mGBA Scripting

Lua scripting for mGBA emulator.

Overview

Starting with version 0.10, mGBA has built-in scripting capabilities. To use scripting, click "Scripting..." from the Tools menu. Currently, only Lua scripting is supported.

Key Features:

• Full memory access (ROM, RAM, MMIO)
• Input manipulation (button presses)
• Save state management
• Callbacks for frame/events
• TCP socket networking
• Console output
• Screenshot capture

Opening Scripting Console

Tools → Scripting...

This opens a console where you can load and run Lua scripts.

Top-Level Objects

Available Objects

-- emu: CoreAdapter instance (available when game loaded)
-- C: Exported constants
-- callbacks: CallbackManager instance
-- console: Console instance
-- util: Basic utility library
-- socket: TCP socket library

Console Output

console.log("Info message")
console.warn("Warning message")
console.error("Error message")

-- Create text buffer
buffer = console.createBuffer("My Buffer")
buffer:print("Text in buffer")
buffer:clear()

Utility Functions

-- Expand bitmask to list
bits = util.expandBitmask(0xFF)  -- {0,1,2,3,4,5,6,7}

-- Make bitmask from list
mask = util.makeBitmask({0, 3, 5})  -- 0x29

Core API

ROM Operations

-- Load ROM file
success = emu.loadFile("path/to/rom.gba")

-- Get game info
title = emu.getGameTitle()  -- "POKEMON FIRE"
code = emu.getGameCode()   -- "AGB-P-FE"
size = emu.romSize()       -- ROM size in bytes
platform = emu.platform()  -- 0=GBA, 1=GB
checksum = emu.checksum()  -- CRC32

Save States

-- Save to file
emu.saveStateFile("path/to/state.state")

-- Load from file
emu.loadStateFile("path/to/state.state")

-- Save to slot (0-9)
emu.saveStateSlot(0)
emu.loadStateSlot(0)

-- Save/load buffer
buffer = emu.saveStateBuffer()
emu.loadStateBuffer(buffer)

-- Flags: SCREENSHOT=1, SAVEDATA=2, CHEATS=4, RTC=8, METADATA=16
-- ALL = 31
emu.saveStateSlot(0, 31)  -- Save everything

Frame Control

-- Run one frame
emu.runFrame()

-- Run one instruction
emu.step()

-- Get current frame number
frame = emu.currentFrame()

-- Get cycle info
cycles = emu.frameCycles()    -- Cycles per frame
freq = emu.frequency()         -- Cycles per second

Input Handling

-- Set all keys at once
emu.setKeys(0x0000)  -- No keys

-- Add keys (OR with existing)
emu.addKeys(C.GBA_KEY.A + C.GBA_KEY.R)

-- Clear keys
emu.clearKeys(C.GBA_KEY.B)

-- Check key state
if emu.getKey(C.GBA_KEY.UP) == 1 then
    print("UP is pressed")
end

-- Get all pressed keys
keys = emu.getKeys()

GBA Key Constants

C.GBA_KEY.A      = 0
C.GBA_KEY.B      = 1
C.GBA_KEY.SELECT = 2
C.GBA_KEY.START  = 3
C.GBA_KEY.RIGHT  = 4
C.GBA_KEY.LEFT   = 5
C.GBA_KEY.UP     = 6
C.GBA_KEY.DOWN   = 7
C.GBA_KEY.R      = 8
C.GBA_KEY.L      = 9

Game Boy Key Constants

C.GB_KEY.A       = 0
C.GB_KEY.B       = 1
C.GB_KEY.SELECT  = 2
C.GB_KEY.START   = 3
C.GB_KEY.RIGHT   = 4
C.GB_KEY.LEFT    = 5
C.GB_KEY.UP      = 6
C.GB_KEY.DOWN    = 7

Memory Access

Reading Memory

-- Read 8/16/32 bit values
value8 = emu.read8(address)
value16 = emu.read16(address)
value32 = emu.read32(address)

-- Read range
data = emu.readRange(address, length)

-- Read from memory domain
rom = emu.memory["cart0"]
value = rom:read8(offset)
data = rom:readRange(offset, length)

Writing Memory

-- Write 8/16/32 bit values
emu.write8(address, value)
emu.write16(address, value)
emu.write32(address, value)

Memory Domains (GBA)

-- Available memory domains
emu.memory["bios"]    -- BIOS (0x00000000)
emu.memory["wram"]     -- EWRAM (0x02000000)
emu.memory["iwram"]    -- IWRAM (0x03000000)
emu.memory["io"]       -- MMIO (0x04000000)
emu.memory["palette"]  -- Palette (0x05000000)
emu.memory["vram"]     -- VRAM (0x06000000)
emu.memory["oam"]      -- OAM (0x07000000)
emu.memory["cart0"]    -- ROM (0x08000000)
emu.memory["cart1"]    -- ROM WS1 (0x0a000000)
emu.memory["cart2"]    -- ROM WS2 (0x0c000000)

Memory Domains (GB)

emu.memory["cart0"]  -- ROM Bank ($0000)
emu.memory["vram"]   -- VRAM ($8000)
emu.memory["sram"]   -- SRAM ($a000)
emu.memory["wram"]   -- WRAM ($c000)
emu.memory["oam"]    -- OAM ($fe00)
emu.memory["io"]     -- MMIO ($ff00)
emu.memory["hram"]   -- HRAM ($ff80)

Callbacks

Adding Callbacks

-- Add callback (returns callback ID)
id = callbacks.add("frame", function()
    -- Called every frame
end)

id = callbacks.add("start", function()
    -- Called when emulation starts
end)

id = callbacks.add("reset", function()
    -- Called when emulation resets
end)

id = callbacks.add("shutdown", function()
    -- Called when emulation stops
end)

-- Remove callback
callbacks.remove(id)

Available Callbacks

-- alarm     - In-game alarm went off
-- crashed   - Emulation crashed
-- frame     - Frame finished
-- keysRead  - About to read key input
-- reset     - Emulation reset
-- savedataUpdated - Save data modified
-- sleep     - Entered low-power mode
-- shutdown  - Powered off
-- start     - Started
-- stop      - Voluntarily shut down

Frame Callback Example

-- Auto-press A every 10 frames
local counter = 0
callbacks.add("frame", function()
    counter = counter + 1
    if counter >= 10 then
        emu.addKeys(C.GBA_KEY.A)
        counter = 0
    else
        emu.clearKeys(C.GBA_KEY.A)
    end
end)

Deep Dives

Detailed reference material is available in the references/ directory. Load these on demand for specific topics:

• Classes & Registers — MemoryDomain class, TextBuffer class, GBA/GB register details
  • references/classes-registers.md
• Networking — Socket API details and constants
  • references/networking.md
• Examples — Complete working scripts (cheats, TAS tools, networked scripts)
  • references/examples.md

Best Practices

1. Always Clear Keys

-- Bad: Keys stay pressed
callbacks.add("frame", function()
    emu.addKeys(C.GBA_KEY.A)
end)

-- Good: Clear after use
callbacks.add("frame", function()
    emu.addKeys(C.GBA_KEY.A)
    emu.clearKeys(C.GBA_KEY.A)
end)

2. Use Frame Callback for Input

-- Input should be handled in frame callback
callbacks.add("frame", function()
    if btnp(6) then  -- UP pressed this frame
        -- Handle input
    end
end)

3. Watch for Crashes

callbacks.add("crashed", function()
    console.error("Emulation crashed!")
    -- Save state before exit
    emu.saveStateFile("crash.state")
end)

4. Reset State on Script Load

-- Clear any previous state when loading
emu.clearKeys(0xFFFF)
callbacks.remove(cbid)  -- Remove old callbacks

Common Issues

Address Not Found

-- Some games use different RAM locations
-- Use mGBA's cheat search or memory viewer to find correct addresses

Keys Not Working

-- Some games poll keys differently
-- Try using addKeys instead of setKeys
emu.addKeys(C.GBA_KEY.A)  -- OR with existing

Socket Connection Timeout

-- Socket connect is blocking!
-- Use connect with timeout or run in separate thread
-- For async, use callbacks and poll()

Complete API Reference

Core Methods Summary

Method	Description
loadFile(path)	Load ROM file
getGameTitle()	Get ROM title
getGameCode()	Get ROM code
romSize()	Get ROM size
platform()	Get platform (GBA=0, GB=1)
checksum(type)	Get ROM checksum
reset()	Reset emulation
runFrame()	Run one frame
step()	Run one instruction
currentFrame()	Get frame number
frameCycles()	Cycles per frame
frequency()	Cycles per second
screenshot(filename)	Save screenshot

Memory Methods Summary

Method	Description
read8(addr)	Read 8-bit value
read16(addr)	Read 16-bit value
read32(addr)	Read 32-bit value
readRange(addr, len)	Read byte range
write8(addr, val)	Write 8-bit value
write16(addr, val)	Write 16-bit value
write32(addr, val)	Write 32-bit value
readRegister(name)	Read CPU register
writeRegister(name, val)	Write CPU register

Input Methods Summary

Method	Description
setKeys(mask)	Set key bitmask
addKeys(mask)	Add keys to current
clearKeys(mask)	Remove keys from current
addKey(key)	Add single key
clearKey(key)	Clear single key
getKey(key)	Get key state
getKeys()	Get all keys as mask

Save State Methods Summary

Method	Description
saveStateFile(path, flags)	Save to file
loadStateFile(path, flags)	Load from file
saveStateSlot(slot, flags)	Save to slot
loadStateSlot(slot, flags)	Load from slot
saveStateBuffer(flags)	Save to buffer
loadStateBuffer(buf, flags)	Load from buffer
autoloadSave()	Load associated save

References

• Official Documentation: https://mgba.io/docs/scripting.html
• mGBA GitHub: https://github.com/mgba-emu/mgba
• Forums: https://forums.mgba.io/
• Discord: https://discord.gg/em2M2sG
• Scripting API Reference: https://mgba.io/docs/scripting.html