Mission Scripting (Overview)

This article handles the scripting technology and background for the GTA 3 game series (including GTA LCS and GTA VCS). It does not cover GTA IV.
For more information about the GTA IV script read the article about the SCO format.

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This article deals with the general overview on the mission scripting in GTA 3D series. Mission scripting is the process of writing scripts: small codes that control many aspects of gameplay. Although most of the game features are hardcoded, still much things could be done via scripting. In fact, every single mission in Grand Theft Auto series comes from the scripts. That is, knowing the format of scripts and having a proper tool, it is possible to change the mission details and even create an absoletely new story plot (although it's considered to be the most complex area in GTA modding, so most often the scripting results in small scripts adding new features in gameplay).

Introduction

The original mission script is looked like this^[*] (taken from Vice City debug.sc file):

IF IS_BUTTON_PRESSED PAD2 RIGHTSHOULDER1
AND flag_create_car = 1
AND button_press_flag = 0
	IF IS_CAR_DEAD magic_car
		DELETE_CAR magic_car
	ELSE
		IF NOT IS_PLAYER_IN_CAR player magic_car
			DELETE_CAR magic_car
		ELSE
			MARK_CAR_AS_NO_LONGER_NEEDED magic_car
		ENDIF
	ENDIF 
	flag_create_car = 0
	initial_car_selected = 0
	button_press_flag = 1
ENDIF

Easy to read and understand, it is fairly basic so anyone with an idea of basic coding (or maybe even English) can understand it. However, very little code came with the game like that. The majority of the mission script comes in a file called main.scm (although in San Andreas there are alternate mains and external scripts, but they all follow the same basic format - hex codes). Example, for the code:

IF IS_CAR_DEAD magic_car
	DELETE_CAR magic_car

The equivalent in the main.scm would look something like this:

D6 00 04 00 19 01 02 45 0E 4D 00 01 FE 3D 87 02 A6 00 02 45 0E

This is how the beginning of the San Andreas mission script looks like:

Byte data	Decompiled data	Decompiled data with description
A4 03   09   4D 41 49 4E 00 00 00 00	03A4: 'MAIN'	03A4: name_thread 'MAIN'
6A 01   04   00   04   00	016A: 0 0	016A: fade 0 time 0
2C 04   05   93 00	042C: 147	042C: set_total_missions_to 147
0D 03   05   BB 00	030D: 187	030D: set_max_progress 187

Cracking the SCM

As has been said, very little of the code was supplied with the game in a decompiled state (only two small files, both test scripts), so how, as asked, do we create our own scripts based on the original? With a decompiler - but how do these work (no decompilers have been provided by Rockstar).

The original SCM format was cracked shortly after the release of GTA 3 (the first game to use this mission coding method), with people having to first figure out what all the sections did (there are 5 segments is an SCM - memory, objects, mission defines, MAIN and missions (GTA SA has more, but only one of these (global variables) has had its use determined), where they started/ended etc, figuring out how many parameters each OpCode had and a lot more. Once this was done, they knew where each OpCode began and ended, so they could split them up to make it more readable, but the data on what each one does was lost in the compiling, so they still only had something that looked like this:

:label035F78
0001: 0?
00D6: 0?
0256: 4??

That doesn't still doesn't mean a lot though, so people had to try figure out what the different OpCodes meant.

(Note: this code is in early Mission Builder format:

:labelxxxxxx means this code was originally at this offset in the mission script (the 'label' is added in by the decompiler)
x? means a one byte number
x?? means a variable stored at this offset from the start

label (i.e. for if we wanted to 'jump' to a label))

Some were easy, the very first line of a decompiled script (besides decompiler headers) looks something like:

The only parameter this command has is a reference to a label, so this is most likely (and in fact is) a jump statement, so we know all 0002s are jumps. Of course, finding what OpCodes do (and in fact finding the original number of parameters took a while to confirm) takes time, you have to have an idea first and then have to test your theory - many OpCodes have still not been named, but with the amount of OpCodes discovered so far, we have a general idea on what the mission script does.

Once the mission script had been cracked, people could write programs to read through it and output it in a form we could understand (based on a format of opcodes, text to say what they do and a list of parameter values - nothing like the original - the opcodes are needed to determine which opcode it is, the describing text is completely ignored). Originally there were two main decompilers, BWME (Barton Waterduck's Mission Editor) and CyQ's disassembler, each with their own compilers (to compile the decompiled code back into an SCM file). BWME quickly became the most commonly used, especially among newer coders, probably due to the fact that the parameters were inter-mixed with the code, so you had something like:

00B1: is_car $car in_cube $lowerx $lowery $lowerz $upperx $uppery $upperz 0

As opposed to the gtaMa/DisAsm format:

is_car_in_cube $car, $lowerx, $lowery, $lowerz, $upperx, $uppery, $upperz, 0

(also note the lack of OpCode in the second example, this builder uses a lookup to find the opcode (if the function is known) instead of just quoting it)

Although you can't see much difference with that example, it can make a lot of difference. Since Barton left the modding community, Seemann created an even more versatile decompiler, the Sanny Builder. It has become the most popular mission builder.

See also

• SCM language
• SCM language III/VC definitions
• Mission Scripting Tools
• Design Your Own Mission

External links

•  GTAForums: Mission Coding Forum
•  GTAForums: Mission Mods Showroom
• ^ Sources of the GTA 3 missions at GTAModding.ru
• GTA 3 Script (scm) Rockstar's original sources found in the iPad version