diff --git a/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java b/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java
index dfe529acd..18d04dac8 100644
--- a/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java
+++ b/src/test/java/dan200/computercraft/core/ComputerTestDelegate.java
@@ -97,7 +97,7 @@ public class ComputerTestDelegate
 
         if( REPORT_PATH.delete() ) ComputerCraft.log.info( "Deleted previous coverage report." );
 
-        Terminal term = new Terminal( 78, 20 );
+        Terminal term = new Terminal( 80, 30 );
         IWritableMount mount = new FileMount( new File( "test-files/mount" ), 10_000_000 );
 
         // Remove any existing files
diff --git a/src/test/resources/test-rom/spec/apis/rednet_spec.lua b/src/test/resources/test-rom/spec/apis/rednet_spec.lua
index f6c942e33..c2b98590d 100644
--- a/src/test/resources/test-rom/spec/apis/rednet_spec.lua
+++ b/src/test/resources/test-rom/spec/apis/rednet_spec.lua
@@ -83,4 +83,90 @@ describe("The rednet library", function()
             expect(rednet.lookup("a_protocol", "a_hostname")):eq(os.getComputerID())
         end)
     end)
+
+    describe("on a fake computers", function()
+        local fake_computer = require "support.fake_computer"
+
+        local function computer_with_rednet(id, fn, options)
+            local computer = fake_computer.make_computer(id, function(_ENV)
+                local fns = { _ENV.rednet.run }
+                if options and options.rep then
+                    fns[#fns + 1] = function() _ENV.dofile("rom/programs/rednet/repeat.lua") end
+                end
+
+                if fn then
+                    fns[#fns + 1] = function()
+                        if options and options.open then
+                            _ENV.rednet.open("back")
+                            _ENV.os.queueEvent("x") _ENV.os.pullEvent("x")
+                        end
+                        return fn(_ENV.rednet, _ENV)
+                    end
+                end
+
+                return parallel.waitForAny(table.unpack(fns))
+            end)
+            local modem = fake_computer.add_modem(computer, "back")
+            fake_computer.add_api(computer, "rom/apis/rednet.lua")
+            return computer, modem
+        end
+
+        it("opens and closes channels", function()
+            local id = math.random(256)
+            local computer = computer_with_rednet(id, function(rednet)
+                expect(rednet.isOpen()):eq(false)
+
+                rednet.open("back")
+                rednet.open("front")
+
+                expect(rednet.isOpen()):eq(true)
+                expect(rednet.isOpen("back")):eq(true)
+                expect(rednet.isOpen("front")):eq(true)
+
+                rednet.close("back")
+                expect(rednet.isOpen("back")):eq(false)
+                expect(rednet.isOpen("front")):eq(true)
+                expect(rednet.isOpen()):eq(true)
+
+                rednet.close()
+
+                expect(rednet.isOpen("back")):eq(false)
+                expect(rednet.isOpen("front")):eq(false)
+                expect(rednet.isOpen()):eq(false)
+            end)
+            fake_computer.add_modem(computer, "front")
+
+            fake_computer.run_all { computer }
+        end)
+
+        it("sends and receives rednet messages", function()
+            local computer_1, modem_1 = computer_with_rednet(1, function(rednet, _ENV)
+                rednet.send(2, "Hello")
+            end, { open = true })
+            local computer_2, modem_2 = computer_with_rednet(2, function(rednet)
+                local id, message = rednet.receive()
+                expect(id):eq(1)
+                expect(message):eq("Hello")
+            end, { open = true })
+            fake_computer.add_modem_edge(modem_1, modem_2)
+
+            fake_computer.run_all { computer_1, computer_2 }
+        end)
+
+        it("repeats messages between computers", function()
+            local computer_1, modem_1 = computer_with_rednet(1, function(rednet, _ENV)
+                rednet.send(3, "Hello")
+            end, { open = true })
+            local computer_2, modem_2 = computer_with_rednet(2, nil, { open = true, rep = true })
+            local computer_3, modem_3 = computer_with_rednet(3, function(rednet)
+                local id, message = rednet.receive()
+                expect(id):eq(1)
+                expect(message):eq("Hello")
+            end, { open = true })
+            fake_computer.add_modem_edge(modem_1, modem_2)
+            fake_computer.add_modem_edge(modem_2, modem_3)
+
+            fake_computer.run_all({ computer_1, computer_2, computer_3 }, { computer_1, computer_3 })
+        end)
+    end)
 end)
diff --git a/src/test/resources/test-rom/spec/support/fake_computer.lua b/src/test/resources/test-rom/spec/support/fake_computer.lua
new file mode 100644
index 000000000..151b536ae
--- /dev/null
+++ b/src/test/resources/test-rom/spec/support/fake_computer.lua
@@ -0,0 +1,146 @@
+local function keys(tbl)
+    local keys = {}
+    for k in pairs(tbl) do keys[#keys + 1] = k end
+    return keys
+end
+
+local safe_globals = {
+    "assert", "bit32", "coroutine", "debug", "error", "fs", "getmetatable", "io", "ipairs", "math", "next", "pairs",
+    "pcall", "print", "printError", "rawequal", "rawget", "rawlen", "rawset", "select", "setmetatable", "string",
+    "table", "term", "tonumber", "tostring", "type", "utf8", "xpcall",
+}
+
+--- Create a fake computer.
+local function make_computer(id, fn)
+    local env = setmetatable({}, _G)
+
+    local peripherals = {}
+    local events = { { n = 1, env } }
+    local function queue_event(...) events[#events + 1] = table.pack(...) end
+
+    for _, k in pairs(safe_globals) do env[k] = _G[k] end
+    env.peripheral = {
+        getNames = function() return keys(peripherals) end,
+        isPresent = function(name) return peripherals[name] ~= nil end,
+        getType = function(name) return peripherals[name] and getmetatable(peripherals[name]).type end,
+        getMethods = function(name) return peripherals[name] and keys(peripherals[name]) end,
+        call = function(name, method, ...)
+            local p = peripherals[name]
+            if p then return p[method](...) end
+            return nil
+        end,
+        wrap = function(name) return peripherals[name] end,
+    }
+    env.os = {
+        getComputerID = function() return id end,
+        queueEvent = queue_event,
+        pullEventRaw = coroutine.yield,
+        pullEvent = function(filter)
+            local event_data = table.pack(coroutine.yield(filter))
+            if event_data[1] == "terminate" then error("Terminated", 0) end
+            return table.unpack(event_data, 1, event_data.n)
+        end,
+        startTimer = function() return 0 end,
+        clock = function() return 0 end,
+    }
+    env.dofile = function(path)
+        local fn, err = loadfile(path, nil, env)
+        if fn then return fn() else error(err, 2) end
+    end
+
+    local co = coroutine.create(fn)
+    local filter = nil
+    local function step()
+        while true do
+            if #events == 0 or coroutine.status(co) == "dead" then return false end
+
+            local ev = table.remove(events, 1)
+            if filter == nil or ev[1] == filter or ev[1] == "terminated" then
+                local ok, result = coroutine.resume(co, table.unpack(ev, 1, ev.n))
+                if not ok then
+                    if type(result) == "table" and result.trace == nil then result.trace = debug.traceback(co) end
+                    error(result, 0)
+                end
+                filter = result
+                return true
+            end
+        end
+    end
+
+    return { env = env, peripherals = peripherals, queue_event = queue_event, step = step, co = co }
+end
+
+--- Add a modem to a computer on a particular side
+local function add_modem(owner, side)
+    local open, adjacent = {}, {}
+    local peripheral = setmetatable({
+        open = function(channel) open[channel] = true end,
+        close = function(channel) open[channel] = false end,
+        closeAll = function(channel) open = {} end,
+        isOpen = function(channel) return open[channel] == true end,
+        transmit = function(channel, reply_channel, payload)
+            for _, adjacent in pairs(adjacent) do
+                if adjacent.open[channel] then
+                    adjacent.owner.queue_event("modem_message", adjacent.side, channel, reply_channel, payload, 123)
+                end
+            end
+        end,
+    }, { type = "modem" })
+    owner.peripherals[side] = peripheral
+    return { adjacent = adjacent, side = side, owner = owner, open = open }
+end
+
+local function add_modem_edge(modem1, modem2)
+    table.insert(modem1.adjacent, modem2)
+    table.insert(modem2.adjacent, modem1)
+end
+
+--- Load an API into the computer's environment.
+local function add_api(computer, path)
+    local name = fs.getName(path)
+    if name:sub(-4) == ".lua" then name = name:sub(1, -5) end
+
+    local child_env = {}
+    setmetatable(child_env, { __index = computer.env })
+    assert(loadfile(path, nil, child_env))()
+
+    local api = {}
+    for k, v in pairs(child_env) do api[k] = v end
+
+    computer.env[name] = api
+end
+
+--- Step all computers forward by one event.
+local function step_all(computers)
+    local any = false
+    for _, computer in pairs(computers) do
+        if computer.step() then any = true end
+    end
+    return any
+end
+
+--- Run all computers until their event queue is empty.
+local function run_all(computers, require_done)
+    while step_all(computers) do end
+
+    if require_done ~= false then
+        if type(require_done) == "table" then
+            for _, v in ipairs(require_done) do require_done[v] = true end
+        end
+
+        for _, computer in pairs(computers) do
+            if coroutine.status(computer.co) ~= "dead" and (type(require_done) ~= "table" or require_done[computer]) then
+                error(debug.traceback(computer.co, "Computer did not shutdown"), 0)
+            end
+        end
+    end
+end
+
+return {
+    make_computer = make_computer,
+    add_modem = add_modem,
+    add_modem_edge = add_modem_edge,
+    add_api = add_api,
+    step_all = step_all,
+    run_all = run_all,
+}