Scripting MAME via LUA

Introduction

It is now possible to externally drive MAME via LUA scripts. This feature initially appeared in version 0.148, when a minimal luaengine was implemented. Nowadays, the LUA interface is rich enough to let you inspect and manipulate devices state, access CPU registers, read and write memory, and draw a custom HUD on screen.

Internally, MAME makes extensive use of luabridge to implement this feature: the idea is to transparently expose as many of the useful internals as possible.

Finally, a warning: The LUA API is not yet declared stable and may suddenly change without prior notice. However, we expose methods to let you know at runtime which API version you are running against, and you can introspect most of the objects at runtime.

Features

The API is not yet complete, but this is a partial list of capabilities currently available to LUA scripts:

  • machine metadata (app version, current rom, rom details)
  • machine control (starting, pausing, resetting, stopping)
  • machine hooks (on frame painting and on user events)
  • devices introspection (device tree listing, memory and register enumeration)
  • screens introspection (screens listing, screen details, frames counting)
  • screen HUD drawing (text, lines, boxes on multiple screens)
  • memory read/write (8/16/32/64 bits, signed and unsigned)
  • registers and states control (states enumeration, get and set)

Usage

MAME supports external scripting via LUA (>= 5.3) scripts, either written on the interactive console or loaded as a file. To reach the console, just run MAME with -console and you will be greeted by a naked > prompt where you can input your script.

To load a whole script at once, store it in a plain text file and pass it via -autoboot_script. Please note that script loading may be delayed (few seconds by default), but you can override the default with the -autoboot_delay argument.

To control the execution of your code, you can use a loop-based or an event-based approach. The former is not encouraged as it is resource-intensive and makes control flow unnecessarily complex. Instead, we suggest to register custom hooks to be invoked on specific events (eg. at each frame rendering).

Walkthrough

Let’s first run MAME in a terminal to reach the LUA console:

$ mame -console YOUR_ROM
M.A.M.E. v0.158 (Feb  5 2015) - Multiple Arcade Machine Emulator
Copyright Nicola Salmoria and the MAME team
Lua 5.3.0  Copyright (C) 1994-2015 Lua.org, PUC-Rio

>

At this point, your game is probably running in demo mode, let’s pause it:

> emu.pause()
>

Even without textual feedback on the console, you’ll notice the game is now paused. In general, commands are quiet and only print back error messages.

You can check at runtime which version of MAME you are running, with:

> print(emu.app_name() .. " " .. emu.app_version())
mame 0.158

We now start exploring screen related methods. First, let’s enumerate available screens:

> for i,v in pairs(manager:machine().screens) do print(i) end
:screen

manager:machine() is the root object of your currently running machine: we will be using this often. screens is a table with all available screens; most machines only have one main screen. In our case, the main and only screen is tagged as :screen, and we can further inspect it:

> -- let's define a shorthand for the main screen
> s = manager:machine().screens[":screen"]
> print(s:width() .. "x" .. s:height())
320x224

We have several methods to draw on the screen a HUD composed of lines, boxes and text:

> -- we define a HUD-drawing function, and then call it
> function draw_hud()
>> s:draw_text(40, 40, "foo"); -- (x0, y0, msg)
>> s:draw_box(20, 20, 80, 80, 0, 0xff00ffff); -- (x0, y0, x1, y1, fill-color, line-color)
>> s:draw_line(20, 20, 80, 80, 0xff00ffff); -- (x0, y0, x1, y1, line-color)
>> end
> draw_hud();

This will draw some useless art on the screen. However, when unpausing the game, your HUD needs to be refreshed otherwise it will just disappear. In order to do this, you have to register your hook to be called on every frame repaint:

> emu.register_frame_done(draw_hud, "frame")

All colors are expected in ARGB format (32b unsigned), while screen origin (0,0) normally corresponds to the top-left corner.

Similarly to screens, you can inspect all the devices attached to a machine:

> for k,v in pairs(manager:machine().devices) do print(k) end
:audiocpu
:maincpu
:saveram
:screen
:palette
[...]

On some of them, you can also inspect and manipulate memory and state:

> cpu = manager:machine().devices[":maincpu"]
> -- enumerate, read and write state registers
> for k,v in pairs(cpu.state) do print(k) end
D5
SP
A4
A3
D0
PC
[...]
> print(cpu.state["D0"].value)
303
> cpu.state["D0"].value = 255
> print(cpu.state["D0"].value)
255
> -- inspect memory
> for k,v in pairs(cpu.spaces) do print(k) end
program
> mem = cpu.spaces["program"]
> print(mem:read_i8(0xC000))
41