPos();
-                    double x = pos.getX() + 0.5;
-                    double y = pos.getY() + 0.75;
-                    double z = pos.getZ() + 0.5;
-                    WorldUtil.dropItemStack( stack, getWorld(), x, y, z );
-                }
+                // Spawn the item in the world
+                BlockPos pos = getPos();
+                double x = pos.getX() + 0.5;
+                double y = pos.getY() + 0.75;
+                double z = pos.getZ() + 0.5;
+                WorldUtil.dropItemStack( stack, getWorld(), x, y, z );
             }
         }
     }
@@ -482,25 +437,22 @@ public final class TilePrinter extends TileGeneric implements DefaultSidedInvent
     private void updateBlockState()
     {
         boolean top = false, bottom = false;
-        synchronized( m_inventory )
+        for( int i = 1; i < 7; i++ )
         {
-            for( int i = 1; i < 7; i++ )
+            ItemStack stack = m_inventory.get( i );
+            if( !stack.isEmpty() && isPaper( stack ) )
             {
-                ItemStack stack = m_inventory.get( i );
-                if( !stack.isEmpty() && isPaper( stack ) )
-                {
-                    top = true;
-                    break;
-                }
+                top = true;
+                break;
             }
-            for( int i = 7; i < 13; i++ )
+        }
+        for( int i = 7; i < 13; i++ )
+        {
+            ItemStack stack = m_inventory.get( i );
+            if( !stack.isEmpty() && isPaper( stack ) )
             {
-                ItemStack stack = m_inventory.get( i );
-                if( !stack.isEmpty() && isPaper( stack ) )
-                {
-                    bottom = true;
-                    break;
-                }
+                bottom = true;
+                break;
             }
         }
 
diff --git a/src/main/java/dan200/computercraft/shared/turtle/FurnaceRefuelHandler.java b/src/main/java/dan200/computercraft/shared/turtle/FurnaceRefuelHandler.java
index 714eb4e69..9c407c9b5 100644
--- a/src/main/java/dan200/computercraft/shared/turtle/FurnaceRefuelHandler.java
+++ b/src/main/java/dan200/computercraft/shared/turtle/FurnaceRefuelHandler.java
@@ -31,9 +31,13 @@ public final class FurnaceRefuelHandler implements TurtleRefuelEvent.Handler
     @Override
     public int refuel( @Nonnull ITurtleAccess turtle, @Nonnull ItemStack currentStack, int slot, int limit )
     {
-        ItemStack stack = turtle.getItemHandler().extractItem( slot, limit, false );
-        int fuelToGive = getFuelPerItem( stack ) * stack.getCount();
+        int fuelSpaceLeft = turtle.getFuelLimit() - turtle.getFuelLevel();
+        int fuelPerItem = getFuelPerItem( turtle.getItemHandler().getStackInSlot( slot ) );
+        int fuelItemLimit = (int) Math.ceil( fuelSpaceLeft / (double) fuelPerItem );
+        if( limit > fuelItemLimit ) limit = fuelItemLimit;
 
+        ItemStack stack = turtle.getItemHandler().extractItem( slot, limit, false );
+        int fuelToGive = fuelPerItem * stack.getCount();
         // Store the replacement item in the inventory
         ItemStack replacementStack = stack.getItem().getContainerItem( stack );
         if( !replacementStack.isEmpty() )
diff --git a/src/main/java/dan200/computercraft/shared/turtle/apis/TurtleAPI.java b/src/main/java/dan200/computercraft/shared/turtle/apis/TurtleAPI.java
index 45ddd84ba..1cb008de4 100644
--- a/src/main/java/dan200/computercraft/shared/turtle/apis/TurtleAPI.java
+++ b/src/main/java/dan200/computercraft/shared/turtle/apis/TurtleAPI.java
@@ -335,9 +335,11 @@ public class TurtleAPI implements ILuaAPI
                 return tryCommand( context, new TurtleInspectCommand( InteractDirection.Up ) );
             case 40: // inspectDown
                 return tryCommand( context, new TurtleInspectCommand( InteractDirection.Down ) );
-            case 41:
+            case 41: // getItemDetail
             {
-                // getItemDetail
+                // FIXME: There's a race condition here if the stack is being modified (mutating NBT, etc...)
+                //  on another thread. The obvious solution is to move this into a command, but some programs rely
+                //  on this having a 0-tick delay.
                 int slot = parseOptionalSlotNumber( args, 0, m_turtle.getSelectedSlot() );
                 ItemStack stack = m_turtle.getInventory().getStackInSlot( slot );
                 if( stack.isEmpty() ) return new Object[] { null };
diff --git a/src/main/java/dan200/computercraft/shared/turtle/blocks/BlockTurtle.java b/src/main/java/dan200/computercraft/shared/turtle/blocks/BlockTurtle.java
index b5572b80c..0e16ef997 100644
--- a/src/main/java/dan200/computercraft/shared/turtle/blocks/BlockTurtle.java
+++ b/src/main/java/dan200/computercraft/shared/turtle/blocks/BlockTurtle.java
@@ -153,7 +153,7 @@ public class BlockTurtle extends BlockComputerBase<TileTurtle> implements IWater
     @Override
     public float getExplosionResistance( BlockState state, IWorldReader world, BlockPos pos, @Nullable Entity exploder, Explosion explosion )
     {
-        if( getFamily() == ComputerFamily.Advanced && (exploder instanceof LivingEntity || exploder instanceof DamagingProjectileEntity) )
+        if( getFamily() == ComputerFamily.Advanced || exploder instanceof LivingEntity || exploder instanceof DamagingProjectileEntity )
         {
             return 2000;
         }
diff --git a/src/main/java/dan200/computercraft/shared/turtle/blocks/TileTurtle.java b/src/main/java/dan200/computercraft/shared/turtle/blocks/TileTurtle.java
index 982697aa5..6a6b72d40 100644
--- a/src/main/java/dan200/computercraft/shared/turtle/blocks/TileTurtle.java
+++ b/src/main/java/dan200/computercraft/shared/turtle/blocks/TileTurtle.java
@@ -49,13 +49,12 @@ import net.minecraftforge.items.wrapper.InvWrapper;
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import java.util.Collections;
 
 import static net.minecraftforge.items.CapabilityItemHandler.ITEM_HANDLER_CAPABILITY;
 
 public class TileTurtle extends TileComputerBase implements ITurtleTile, DefaultInventory
 {
-    // Statics
-
     public static final int INVENTORY_SIZE = 16;
     public static final int INVENTORY_WIDTH = 4;
     public static final int INVENTORY_HEIGHT = 4;
@@ -70,8 +69,6 @@ public class TileTurtle extends TileComputerBase implements ITurtleTile, Default
         type -> new TileTurtle( type, ComputerFamily.Advanced )
     );
 
-    // Members
-
     enum MoveState
     {
         NOT_MOVED,
@@ -79,25 +76,20 @@ public class TileTurtle extends TileComputerBase implements ITurtleTile, Default
         MOVED
     }
 
-    private NonNullList<ItemStack> m_inventory;
-    private NonNullList<ItemStack> m_previousInventory;
+    private final NonNullList<ItemStack> m_inventory = NonNullList.withSize( INVENTORY_SIZE, ItemStack.EMPTY );
+    private final NonNullList<ItemStack> m_previousInventory = NonNullList.withSize( INVENTORY_SIZE, ItemStack.EMPTY );
     private final IItemHandlerModifiable m_itemHandler = new InvWrapper( this );
     private LazyOptional<IItemHandlerModifiable> itemHandlerCap;
-    private boolean m_inventoryChanged;
-    private TurtleBrain m_brain;
-    private MoveState m_moveState;
+    private boolean m_inventoryChanged = false;
+    private TurtleBrain m_brain = new TurtleBrain( this );
+    private MoveState m_moveState = MoveState.NOT_MOVED;
 
     public TileTurtle( TileEntityType<? extends TileGeneric> type, ComputerFamily family )
     {
         super( type, family );
-        m_inventory = NonNullList.withSize( INVENTORY_SIZE, ItemStack.EMPTY );
-        m_previousInventory = NonNullList.withSize( INVENTORY_SIZE, ItemStack.EMPTY );
-        m_inventoryChanged = false;
-        m_brain = new TurtleBrain( this );
-        m_moveState = MoveState.NOT_MOVED;
     }
 
-    public boolean hasMoved()
+    private boolean hasMoved()
     {
         return m_moveState == MoveState.MOVED;
     }
@@ -237,18 +229,15 @@ public class TileTurtle extends TileComputerBase implements ITurtleTile, Default
     {
         super.tick();
         m_brain.update();
-        synchronized( m_inventory )
+        if( !getWorld().isRemote && m_inventoryChanged )
         {
-            if( !getWorld().isRemote && m_inventoryChanged )
-            {
-                ServerComputer computer = getServerComputer();
-                if( computer != null ) computer.queueEvent( "turtle_inventory" );
+            ServerComputer computer = getServerComputer();
+            if( computer != null ) computer.queueEvent( "turtle_inventory" );
 
-                m_inventoryChanged = false;
-                for( int n = 0; n < getSizeInventory(); n++ )
-                {
-                    m_previousInventory.set( n, InventoryUtil.copyItem( getStackInSlot( n ) ) );
-                }
+            m_inventoryChanged = false;
+            for( int n = 0; n < getSizeInventory(); n++ )
+            {
+                m_previousInventory.set( n, InventoryUtil.copyItem( getStackInSlot( n ) ) );
             }
         }
     }
@@ -288,8 +277,8 @@ public class TileTurtle extends TileComputerBase implements ITurtleTile, Default
 
         // Read inventory
         ListNBT nbttaglist = nbt.getList( "Items", Constants.NBT.TAG_COMPOUND );
-        m_inventory = NonNullList.withSize( INVENTORY_SIZE, ItemStack.EMPTY );
-        m_previousInventory = NonNullList.withSize( INVENTORY_SIZE, ItemStack.EMPTY );
+        m_inventory.clear();
+        m_previousInventory.clear();
         for( int i = 0; i < nbttaglist.size(); i++ )
         {
             CompoundNBT tag = nbttaglist.getCompound( i );
@@ -396,7 +385,7 @@ public class TileTurtle extends TileComputerBase implements ITurtleTile, Default
         return m_brain.getToolRenderAngle( side, f );
     }
 
-    public void setOwningPlayer( GameProfile player )
+    void setOwningPlayer( GameProfile player )
     {
         m_brain.setOwningPlayer( player );
         markDirty();
@@ -424,109 +413,76 @@ public class TileTurtle extends TileComputerBase implements ITurtleTile, Default
     @Override
     public ItemStack getStackInSlot( int slot )
     {
-        if( slot >= 0 && slot < INVENTORY_SIZE )
-        {
-            synchronized( m_inventory )
-            {
-                return m_inventory.get( slot );
-            }
-        }
-        return ItemStack.EMPTY;
+        return slot >= 0 && slot < INVENTORY_SIZE ? m_inventory.get( slot ) : ItemStack.EMPTY;
     }
 
     @Nonnull
     @Override
     public ItemStack removeStackFromSlot( int slot )
     {
-        synchronized( m_inventory )
-        {
-            ItemStack result = getStackInSlot( slot );
-            setInventorySlotContents( slot, ItemStack.EMPTY );
-            return result;
-        }
+        ItemStack result = getStackInSlot( slot );
+        setInventorySlotContents( slot, ItemStack.EMPTY );
+        return result;
     }
 
     @Nonnull
     @Override
     public ItemStack decrStackSize( int slot, int count )
     {
-        if( count == 0 )
+        if( count == 0 ) return ItemStack.EMPTY;
+
+        ItemStack stack = getStackInSlot( slot );
+        if( stack.isEmpty() ) return ItemStack.EMPTY;
+
+        if( stack.getCount() <= count )
         {
-            return ItemStack.EMPTY;
+            setInventorySlotContents( slot, ItemStack.EMPTY );
+            return stack;
         }
 
-        synchronized( m_inventory )
-        {
-            ItemStack stack = getStackInSlot( slot );
-            if( stack.isEmpty() )
-            {
-                return ItemStack.EMPTY;
-            }
-
-            if( stack.getCount() <= count )
-            {
-                setInventorySlotContents( slot, ItemStack.EMPTY );
-                return stack;
-            }
-
-            ItemStack part = stack.split( count );
-            onInventoryDefinitelyChanged();
-            return part;
-        }
+        ItemStack part = stack.split( count );
+        onInventoryDefinitelyChanged();
+        return part;
     }
 
     @Override
     public void setInventorySlotContents( int i, @Nonnull ItemStack stack )
     {
-        if( i >= 0 && i < INVENTORY_SIZE )
+        if( i >= 0 && i < INVENTORY_SIZE && !InventoryUtil.areItemsEqual( stack, m_inventory.get( i ) ) )
         {
-            synchronized( m_inventory )
-            {
-                if( !InventoryUtil.areItemsEqual( stack, m_inventory.get( i ) ) )
-                {
-                    m_inventory.set( i, stack );
-                    onInventoryDefinitelyChanged();
-                }
-            }
+            m_inventory.set( i, stack );
+            onInventoryDefinitelyChanged();
         }
     }
 
     @Override
     public void clear()
     {
-        synchronized( m_inventory )
+        boolean changed = false;
+        for( int i = 0; i < INVENTORY_SIZE; i++ )
         {
-            boolean changed = false;
-            for( int i = 0; i < INVENTORY_SIZE; i++ )
+            if( !m_inventory.get( i ).isEmpty() )
             {
-                if( !m_inventory.get( i ).isEmpty() )
-                {
-                    m_inventory.set( i, ItemStack.EMPTY );
-                    changed = true;
-                }
-            }
-            if( changed )
-            {
-                onInventoryDefinitelyChanged();
+                m_inventory.set( i, ItemStack.EMPTY );
+                changed = true;
             }
         }
+
+        if( changed ) onInventoryDefinitelyChanged();
     }
 
     @Override
     public void markDirty()
     {
         super.markDirty();
-        synchronized( m_inventory )
+        if( !m_inventoryChanged )
         {
-            if( !m_inventoryChanged )
+            for( int n = 0; n < getSizeInventory(); n++ )
             {
-                for( int n = 0; n < getSizeInventory(); n++ )
+                if( !ItemStack.areItemStacksEqual( getStackInSlot( n ), m_previousInventory.get( n ) ) )
                 {
-                    if( !ItemStack.areItemStacksEqual( getStackInSlot( n ), m_previousInventory.get( n ) ) )
-                    {
-                        m_inventoryChanged = true;
-                        break;
-                    }
+                    m_inventoryChanged = true;
+                    break;
                 }
             }
         }
@@ -587,8 +543,8 @@ public class TileTurtle extends TileComputerBase implements ITurtleTile, Default
     public void transferStateFrom( TileTurtle copy )
     {
         super.transferStateFrom( copy );
-        m_inventory = copy.m_inventory;
-        m_previousInventory = copy.m_previousInventory;
+        Collections.copy( m_inventory, copy.m_inventory );
+        Collections.copy( m_previousInventory, copy.m_previousInventory );
         m_inventoryChanged = copy.m_inventoryChanged;
         m_brain = copy.m_brain;
         m_brain.setOwner( this );
diff --git a/src/main/java/dan200/computercraft/shared/turtle/upgrades/TurtleTool.java b/src/main/java/dan200/computercraft/shared/turtle/upgrades/TurtleTool.java
index 5f82731b9..153954e56 100644
--- a/src/main/java/dan200/computercraft/shared/turtle/upgrades/TurtleTool.java
+++ b/src/main/java/dan200/computercraft/shared/turtle/upgrades/TurtleTool.java
@@ -248,7 +248,7 @@ public class TurtleTool extends AbstractTurtleUpgrade
         boolean canHarvest = state.canHarvestBlock( world, blockPosition, turtlePlayer );
         boolean canBreak = state.removedByPlayer( world, blockPosition, turtlePlayer, canHarvest, fluidState );
         if( canBreak ) state.getBlock().onPlayerDestroy( world, blockPosition, state );
-        if( canHarvest )
+        if( canHarvest && canBreak )
         {
             state.getBlock().harvestBlock( world, turtlePlayer, blockPosition, state, tile, turtlePlayer.getHeldItemMainhand() );
         }
diff --git a/src/main/resources/assets/computercraft/lua/rom/modules/main/cc/expect.lua b/src/main/resources/assets/computercraft/lua/rom/modules/main/cc/expect.lua
new file mode 100644
index 000000000..14db7231c
--- /dev/null
+++ b/src/main/resources/assets/computercraft/lua/rom/modules/main/cc/expect.lua
@@ -0,0 +1,46 @@
+--- The @{craftos.expect} library provides helper functions for verifying that
+-- function arguments are well-formed and of the correct type.
+--
+-- @module craftos.expect
+
+local native_select, native_type = select, type
+
+--- Expect an argument to have a specific type.
+--
+-- @tparam int index The 1-based argument index.
+-- @param value The argument's value.
+-- @tparam string ... The allowed types of the argument.
+-- @throws If the value is not one of the allowed types.
+local function expect(index, value, ...)
+    local t = native_type(value)
+    for i = 1, native_select("#", ...) do
+        if t == native_select(i, ...) then return true end
+    end
+
+    local types = table.pack(...)
+    for i = types.n, 1, -1 do
+        if types[i] == "nil" then table.remove(types, i) end
+    end
+
+    local type_names
+    if #types <= 1 then
+        type_names = tostring(...)
+    else
+        type_names = table.concat(types, ", ", 1, #types - 1) .. " or " .. types[#types]
+    end
+
+    -- If we can determine the function name with a high level of confidence, try to include it.
+    local name
+    if native_type(debug) == "table" and native_type(debug.getinfo) == "function" then
+        local ok, info = pcall(debug.getinfo, 3, "nS")
+        if ok and info.name and #info.name ~= "" and info.what ~= "C" then name = info.name end
+    end
+
+    if name then
+        error( ("bad argument #%d to '%s' (expected %s, got %s)"):format(index, name, type_names, t), 3 )
+    else
+        error( ("bad argument #%d (expected %s, got %s)"):format(index, type_names, t), 3 )
+    end
+end
+
+return { expect = expect }
diff --git a/src/main/resources/data/computercraft/lua/bios.lua b/src/main/resources/data/computercraft/lua/bios.lua
index 5f24d94cb..417b55bc9 100644
--- a/src/main/resources/data/computercraft/lua/bios.lua
+++ b/src/main/resources/data/computercraft/lua/bios.lua
@@ -1,48 +1,19 @@
-local native_select, native_type = select, type
-
---- Expect an argument to have a specific type.
+-- Load in expect from the module path.
 --
--- @tparam int index The 1-based argument index.
--- @param value The argument's value.
--- @tparam string ... The allowed types of the argument.
--- @throws If the value is not one of the allowed types.
-local function expect(index, value, ...)
-    local t = native_type(value)
-    for i = 1, native_select("#", ...) do
-        if t == native_select(i, ...) then return true end
-    end
+-- Ideally we'd use require, but that is part of the shell, and so is not
+-- available to the BIOS or any APIs. All APIs load this using dofile, but that
+-- has not been defined at this point.
+local expect
 
-    local types = table.pack(...)
-    for i = types.n, 1, -1 do
-        if types[i] == "nil" then table.remove(types, i) end
-    end
+do
+    local h = fs.open("rom/modules/main/cc/expect.lua", "r")
+    local f, err = loadstring(h.readAll(), "@expect.lua")
+    h.close()
 
-    local type_names
-    if #types <= 1 then
-        type_names = tostring(...)
-    else
-        type_names = table.concat(types, ", ", 1, #types - 1) .. " or " .. types[#types]
-    end
-
-    -- If we can determine the function name with a high level of confidence, try to include it.
-    local name
-    if native_type(debug) == "table" and native_type(debug.getinfo) == "function" then
-        local ok, info = pcall(debug.getinfo, 3, "nS")
-        if ok and info.name and #info.name ~= "" and info.what ~= "C" then name = info.name end
-    end
-
-    if name then
-        error( ("bad argument #%d to '%s' (expected %s, got %s)"):format(index, name, type_names, t), 3 )
-    else
-        error( ("bad argument #%d (expected %s, got %s)"):format(index, type_names, t), 3 )
-    end
+    if not f then error(err) end
+    expect = f().expect
 end
 
--- We expose expect in the global table as APIs need to access it, but give it
--- a non-identifier name - meaning it does not show up in auto-completion.
--- expect is an internal function, and should not be used by users.
-_G["~expect"] = expect
-
 if _VERSION == "Lua 5.1" then
     -- If we're on Lua 5.1, install parts of the Lua 5.2/5.3 API so that programs can be written against it
     local type = type
@@ -568,23 +539,28 @@ function read( _sReplaceChar, _tHistory, _fnComplete, _sDefault )
     return sLine
 end
 
-function loadfile( _sFile, _tEnv )
-    expect(1, _sFile, "string")
-    expect(2, _tEnv, "table", "nil")
-
-    local file = fs.open( _sFile, "r" )
-    if file then
-        local func, err = load( file.readAll(), "@" .. fs.getName( _sFile ), "t", _tEnv )
-        file.close()
-        return func, err
+function loadfile( filename, mode, env )
+    -- Support the previous `loadfile(filename, env)` form instead.
+    if type(mode) == "table" and env == nil then
+        mode, env = nil, mode
     end
-    return nil, "File not found"
+
+    expect(1, filename, "string")
+    expect(2, mode, "string", "nil")
+    expect(3, env, "table", "nil")
+
+    local file = fs.open( filename, "r" )
+    if not file then return nil, "File not found" end
+
+    local func, err = load( file.readAll(), "@" .. fs.getName( filename ), mode, env )
+    file.close()
+    return func, err
 end
 
 function dofile( _sFile )
     expect(1, _sFile, "string")
 
-    local fnFile, e = loadfile( _sFile, _G )
+    local fnFile, e = loadfile( _sFile, nil, _G )
     if fnFile then
         return fnFile()
     else
@@ -600,7 +576,7 @@ function os.run( _tEnv, _sPath, ... )
     local tArgs = table.pack( ... )
     local tEnv = _tEnv
     setmetatable( tEnv, { __index = _G } )
-    local fnFile, err = loadfile( _sPath, tEnv )
+    local fnFile, err = loadfile( _sPath, nil, tEnv )
     if fnFile then
         local ok, err = pcall( function()
             fnFile( table.unpack( tArgs, 1, tArgs.n ) )
@@ -634,7 +610,7 @@ function os.loadAPI( _sPath )
 
     local tEnv = {}
     setmetatable( tEnv, { __index = _G } )
-    local fnAPI, err = loadfile( _sPath, tEnv )
+    local fnAPI, err = loadfile( _sPath, nil, tEnv )
     if fnAPI then
         local ok, err = pcall( fnAPI )
         if not ok then
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/colors.lua b/src/main/resources/data/computercraft/lua/rom/apis/colors.lua
index c113d324a..969f984e5 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/colors.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/colors.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 -- Colors
 white = 1
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/gps.lua b/src/main/resources/data/computercraft/lua/rom/apis/gps.lua
index d27ba1beb..08f698eb2 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/gps.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/gps.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 CHANNEL_GPS = 65534
 
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/help.lua b/src/main/resources/data/computercraft/lua/rom/apis/help.lua
index 63ca4d1d6..b4e69921f 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/help.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/help.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 local sPath = "/rom/help"
 
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/io.lua b/src/main/resources/data/computercraft/lua/rom/apis/io.lua
index cd8a02acb..efbc13b40 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/io.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/io.lua
@@ -1,6 +1,6 @@
 -- Definition for the IO API
 
-local expect, typeOf = _G["~expect"], _G.type
+local expect, typeOf = dofile("rom/modules/main/cc/expect.lua").expect, _G.type
 
 --- If we return nil then close the file, as we've reached the end.
 -- We use this weird wrapper function as we wish to preserve the varargs
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/keys.lua b/src/main/resources/data/computercraft/lua/rom/apis/keys.lua
index ffe2ddcf7..06dc1c6d1 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/keys.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/keys.lua
@@ -8,7 +8,7 @@
 -- taught me anything, it's that emulating LWJGL's weird key handling is nigh-on
 -- impossible.
 
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 local tKeys = {}
 tKeys[32] = 'space'
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/paintutils.lua b/src/main/resources/data/computercraft/lua/rom/apis/paintutils.lua
index fe475a6fa..0988945e0 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/paintutils.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/paintutils.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 local function drawPixelInternal( xPos, yPos )
     term.setCursorPos( xPos, yPos )
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/peripheral.lua b/src/main/resources/data/computercraft/lua/rom/apis/peripheral.lua
index c34a59eb8..3f4fdc150 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/peripheral.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/peripheral.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 local native = peripheral
 
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/rednet.lua b/src/main/resources/data/computercraft/lua/rom/apis/rednet.lua
index 8867e4caf..5af3a4706 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/rednet.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/rednet.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 CHANNEL_BROADCAST = 65535
 CHANNEL_REPEAT = 65533
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/settings.lua b/src/main/resources/data/computercraft/lua/rom/apis/settings.lua
index 3290a03d3..d60510937 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/settings.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/settings.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 local tSettings = {}
 
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/term.lua b/src/main/resources/data/computercraft/lua/rom/apis/term.lua
index d93c3f367..e9ad00408 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/term.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/term.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 local native = (term.native and term.native()) or term
 local redirectTarget = native
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/textutils.lua b/src/main/resources/data/computercraft/lua/rom/apis/textutils.lua
index ba9f5e033..447babb44 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/textutils.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/textutils.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 function slowWrite( sText, nRate )
     expect(2, nRate, "number", "nil")
diff --git a/src/main/resources/data/computercraft/lua/rom/apis/window.lua b/src/main/resources/data/computercraft/lua/rom/apis/window.lua
index 6d9ab38ec..b1e1b3db4 100644
--- a/src/main/resources/data/computercraft/lua/rom/apis/window.lua
+++ b/src/main/resources/data/computercraft/lua/rom/apis/window.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 local tHex = {
     [ colors.white ] = "0",
@@ -388,6 +388,16 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
         return nBackgroundColor
     end
 
+    function window.getLine(y)
+        if type(y) ~= "number" then expect(1, y, "number") end
+
+        if y < 1 or y > nHeight then
+            error("Line is out of range.", 2)
+        end
+
+        return tLines[y].text, tLines[y].textColor, tLines[y].backgroundColor
+    end
+
     -- Other functions
     function window.setVisible( bVis )
         if type(bVis) ~= "boolean" then expect(1, bVis, "boolean") end
diff --git a/src/main/resources/data/computercraft/lua/rom/help/changelog.txt b/src/main/resources/data/computercraft/lua/rom/help/changelog.txt
index a9e543539..c9fb3ad1b 100644
--- a/src/main/resources/data/computercraft/lua/rom/help/changelog.txt
+++ b/src/main/resources/data/computercraft/lua/rom/help/changelog.txt
@@ -1,3 +1,20 @@
+New features in CC: Tweaked 1.84.0
+
+* Improve validation in rename, copy and delete programs
+* Add window.getLine - the inverse of blit
+* turtle.refuel no longer consumes more fuel than needed
+* Add "cc.expect" module, for improved argument type checks
+* Mount the ROM from all mod jars, not just CC's
+
+And several bug fixes:
+* Ensure file error messages use the absolute correct path
+* Fix NPE when closing a file multiple times.
+* Do not load chunks when calling writeDescription.
+* Fix the signature of loadfile
+* Fix turtles harvesting blocks multiple times
+* Improve thread-safety of various peripherals
+* Prevent printed pages having massive/malformed titles
+
 # New features in CC: Tweaked 1.83.1
 
 * Add several new MOTD messages (JakobDev)
diff --git a/src/main/resources/data/computercraft/lua/rom/help/whatsnew.txt b/src/main/resources/data/computercraft/lua/rom/help/whatsnew.txt
index 55e2ac007..20957867e 100644
--- a/src/main/resources/data/computercraft/lua/rom/help/whatsnew.txt
+++ b/src/main/resources/data/computercraft/lua/rom/help/whatsnew.txt
@@ -1,10 +1,18 @@
-New features in CC: Tweaked 1.83.1
+New features in CC: Tweaked 1.84.0
 
-* Add several new MOTD messages (JakobDev)
+* Improve validation in rename, copy and delete programs
+* Add window.getLine - the inverse of blit
+* turtle.refuel no longer consumes more fuel than needed
+* Add "cc.expect" module, for improved argument type checks
+* Mount the ROM from all mod jars, not just CC's
 
 And several bug fixes:
-* Fix type check in `rednet.lookup`
-* Error if turtle and pocket computer programs are run on the wrong system (JakobDev)
-* Do not discard varargs after a nil.
+* Ensure file error messages use the absolute correct path
+* Fix NPE when closing a file multiple times.
+* Do not load chunks when calling writeDescription.
+* Fix the signature of loadfile
+* Fix turtles harvesting blocks multiple times
+* Improve thread-safety of various peripherals
+* Prevent printed pages having massive/malformed titles
 
 Type "help changelog" to see the full version history.
diff --git a/src/main/resources/data/computercraft/lua/rom/help/window.txt b/src/main/resources/data/computercraft/lua/rom/help/window.txt
index 7fcf5d43e..ea3ea3d78 100644
--- a/src/main/resources/data/computercraft/lua/rom/help/window.txt
+++ b/src/main/resources/data/computercraft/lua/rom/help/window.txt
@@ -23,3 +23,4 @@ getPosition()
 reposition( x, y, width, height )
 getPaletteColor( color )
 setPaletteColor( color, r, g, b )
+getLine()
diff --git a/src/main/resources/data/computercraft/lua/rom/programs/advanced/multishell.lua b/src/main/resources/data/computercraft/lua/rom/programs/advanced/multishell.lua
index 00988eb27..51c1c1753 100644
--- a/src/main/resources/data/computercraft/lua/rom/programs/advanced/multishell.lua
+++ b/src/main/resources/data/computercraft/lua/rom/programs/advanced/multishell.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 -- Setup process switching
 local parentTerm = term.current()
diff --git a/src/main/resources/data/computercraft/lua/rom/programs/edit.lua b/src/main/resources/data/computercraft/lua/rom/programs/edit.lua
index 43910d56a..e481dd9cd 100644
--- a/src/main/resources/data/computercraft/lua/rom/programs/edit.lua
+++ b/src/main/resources/data/computercraft/lua/rom/programs/edit.lua
@@ -329,7 +329,7 @@ local tMenuFuncs = {
                 printer.setPageTitle( sName.." (page "..nPage..")" )
             end
 
-            while not printer.newPage()    do
+            while not printer.newPage() do
                 if printer.getInkLevel() < 1 then
                     sStatus = "Printer out of ink, please refill"
                 elseif printer.getPaperLevel() < 1 then
@@ -342,7 +342,6 @@ local tMenuFuncs = {
                 redrawMenu()
                 term.redirect( printerTerminal )
 
-                local timer = os.startTimer(0.5)
                 sleep(0.5)
             end
 
diff --git a/src/main/resources/data/computercraft/lua/rom/programs/shell.lua b/src/main/resources/data/computercraft/lua/rom/programs/shell.lua
index bc95dbf2e..3c15e04b3 100644
--- a/src/main/resources/data/computercraft/lua/rom/programs/shell.lua
+++ b/src/main/resources/data/computercraft/lua/rom/programs/shell.lua
@@ -1,4 +1,4 @@
-local expect = _G["~expect"]
+local expect = dofile("rom/modules/main/cc/expect.lua").expect
 
 local multishell = multishell
 local parentShell = shell
@@ -56,7 +56,7 @@ local function createShellEnv( sDir )
                     sPath = fs.combine(sDir, sPath)
                 end
                 if fs.exists(sPath) and not fs.isDir(sPath) then
-                    local fnFile, sError = loadfile( sPath, tEnv )
+                    local fnFile, sError = loadfile( sPath, nil, tEnv )
                     if fnFile then
                         return fnFile, sPath
                     else
diff --git a/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java b/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java
index 86cbd6c03..7f083c7ee 100644
--- a/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java
+++ b/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java
@@ -90,7 +90,7 @@ public class ComputerTestDelegate
         try( WritableByteChannel channel = mount.openChannelForWrite( "startup.lua" );
              Writer writer = Channels.newWriter( channel, StandardCharsets.UTF_8.newEncoder(), -1 ) )
         {
-            writer.write( "loadfile('test/mcfly.lua', _ENV)('test/spec') cct_test.finish()" );
+            writer.write( "loadfile('test/mcfly.lua', nil, _ENV)('test/spec') cct_test.finish()" );
         }
 
         computer = new Computer( new BasicEnvironment( mount ), term, 0 );
diff --git a/src/test/java/dan200/computercraft/core/computer/ComputerBootstrap.java b/src/test/java/dan200/computercraft/core/computer/ComputerBootstrap.java
index 79b67eba2..a0e044bb2 100644
--- a/src/test/java/dan200/computercraft/core/computer/ComputerBootstrap.java
+++ b/src/test/java/dan200/computercraft/core/computer/ComputerBootstrap.java
@@ -33,7 +33,7 @@ public class ComputerBootstrap
     {
         MemoryMount mount = new MemoryMount()
             .addFile( "test.lua", program )
-            .addFile( "startup", "assertion.assert(pcall(loadfile('test.lua', _ENV))) os.shutdown()" );
+            .addFile( "startup", "assertion.assert(pcall(loadfile('test.lua', nil, _ENV))) os.shutdown()" );
 
         run( mount, x -> { } );
     }
diff --git a/src/test/resources/test-rom/mcfly.lua b/src/test/resources/test-rom/mcfly.lua
index 38ebf9e83..e46d2dc4e 100644
--- a/src/test/resources/test-rom/mcfly.lua
+++ b/src/test/resources/test-rom/mcfly.lua
@@ -27,18 +27,58 @@ local function check(func, arg, ty, val)
     end
 end
 
+--- A stub - wraps a value within a a table,
+local stub_mt = {}
+stub_mt.__index = stub_mt
+
+--- Revert this stub, restoring the previous value.
+--
+-- Note, a stub can only be reverted once.
+function stub_mt:revert()
+    if not self.active then return end
+
+    self.active = false
+    rawset(self.stubbed_in, self.key, self.original)
+end
+
 local active_stubs = {}
 
---- Stub a global variable with a specific value
---
--- @tparam string var The variable to stub
--- @param value The value to stub it with
-local function stub(tbl, var, value)
-    check('stub', 1, 'table', tbl)
-    check('stub', 2, 'string', var)
+local function default_stub() end
 
-    table.insert(active_stubs, { tbl = tbl, var = var, value = tbl[var] })
-    rawset(tbl, var, value)
+--- Stub a table entry with a new value.
+--
+-- @tparam table
+-- @tparam string key The variable to stub
+-- @param[opt] value The value to stub it with. If this is a function, one can
+-- use the various stub expectation methods to determine what it was called
+-- with. Defaults to an empty function - pass @{nil} in explicitly to set the
+-- value to nil.
+-- @treturn Stub The resulting stub
+local function stub(tbl, key, ...)
+    check('stub', 1, 'table', tbl)
+    check('stub', 2, 'string', key)
+
+    local stub = setmetatable({
+        active = true,
+        stubbed_in = tbl,
+        key = key,
+        original = rawget(tbl, key),
+    }, stub_mt)
+
+    local value = ...
+    if select('#', ...) == 0 then value = default_stub end
+    if type(value) == "function" then
+        local arguments, delegate = {}, value
+        stub.arguments = arguments
+        value = function(...)
+            arguments[#arguments + 1] = table.pack(...)
+            return delegate(...)
+        end
+    end
+
+    table.insert(active_stubs, stub)
+    rawset(tbl, key, value)
+    return stub
 end
 
 --- Capture the current global state of the computer
@@ -51,16 +91,14 @@ local function push_state()
         output = io.output(),
         dir = shell.dir(),
         path = shell.path(),
+        aliases = shell.aliases(),
         stubs = stubs,
     }
 end
 
 --- Restore the global state of the computer to a previous version
 local function pop_state(state)
-    for i = #active_stubs, 1, -1 do
-        local stub = active_stubs[i]
-        rawset(stub.tbl, stub.var, stub.value)
-    end
+    for i = #active_stubs, 1, -1 do active_stubs[i]:revert() end
 
     active_stubs = state.stubs
 
@@ -69,6 +107,14 @@ local function pop_state(state)
     io.output(state.output)
     shell.setDir(state.dir)
     shell.setPath(state.path)
+
+    local aliases = shell.aliases()
+    for k in pairs(aliases) do
+        if not state.aliases[k] then shell.clearAlias(k) end
+    end
+    for k, v in pairs(state.aliases) do
+        if aliases[k] ~= v then shell.setAlias(k, v) end
+    end
 end
 
 local error_mt = { __tostring = function(self) return self.message end }
@@ -210,6 +256,16 @@ local function matches(eq, exact, left, right)
     return true
 end
 
+local function pairwise_equal(left, right)
+    if left.n ~= right.n then return false end
+
+    for i = 1, left.n do
+        if left[i] ~= right[i] then return false end
+    end
+
+    return true
+end
+
 --- Assert that this expectation is structurally equivalent to
 -- the provided object.
 --
@@ -236,6 +292,70 @@ function expect_mt:matches(value)
     return self
 end
 
+--- Assert that this stub was called a specific number of times.
+--
+-- @tparam[opt] number The exact number of times the function must be called.
+-- If not given just require the function to be called at least once.
+-- @raises If this function was not called the expected number of times.
+function expect_mt:called(times)
+    if getmetatable(self.value) ~= stub_mt or self.value.arguments == nil then
+        fail(("Expected stubbed function, got %s"):format(type(self.value)))
+    end
+
+    local called = #self.value.arguments
+
+    if times == nil then
+        if called == 0 then
+            fail("Expected stub to be called\nbut it was not.")
+        end
+    else
+        check('stub', 1, 'number', times)
+        if called ~= times then
+            fail(("Expected stub to be called %d times\nbut was called %d times."):format(times, called))
+        end
+    end
+
+    return self
+end
+
+local function called_with_check(eq, self, ...)
+    if getmetatable(self.value) ~= stub_mt or self.value.arguments == nil then
+        fail(("Expected stubbed function, got %s"):format(type(self.value)))
+    end
+
+    local exp_args = table.pack(...)
+    local actual_args = self.value.arguments
+    for i = 1, #actual_args do
+        if eq(actual_args[i], exp_args) then return self end
+    end
+
+    local head = ("Expected stub to be called with %s\nbut was"):format(format(exp_args))
+    if #actual_args == 0 then
+        fail(head .. " not called at all")
+    elseif #actual_args == 1 then
+        fail(("%s called with %s."):format(head, format(actual_args[1])))
+    else
+        local lines = { head .. " called with:" }
+        for i = 1, #actual_args do lines[i + 1] = " - " .. format(actual_args[i]) end
+
+        fail(table.concat(lines, "\n"))
+    end
+end
+
+--- Assert that this stub was called with a set of arguments
+--
+-- Arguments are compared using exact equality.
+function expect_mt:called_with(...)
+    return called_with_check(pairwise_equal, self, ...)
+end
+
+--- Assert that this stub was called with a set of arguments
+--
+-- Arguments are compared using matching.
+function expect_mt:called_with_matching(...)
+    return called_with_check(matches, self, ...)
+end
+
 local expect = setmetatable( {
     --- Construct an expectation on the error message calling this function
     -- produces
@@ -381,7 +501,7 @@ do
             if fs.isDir(file) then
                 run_in(file)
             elseif file:sub(-#suffix) == suffix then
-                local fun, err = loadfile(file, env)
+                local fun, err = loadfile(file, nil, env)
                 if not fun then
                     do_test { name = file:sub(#root_dir + 2), error = { message = err } }
                 else
diff --git a/src/test/resources/test-rom/spec/apis/window_spec.lua b/src/test/resources/test-rom/spec/apis/window_spec.lua
index 16fe229e1..2fb6d119d 100644
--- a/src/test/resources/test-rom/spec/apis/window_spec.lua
+++ b/src/test/resources/test-rom/spec/apis/window_spec.lua
@@ -120,4 +120,21 @@ describe("The window library", function()
             expect.error(w.reposition, 1, 1, 1, nil):eq("bad argument #4 (expected number, got nil)")
         end)
     end)
+
+    describe("Window.getLine", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.getLine(1)
+            local _, y = w.getSize()
+            expect.error(w.getLine, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(w.getLine, 0):eq("Line is out of range.")
+            expect.error(w.getLine, y + 1):eq("Line is out of range.")
+        end)
+
+        it("provides a line's contents", function()
+            local w = mk()
+            w.blit("test", "aaaa", "4444")
+            expect({ w.getLine(1) }):same { "test ", "aaaa0", "4444f" }
+        end)
+    end)
 end)
diff --git a/src/test/resources/test-rom/spec/base_spec.lua b/src/test/resources/test-rom/spec/base_spec.lua
index 191d87eab..43de8239d 100644
--- a/src/test/resources/test-rom/spec/base_spec.lua
+++ b/src/test/resources/test-rom/spec/base_spec.lua
@@ -1,36 +1,4 @@
 describe("The Lua base library", function()
-    describe("expect", function()
-        local e = _G["~expect"]
-
-        it("checks a single type", function()
-            expect(e(1, "test", "string")):eq(true)
-            expect(e(1, 2, "number")):eq(true)
-
-            expect.error(e, 1, nil, "string"):eq("bad argument #1 (expected string, got nil)")
-            expect.error(e, 2, 1, "nil"):eq("bad argument #2 (expected nil, got number)")
-        end)
-
-        it("checks multiple types", function()
-            expect(e(1, "test", "string", "number")):eq(true)
-            expect(e(1, 2, "string", "number")):eq(true)
-
-            expect.error(e, 1, nil, "string", "number"):eq("bad argument #1 (expected string or number, got nil)")
-            expect.error(e, 2, false, "string", "table", "number", "nil")
-                  :eq("bad argument #2 (expected string, table or number, got boolean)")
-        end)
-
-        it("includes the function name", function()
-            local function worker()
-                expect(e(1, nil, "string")):eq(true)
-            end
-            local function trampoline()
-                worker()
-            end
-
-            expect.error(trampoline):eq("base_spec.lua:27: bad argument #1 to 'worker' (expected string, got nil)")
-        end)
-    end)
-
     describe("sleep", function()
         it("validates arguments", function()
             sleep(0)
@@ -48,18 +16,43 @@ describe("The Lua base library", function()
     end)
 
     describe("loadfile", function()
+        local function make_file()
+            local tmp = fs.open("test-files/out.lua", "w")
+            tmp.write("return _ENV")
+            tmp.close()
+        end
+
         it("validates arguments", function()
             loadfile("")
-            loadfile("", {})
+            loadfile("", "")
+            loadfile("", "", {})
 
             expect.error(loadfile, nil):eq("bad argument #1 (expected string, got nil)")
-            expect.error(loadfile, "", false):eq("bad argument #2 (expected table, got boolean)")
+            expect.error(loadfile, "", false):eq("bad argument #2 (expected string, got boolean)")
+            expect.error(loadfile, "", "", false):eq("bad argument #3 (expected table, got boolean)")
         end)
 
         it("prefixes the filename with @", function()
             local info = debug.getinfo(loadfile("/rom/startup.lua"), "S")
             expect(info):matches { short_src = "startup.lua", source = "@startup.lua" }
         end)
+
+        it("loads a file with the global environment", function()
+            make_file()
+            expect(loadfile("test-files/out.lua")()):eq(_G)
+        end)
+
+        it("loads a file with a specific environment", function()
+            make_file()
+            local env = {}
+            expect(loadfile("test-files/out.lua", nil, env)()):eq(env)
+        end)
+
+        it("supports the old-style argument form", function()
+            make_file()
+            local env = {}
+            expect(loadfile("test-files/out.lua", env)()):eq(env)
+        end)
     end)
 
     describe("dofile", function()
diff --git a/src/test/resources/test-rom/spec/modules/cc/expect_spec.lua b/src/test/resources/test-rom/spec/modules/cc/expect_spec.lua
new file mode 100644
index 000000000..36f076f60
--- /dev/null
+++ b/src/test/resources/test-rom/spec/modules/cc/expect_spec.lua
@@ -0,0 +1,31 @@
+describe("cc.expect", function()
+    local e = require("cc.expect")
+
+    it("checks a single type", function()
+        expect(e.expect(1, "test", "string")):eq(true)
+        expect(e.expect(1, 2, "number")):eq(true)
+
+        expect.error(e.expect, 1, nil, "string"):eq("bad argument #1 (expected string, got nil)")
+        expect.error(e.expect, 2, 1, "nil"):eq("bad argument #2 (expected nil, got number)")
+    end)
+
+    it("checks multiple types", function()
+        expect(e.expect(1, "test", "string", "number")):eq(true)
+        expect(e.expect(1, 2, "string", "number")):eq(true)
+
+        expect.error(e.expect, 1, nil, "string", "number"):eq("bad argument #1 (expected string or number, got nil)")
+        expect.error(e.expect, 2, false, "string", "table", "number", "nil")
+              :eq("bad argument #2 (expected string, table or number, got boolean)")
+    end)
+
+    it("includes the function name", function()
+        local function worker()
+            expect(e.expect(1, nil, "string")):eq(true)
+        end
+        local function trampoline()
+            worker()
+        end
+
+        expect.error(trampoline):eq("expect_spec.lua:26: bad argument #1 to 'worker' (expected string, got nil)")
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/advanced/bg_spec.lua b/src/test/resources/test-rom/spec/programs/advanced/bg_spec.lua
new file mode 100644
index 000000000..a53dc2f2d
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/advanced/bg_spec.lua
@@ -0,0 +1,11 @@
+local capture = require "test_helpers".capture_program
+
+describe("The bg program", function()
+    it("opens a tab in the background", function()
+        local openTab = stub(shell, "openTab", function() return 12 end)
+        local switchTab = stub(shell, "switchTab")
+        capture(stub, "bg")
+        expect(openTab):called_with("shell")
+        expect(switchTab):called(0)
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/advanced/fg_spec.lua b/src/test/resources/test-rom/spec/programs/advanced/fg_spec.lua
new file mode 100644
index 000000000..8e5cc4f26
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/advanced/fg_spec.lua
@@ -0,0 +1,11 @@
+local capture = require "test_helpers".capture_program
+
+describe("The fg program", function()
+    it("opens the shell in the foreground", function()
+        local openTab = stub(shell, "openTab", function() return 12 end)
+        local switchTab = stub(shell, "switchTab")
+        capture(stub, "fg")
+        expect(openTab):called_with("shell")
+        expect(switchTab):called_with(12)
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/alias_spec.lua b/src/test/resources/test-rom/spec/programs/alias_spec.lua
new file mode 100644
index 000000000..95a2c2556
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/alias_spec.lua
@@ -0,0 +1,28 @@
+local capture = require "test_helpers".capture_program
+
+describe("The alias program", function()
+    it("displays its usage when given too many arguments", function()
+        expect(capture(stub, "alias a b c"))
+            :matches { ok = true, output = "Usage: alias <alias> <program>\n", error = "" }
+    end)
+
+    it("lists aliases", function()
+        local pagedTabulate = stub(textutils, "pagedTabulate", function(x) print(table.unpack(x)) end)
+        stub(shell, "aliases", function() return { cp = "copy" } end)
+        expect(capture(stub, "alias"))
+            :matches { ok = true, output = "cp:copy\n", error = "" }
+        expect(pagedTabulate):called_with_matching({ "cp:copy" })
+    end)
+
+    it("sets an alias", function()
+        local setAlias = stub(shell, "setAlias")
+        capture(stub, "alias test Hello")
+        expect(setAlias):called_with("test", "Hello")
+    end)
+
+    it("clears an alias", function()
+        local clearAlias = stub(shell, "clearAlias")
+        capture(stub, "alias test")
+        expect(clearAlias):called_with("test")
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/cd_spec.lua b/src/test/resources/test-rom/spec/programs/cd_spec.lua
index 2826b8007..348d62944 100644
--- a/src/test/resources/test-rom/spec/programs/cd_spec.lua
+++ b/src/test/resources/test-rom/spec/programs/cd_spec.lua
@@ -1,19 +1,18 @@
 local capture = require "test_helpers".capture_program
 
 describe("The cd program", function()
-
-    it("cd into a directory", function()
-        shell.run("cd /rom/programs")
-        
-        expect(shell.dir()):eq("rom/programs")
+    it("changes into a directory", function()
+        local setDir = stub(shell, "setDir")
+        capture(stub, "cd /rom/programs")
+        expect(setDir):called_with("rom/programs")
     end)
 
-    it("cd into a not existing directory", function()
+    it("does not move into a non-existent directory", function()
         expect(capture(stub, "cd /rom/nothing"))
             :matches { ok = true, output = "Not a directory\n", error = "" }
     end)
-    
-    it("displays the usage with no arguments", function()
+
+    it("displays the usage when given no arguments", function()
         expect(capture(stub, "cd"))
             :matches { ok = true, output = "Usage: cd <path>\n", error = "" }
     end)
diff --git a/src/test/resources/test-rom/spec/programs/clear_spec.lua b/src/test/resources/test-rom/spec/programs/clear_spec.lua
new file mode 100644
index 000000000..f339c83ce
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/clear_spec.lua
@@ -0,0 +1,13 @@
+local capture = require "test_helpers".capture_program
+
+describe("The clear program", function()
+    it("clears the screen", function()
+        local clear = stub(term, "clear")
+        local setCursorPos = stub(term, "setCursorPos")
+
+        capture(stub, "clear")
+
+        expect(clear):called(1)
+        expect(setCursorPos):called_with(1, 1)
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/command/commands_spec.lua b/src/test/resources/test-rom/spec/programs/command/commands_spec.lua
new file mode 100644
index 000000000..d7261b9d1
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/command/commands_spec.lua
@@ -0,0 +1,20 @@
+local capture = require "test_helpers".capture_program
+
+describe("The commands program", function()
+    it("displays an error without the commands api", function()
+        stub(_G, "commands", nil)
+        expect(capture(stub, "/rom/programs/command/commands.lua"))
+            :matches { ok = true, output = "", error = "Requires a Command Computer.\n" }
+    end)
+
+    it("lists commands", function()
+        local pagedTabulate = stub(textutils, "pagedTabulate", function(x) print(table.unpack(x)) end)
+        stub(_G, "commands", {
+            list = function() return { "computercraft" } end
+        })
+
+        expect(capture(stub, "/rom/programs/command/commands.lua"))
+            :matches { ok = true, output = "Available commands:\ncomputercraft\n", error = "" }
+        expect(pagedTabulate):called_with_matching({ "computercraft" })
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/command/exec_spec.lua b/src/test/resources/test-rom/spec/programs/command/exec_spec.lua
new file mode 100644
index 000000000..869222b2c
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/command/exec_spec.lua
@@ -0,0 +1,33 @@
+local capture = require "test_helpers".capture_program
+
+describe("The exec program", function()
+    it("displays an error without the commands api", function()
+        stub(_G, "commands", nil)
+        expect(capture(stub, "/rom/programs/command/exec.lua"))
+            :matches { ok = true, output = "", error = "Requires a Command Computer.\n" }
+    end)
+
+    it("displays its usage when given no argument", function()
+        stub(_G, "commands", {})
+        expect(capture(stub, "/rom/programs/command/exec.lua"))
+            :matches { ok = true, output = "", error = "Usage: exec <command>\n" }
+    end)
+
+    it("runs a command", function()
+        stub(_G, "commands", {
+            exec = function() return true, {"Hello World!"} end
+        })
+
+        expect(capture(stub, "/rom/programs/command/exec.lua computercraft"))
+            :matches { ok = true, output = "Success\nHello World!\n", error = "" }
+    end)
+
+    it("reports command failures", function()
+        stub(_G,"commands",{
+            exec = function() return false, {"Hello World!"} end
+        })
+
+        expect(capture(stub, "/rom/programs/command/exec.lua computercraft"))
+            :matches { ok = true, output = "Hello World!\n", error = "Failed\n" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/drive_spec.lua b/src/test/resources/test-rom/spec/programs/drive_spec.lua
new file mode 100644
index 000000000..d175a60f7
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/drive_spec.lua
@@ -0,0 +1,16 @@
+local capture = require "test_helpers".capture_program
+
+describe("The drive program", function()
+    it("run the program", function()
+        local getFreeSpace = stub(fs, "getFreeSpace", function() return 1234e4 end)
+
+        expect(capture(stub, "drive"))
+            :matches { ok = true, output = "hdd (12.3MB remaining)\n", error = "" }
+        expect(getFreeSpace):called(1):called_with("")
+    end)
+
+    it("fails on a non-existent path", function()
+        expect(capture(stub, "drive /rom/nothing"))
+            :matches { ok = true, output = "No such path\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/edit_spec.lua b/src/test/resources/test-rom/spec/programs/edit_spec.lua
index dc8bcf9d0..137c39c83 100644
--- a/src/test/resources/test-rom/spec/programs/edit_spec.lua
+++ b/src/test/resources/test-rom/spec/programs/edit_spec.lua
@@ -1,10 +1,9 @@
 local capture = require "test_helpers".capture_program
+local testFile = require "test_helpers".testFile
 
 describe("The edit program", function()
 
-    it("displays its usage when given no argument", function()
-        multishell = nil
-        
+    it("displays its usage when given no argument", function()      
         expect(capture(stub, "edit"))
             :matches { ok = true, output = "Usage: edit <path>\n", error = "" }
     end)
diff --git a/src/test/resources/test-rom/spec/programs/eject_spec.lua b/src/test/resources/test-rom/spec/programs/eject_spec.lua
new file mode 100644
index 000000000..e524a8625
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/eject_spec.lua
@@ -0,0 +1,13 @@
+local capture = require "test_helpers".capture_program
+
+describe("The eject program", function()
+    it("displays its usage when given no argument", function()
+        expect(capture(stub, "eject"))
+            :matches { ok = true, output = "Usage: eject <drive>\n", error = "" }
+    end)
+
+    it("fails when trying to eject a non-drive", function()
+        expect(capture(stub, "eject /rom"))
+            :matches { ok = true, output = "Nothing in /rom drive\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/exit_spec.lua b/src/test/resources/test-rom/spec/programs/exit_spec.lua
new file mode 100644
index 000000000..301461087
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/exit_spec.lua
@@ -0,0 +1,9 @@
+local capture = require "test_helpers".capture_program
+
+describe("The exit program", function()
+    it("exits the shell", function()
+        local exit = stub(shell, "exit")
+        expect(capture(stub, "exit")):matches { ok = true, combined = "" }
+        expect(exit):called(1)
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/fun/advanced/paint_spec.lua b/src/test/resources/test-rom/spec/programs/fun/advanced/paint_spec.lua
new file mode 100644
index 000000000..086404add
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/fun/advanced/paint_spec.lua
@@ -0,0 +1,8 @@
+local capture = require "test_helpers".capture_program
+
+describe("The paint program", function()
+    it("displays its usage when given no arguments", function()
+        expect(capture(stub, "paint"))
+            :matches { ok = true, output = "Usage: paint <path>\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/fun/dj_spec.lua b/src/test/resources/test-rom/spec/programs/fun/dj_spec.lua
new file mode 100644
index 000000000..4464d4e8c
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/fun/dj_spec.lua
@@ -0,0 +1,13 @@
+local capture = require "test_helpers".capture_program
+
+describe("The dj program", function()
+    it("displays its usage when given too many arguments", function()
+        expect(capture(stub, "dj a b c"))
+            :matches { ok = true, output = "Usages:\ndj play\ndj play <drive>\ndj stop\n", error = "" }
+    end)
+
+    it("fails when no disks are present", function()
+        expect(capture(stub, "dj"))
+            :matches { ok = true, output = "No Music Discs in attached disk drives\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/fun/hello_spec.lua b/src/test/resources/test-rom/spec/programs/fun/hello_spec.lua
new file mode 100644
index 000000000..25db085bc
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/fun/hello_spec.lua
@@ -0,0 +1,10 @@
+local capture = require "test_helpers".capture_program
+
+describe("The hello program", function()
+    it("says hello", function()
+        local slowPrint = stub(textutils, "slowPrint", function(...) return print(...) end)
+        expect(capture(stub, "hello"))
+            :matches { ok = true, output = "Hello World!\n", error = "" }
+        expect(slowPrint):called(1)
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/gps_spec.lua b/src/test/resources/test-rom/spec/programs/gps_spec.lua
new file mode 100644
index 000000000..245f259c9
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/gps_spec.lua
@@ -0,0 +1,23 @@
+local capture = require "test_helpers".capture_program
+
+describe("The gps program", function()
+    it("displays its usage when given no arguments", function()
+        expect(capture(stub, "gps"))
+            :matches { ok = true, output = "Usages:\ngps host\ngps host <x> <y> <z>\ngps locate\n", error = "" }
+    end)
+
+    it("fails on a pocket computer", function()
+        stub(_G, "pocket", {})
+
+        expect(capture(stub, "gps host"))
+            :matches { ok = true, output = "GPS Hosts must be stationary\n", error = "" }
+    end)
+
+    it("can locate the computer", function()
+        local locate = stub(gps, "locate", function() print("Some debugging information.") end)
+
+        expect(capture(stub, "gps locate"))
+            :matches { ok = true, output = "Some debugging information.\n", error = "" }
+        expect(locate):called_with(2, true)
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/help_spec.lua b/src/test/resources/test-rom/spec/programs/help_spec.lua
new file mode 100644
index 000000000..79e4e4742
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/help_spec.lua
@@ -0,0 +1,8 @@
+local capture = require "test_helpers".capture_program
+
+describe("The help program", function()
+    it("errors when there is no such help file", function()
+        expect(capture(stub, "help nothing"))
+            :matches { ok = true, output = "No help available\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/label_spec.lua b/src/test/resources/test-rom/spec/programs/label_spec.lua
new file mode 100644
index 000000000..5762f8fbe
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/label_spec.lua
@@ -0,0 +1,34 @@
+local capture = require "test_helpers".capture_program
+
+describe("The label program", function()
+    it("displays its usage when given no arguments", function()
+        expect(capture(stub, "label"))
+            :matches { ok = true, output = "Usages:\nlabel get\nlabel get <drive>\nlabel set <text>\nlabel set <drive> <text>\nlabel clear\nlabel clear <drive>\n", error = "" }
+    end)
+
+    describe("displays the computer's label", function()
+        it("when it is not labelled", function()
+            stub(os, "getComputerLabel", function() return nil end)
+            expect(capture(stub, "label get"))
+                :matches { ok = true, output = "No Computer label\n", error = "" }
+        end)
+
+        it("when it is labelled", function()
+            stub(os, "getComputerLabel", function() return "Test" end)
+            expect(capture(stub, "label get"))
+                :matches { ok = true, output = "Computer label is \"Test\"\n", error = "" }
+        end)
+    end)
+
+    it("sets the computer's label", function()
+        local setComputerLabel = stub(os, "setComputerLabel")
+        capture(stub, "label set Test")
+        expect(setComputerLabel):called_with("Test")
+    end)
+
+    it("clears the computer's label", function()
+        local setComputerLabel = stub(os, "setComputerLabel")
+        capture(stub, "label clear")
+        expect(setComputerLabel):called_with(nil)
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/list_spec.lua b/src/test/resources/test-rom/spec/programs/list_spec.lua
new file mode 100644
index 000000000..f9f5f0b0b
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/list_spec.lua
@@ -0,0 +1,22 @@
+local capture = require "test_helpers".capture_program
+
+describe("The list program", function()
+    it("lists files", function()
+        local pagedTabulate = stub(textutils, "pagedTabulate")
+        capture(stub, "list /rom")
+        expect(pagedTabulate):called_with_matching(
+            colors.green, { "apis", "autorun", "help", "modules", "programs" },
+            colors.white, { "motd.txt", "startup.lua" }
+        )
+    end)
+
+    it("fails on a non-existent directory", function()
+        expect(capture(stub, "list /rom/nothing"))
+            :matches { ok = true, output = "", error = "Not a directory\n" }
+    end)
+
+    it("fails on a file", function()
+        expect(capture(stub, "list /rom/startup.lua"))
+            :matches { ok = true, output = "", error = "Not a directory\n" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/monitor_spec.lua b/src/test/resources/test-rom/spec/programs/monitor_spec.lua
new file mode 100644
index 000000000..04fbb613a
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/monitor_spec.lua
@@ -0,0 +1,8 @@
+local capture = require "test_helpers".capture_program
+
+describe("The monitor program", function()
+    it("displays its usage when given no arguments", function()
+        expect(capture(stub, "monitor"))
+            :matches { ok = true, output = "Usage: monitor <name> <program> <arguments>\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/peripherals_spec.lua b/src/test/resources/test-rom/spec/programs/peripherals_spec.lua
new file mode 100644
index 000000000..f52961a13
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/peripherals_spec.lua
@@ -0,0 +1,8 @@
+local capture = require "test_helpers".capture_program
+
+describe("The peripherals program", function()
+    it("says when there are no peripherals", function()
+        expect(capture(stub, "peripherals" ))
+            :matches { ok = true, output = "Attached Peripherals:\nNone\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/pocket/equip_spec.lua b/src/test/resources/test-rom/spec/programs/pocket/equip_spec.lua
new file mode 100644
index 000000000..0c3fb3183
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/pocket/equip_spec.lua
@@ -0,0 +1,27 @@
+local capture = require "test_helpers".capture_program
+
+describe("The pocket equip program", function()
+    it("errors when not a pocket computer", function()
+        stub(_G, "pocket", nil)
+        expect(capture(stub, "/rom/programs/pocket/equip.lua"))
+            :matches { ok = true, output = "", error = "Requires a Pocket Computer\n" }
+    end)
+
+    it("can equip an upgrade", function()
+        stub(_G, "pocket", {
+            equipBack = function() return true end
+        })
+
+        expect(capture(stub, "/rom/programs/pocket/equip.lua"))
+            :matches { ok = true, output = "Item equipped\n", error = "" }
+    end)
+
+    it("handles when an upgrade cannot be equipped", function()
+        stub(_G, "pocket", {
+            equipBack = function() return false, "Cannot equip this item." end
+        })
+
+        expect(capture(stub, "/rom/programs/pocket/equip.lua"))
+            :matches { ok = true, output = "", error = "Cannot equip this item.\n" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/pocket/unequip_spec.lua b/src/test/resources/test-rom/spec/programs/pocket/unequip_spec.lua
new file mode 100644
index 000000000..85a1e640f
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/pocket/unequip_spec.lua
@@ -0,0 +1,27 @@
+local capture = require "test_helpers".capture_program
+
+describe("The pocket unequip program", function()
+    it("errors when not a pocket computer", function()
+        stub(_G, "pocket", nil)
+        expect(capture(stub, "/rom/programs/pocket/unequip.lua"))
+            :matches { ok = true, output = "", error = "Requires a Pocket Computer\n" }
+    end)
+
+    it("unequips an upgrade", function()
+        stub(_G, "pocket", {
+            unequipBack = function() return true end
+        })
+
+        expect(capture(stub, "/rom/programs/pocket/unequip.lua"))
+            :matches { ok = true, output = "Item unequipped\n", error = "" }
+    end)
+
+    it("handles when an upgrade cannot be equipped", function()
+        stub(_G, "pocket", {
+            unequipBack = function() return false, "Nothing to remove." end
+        })
+
+        expect(capture(stub, "/rom/programs/pocket/unequip.lua"))
+            :matches { ok = true, output = "", error = "Nothing to remove.\n" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/programs_spec.lua b/src/test/resources/test-rom/spec/programs/programs_spec.lua
new file mode 100644
index 000000000..8f19bc83e
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/programs_spec.lua
@@ -0,0 +1,14 @@
+local capture = require "test_helpers".capture_program
+
+describe("The programs program", function()
+    it("list programs", function()
+        local programs = stub(shell, "programs", function() return { "some", "programs" } end)
+        local pagedTabulate = stub(textutils, "pagedTabulate", function(x) print(table.unpack(x)) end)
+
+        expect(capture(stub, "/rom/programs/programs.lua"))
+            :matches { ok = true, output = "some programs\n", error = "" }
+
+        expect(programs):called_with(false)
+        expect(pagedTabulate):called_with_matching({ "some", "programs" })
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/reboot_spec.lua b/src/test/resources/test-rom/spec/programs/reboot_spec.lua
new file mode 100644
index 000000000..d2a877d9d
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/reboot_spec.lua
@@ -0,0 +1,14 @@
+local capture = require "test_helpers".capture_program
+
+describe("The reboot program", function()
+    it("sleeps and then reboots", function()
+        local sleep = stub(_G, "sleep")
+        local reboot = stub(os, "reboot")
+
+        expect(capture(stub, "reboot"))
+            :matches { ok = true, output = "Goodbye\n", error = "" }
+
+        expect(sleep):called_with(1)
+        expect(reboot):called()
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/redstone_spec.lua b/src/test/resources/test-rom/spec/programs/redstone_spec.lua
new file mode 100644
index 000000000..fc687ac7e
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/redstone_spec.lua
@@ -0,0 +1,8 @@
+local capture = require "test_helpers".capture_program
+
+describe("The redstone program", function()
+    it("displays its usage when given no arguments", function()
+        expect(capture(stub, "redstone"))
+            :matches { ok = true, output = "Usages:\nredstone probe\nredstone set <side> <value>\nredstone set <side> <color> <value>\nredstone pulse <side> <count> <period>\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/shutdown_spec.lua b/src/test/resources/test-rom/spec/programs/shutdown_spec.lua
new file mode 100644
index 000000000..4cfa37ba1
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/shutdown_spec.lua
@@ -0,0 +1,15 @@
+local capture = require "test_helpers".capture_program
+
+describe("The shutdown program", function()
+
+    it("run the program", function()
+        local sleep = stub(_G, "sleep")
+        local shutdown = stub(os, "shutdown")
+
+        expect(capture(stub, "shutdown"))
+            :matches { ok = true, output = "Goodbye\n", error = "" }
+
+        expect(sleep):called_with(1)
+        expect(shutdown):called()
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/turtle/craft_spec.lua b/src/test/resources/test-rom/spec/programs/turtle/craft_spec.lua
new file mode 100644
index 000000000..43b7ea982
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/turtle/craft_spec.lua
@@ -0,0 +1,69 @@
+local capture = require "test_helpers".capture_program
+
+describe("The craft program", function()
+    it("errors when not a turtle", function()
+        stub(_G, "turtle", nil)
+
+        expect(capture(stub, "/rom/programs/turtle/craft.lua"))
+            :matches { ok = true, output = "", error = "Requires a Turtle\n" }
+    end)
+
+    it("fails when turtle.craft() is unavailable", function()
+        stub(_G, "turtle", {})
+
+        expect(capture(stub, "/rom/programs/turtle/craft.lua"))
+            :matches { ok = true, output = "Requires a Crafty Turtle\n", error = "" }
+    end)
+
+    it("displays its usage when given no arguments", function()
+        stub(_G, "turtle", { craft = function() end })
+
+        expect(capture(stub, "/rom/programs/turtle/craft.lua"))
+            :matches { ok = true, output = "Usage: craft [number]\n", error = "" }
+    end)
+
+    it("crafts multiple items", function()
+        local item_count = 3
+        stub(_G, "turtle", {
+            craft = function()
+                item_count = 1
+                return true
+            end,
+            getItemCount = function() return item_count end,
+            getSelectedSlot = function() return 1 end,
+        })
+
+        expect(capture(stub, "/rom/programs/turtle/craft.lua 2"))
+            :matches { ok = true, output = "2 items crafted\n", error = "" }
+    end)
+
+    it("craft a single item", function()
+        local item_count = 2
+        stub(_G,"turtle",{
+            craft = function()
+                item_count = 1
+                return true
+            end,
+            getItemCount = function() return item_count end,
+            getSelectedSlot = function() return 1 end,
+        })
+
+        expect(capture(stub, "/rom/programs/turtle/craft.lua 1"))
+            :matches { ok = true, output = "1 item crafted\n", error = "" }
+    end)
+
+     it("crafts no items", function()
+        local item_count = 2
+        stub(_G,"turtle",{
+            craft = function()
+                item_count = 1
+                return false
+            end,
+            getItemCount = function() return item_count end,
+            getSelectedSlot = function() return 1 end,
+        })
+
+        expect(capture(stub, "/rom/programs/turtle/craft.lua 1"))
+            :matches { ok = true, output = "No items crafted\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/turtle/equip_spec.lua b/src/test/resources/test-rom/spec/programs/turtle/equip_spec.lua
new file mode 100644
index 000000000..2e637fc82
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/turtle/equip_spec.lua
@@ -0,0 +1,89 @@
+local capture = require "test_helpers".capture_program
+
+describe("The turtle equip program", function()
+    it("errors when not a turtle", function()
+        stub(_G, "turtle", nil)
+
+        expect(capture(stub, "/rom/programs/turtle/equip.lua"))
+            :matches { ok = true, output = "", error = "Requires a Turtle\n" }
+    end)
+
+
+    it("displays its usage when given no arguments", function()
+        stub(_G, "turtle", {})
+
+        expect(capture(stub, "/rom/programs/turtle/equip.lua"))
+            :matches { ok = true, output = "Usage: equip <slot> <side>\n", error = "" }
+    end)
+
+    it("equip nothing", function()
+        stub(_G, "turtle", {
+            select = function() end,
+            getItemCount = function() return 0 end,
+        })
+
+        expect(capture(stub, "/rom/programs/turtle/equip.lua 1 left"))
+            :matches { ok = true, output = "Nothing to equip\n", error = "" }
+        expect(capture(stub, "/rom/programs/turtle/equip.lua 1 right"))
+            :matches { ok = true, output = "Nothing to equip\n", error = "" }
+    end)
+
+    it("swaps existing upgrades", function()
+        stub(_G,"turtle",{
+            select = function() end,
+            getItemCount = function() return 1 end,
+            equipLeft = function() return true end,
+            equipRight = function() return true end,
+        })
+
+        expect(capture(stub, "/rom/programs/turtle/equip.lua 1 left"))
+            :matches { ok = true, output = "Items swapped\n", error = "" }
+        expect(capture(stub, "/rom/programs/turtle/equip.lua 1 right"))
+            :matches { ok = true, output = "Items swapped\n", error = "" }
+    end)
+
+    describe("equips a new upgrade", function()
+        local function setup()
+            local item_count = 1
+            stub(_G,"turtle",{
+                select = function() end,
+                getItemCount = function() return item_count end,
+                equipLeft = function()
+                    item_count  = 0
+                    return true
+                end,
+                equipRight = function()
+                    item_count = 0
+                    return true
+                end,
+            })
+        end
+
+        it("on the left", function()
+            setup()
+            expect(capture(stub, "/rom/programs/turtle/equip.lua 1 left"))
+                :matches { ok = true, output = "Item equipped\n", error = "" }
+        end)
+
+        it("on the right", function()
+            setup()
+            expect(capture(stub, "/rom/programs/turtle/equip.lua 1 right"))
+                :matches { ok = true, output = "Item equipped\n", error = "" }
+        end)
+    end)
+
+    it("handles when an upgrade cannot be equipped", function()
+        stub(_G,"turtle",{
+            select = function() end,
+            getItemCount = function() return 1 end,
+            equipLeft = function() return false end,
+            equipRight = function() return false end,
+        })
+
+        expect(capture(stub, "/rom/programs/turtle/equip.lua 1 left"))
+            :matches { ok = true, output = "Item not equippable\n", error = "" }
+        expect(capture(stub, "/rom/programs/turtle/equip.lua 1 right"))
+            :matches { ok = true, output = "Item not equippable\n", error = "" }
+    end)
+
+end)
diff --git a/src/test/resources/test-rom/spec/programs/turtle/refuel_spec.lua b/src/test/resources/test-rom/spec/programs/turtle/refuel_spec.lua
new file mode 100644
index 000000000..3b6283260
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/turtle/refuel_spec.lua
@@ -0,0 +1,62 @@
+local capture = require "test_helpers".capture_program
+
+describe("The refuel program", function()
+    local function setup_turtle(fuel_level, fuel_limit, item_count)
+        stub(_G, "turtle", {
+            getFuelLevel = function()
+                return fuel_level
+            end,
+            getItemCount = function()
+                return item_count
+            end,
+            refuel = function(nLimit)
+                item_count = item_count - nLimit
+                fuel_level = fuel_level + nLimit
+            end,
+            select = function()
+            end,
+            getFuelLimit = function()
+                return fuel_limit
+            end
+        })
+    end
+
+    it("errors when not a turtle", function()
+        stub(_G, "turtle", nil)
+
+        expect(capture(stub, "/rom/programs/turtle/refuel.lua"))
+            :matches { ok = true, output = "", error = "Requires a Turtle\n" }
+    end)
+
+
+    it("displays its usage when given too many argument", function()
+        setup_turtle(0, 5, 0)
+        expect(capture(stub, "/rom/programs/turtle/refuel.lua a b"))
+            :matches { ok = true, output = "Usage: refuel [number]\n", error = "" }
+    end)
+
+    it("requires a numeric argument", function()
+       setup_turtle(0, 0, 0)
+       expect(capture(stub, "/rom/programs/turtle/refuel.lua nothing"))
+           :matches { ok = true, output = "Invalid limit, expected a number or \"all\"\n", error = "" }
+    end)
+
+    it("refuels the turtle", function()
+       setup_turtle(0, 10, 5)
+
+       expect(capture(stub, "/rom/programs/turtle/refuel.lua 5"))
+           :matches { ok = true, output = "Fuel level is 5\n", error = "" }
+    end)
+
+    it("reports when the fuel limit is reached", function()
+       setup_turtle(0,5,5)
+       expect(capture(stub, "/rom/programs/turtle/refuel.lua 5"))
+           :matches { ok = true, output = "Fuel level is 5\nFuel limit reached\n", error = "" }
+    end)
+
+    it("reports when the fuel level is unlimited", function()
+       setup_turtle("unlimited",5,5)
+       expect(capture(stub, "/rom/programs/turtle/refuel.lua 5"))
+           :matches { ok = true, output = "Fuel level is unlimited\n", error = "" }
+    end)
+end)
diff --git a/src/test/resources/test-rom/spec/programs/turtle/unequip_spec.lua b/src/test/resources/test-rom/spec/programs/turtle/unequip_spec.lua
new file mode 100644
index 000000000..1cb97b440
--- /dev/null
+++ b/src/test/resources/test-rom/spec/programs/turtle/unequip_spec.lua
@@ -0,0 +1,69 @@
+local capture = require "test_helpers".capture_program
+
+describe("The turtle unequip program", function()
+    it("errors when not a turtle", function()
+        stub(_G, "turtle", nil)
+
+        expect(capture(stub, "/rom/programs/turtle/unequip.lua"))
+            :matches { ok = true, output = "", error = "Requires a Turtle\n" }
+    end)
+
+
+    it("displays its usage when given no arguments", function()
+        stub(_G, "turtle", {})
+
+        expect(capture(stub, "/rom/programs/turtle/unequip.lua"))
+            :matches { ok = true, output = "Usage: unequip <side>\n", error = "" }
+    end)
+
+    it("says when nothing was unequipped", function()
+        stub(_G,"turtle",{
+            select = function() end,
+            getItemCount = function() return 0 end,
+            equipRight = function() return true end,
+            equipLeft = function() return true end
+        })
+
+        expect(capture(stub, "/rom/programs/turtle/unequip.lua left"))
+            :matches { ok = true, output = "Nothing to unequip\n", error = "" }
+        expect(capture(stub, "/rom/programs/turtle/unequip.lua right"))
+            :matches { ok = true, output = "Nothing to unequip\n", error = "" }
+    end)
+
+    it("unequips a upgrade", function()
+        local item_count = 0
+        stub(_G,"turtle",{
+            select = function() end,
+            getItemCount = function() return item_count end,
+            equipRight = function()
+                item_count = 1
+                return true
+            end,
+            equipLeft = function()
+                item_count = 1
+                return true
+            end
+        })
+
+        expect(capture(stub, "/rom/programs/turtle/unequip.lua left"))
+            :matches { ok = true, output = "Item unequipped\n", error = "" }
+        item_count = 0
+        expect(capture(stub, "/rom/programs/turtle/unequip.lua right"))
+            :matches { ok = true, output = "Item unequipped\n", error = "" }
+    end)
+
+    it("fails when the turtle is full", function()
+        stub(_G,"turtle",{
+            select = function() end,
+            getItemCount = function() return 1 end,
+            equipRight = function() return true end,
+            equipLeft = function() return true end
+        })
+
+        expect(capture(stub, "/rom/programs/turtle/unequip.lua left"))
+            :matches { ok = true, output = "No space to unequip item\n", error = "" }
+        expect(capture(stub, "/rom/programs/turtle/unequip.lua right"))
+            :matches { ok = true, output = "No space to unequip item\n", error = "" }
+    end)
+
+end)