Mission Scripting (Overview)

Introduction

The original Mission Script looked something 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

As you can see, this is nowhere near as easy to understand (in fact, it's pretty hard to understand, even for someone who knows the SCM format). The original code (which is easy for us to understand) is compiled into this format which is easy for the game to understand. Mission script, when compiled, is an opcode based language, this basically means all the commands are represented by (hex (base 16)) numbers, e.g. the command to tell the engine to wait is command 0001. In the original script, all you would need to do to make the game wait is type wait, and the length (in milliseconds) you want the game to wait for, just with a simple 'WAIT 0' command. This is then compiled into commands the game can understand:

01

00

04

00

The first number is the SECOND HALF of the opcode

the second is the FIRST HALF of the opcode,

the third is the data type (more on these later)

and the fourth is the amount of time (in milliseconds in hex - this is a parameter,

any additional data to define the usage of an OpCode is a parameter) for the game to wait, but for anything but the most advanced editing (very few people know how to directly edit the SCM in its raw form), no knowledge of the compiled version is required. The third and fourth numbers can repeat up to 16 times, depending on amount of parameter that the function takes.

Special opcodes

There are special opcodes, which do not follow the usual parttype structure such as:
004F and 0913 have variable amount of parameters, they are ended with a parttype of $00. Maximum amount of parameters equals to the number of local variables of thread: 16 for GTA3/VC; 32 for SA.
05B6 - defines a table (stream of data, 4 * 32 bytes long strings)

Data types

Data types describe what kind of data is stored in that parameter and the size of data:

data type	size (bytes)	description
01	4	immediate 32 bit signed int
02	2	global int/float var
03	2	local int/float var
04	1	immediate 8 bit signed int
05	2	immediate 16 bit signed int
06	4	immediate 32 bit float
07	6	global int/float var array
08	6	local int/float var array
09	8	immediate 8 byte string
10	2	global 8 byte string var
11	2	local 8 byte string var
12	6	global 8 byte string var array
13	6	local 8 byte string var array
14	1+x	immediate varlen string - first you read 1 byte which gives you length of the rest which is text
15	16	immediate 16 byte string
16	2	global varlen string var
17	2	local varlen string var
18	6	global varlen string var array
19	6	local varlen string var array

• Varlen var array are made of 3 uint16s.

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 (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

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 0002's 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 are 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.

The Tools

There are three main builders for GTA 3, VC, and SA, and one for LCS and VCS.

Mission Builder

(BWME - note: although BWME was a slightly different tool, I shall be referring to this as that):

This tool uses only OpCodes to compile the code, all the text on the line is ignored. Traditionally, it decompiles to a file called main.scm.txt, which is just a big text file with all the code in it, expanded to be readable. This tool is used by the vast majority of mission coders as it is the most abundant and as most source code online is written for it, most people use it, the more people use it, the more code there is for it, so the more people use it.

Code format

Early builders used data type identifiers on all numbers, these were

 ? - small int (data type 04)
& - medium int (data type 05)
&& - big int (data type 01)<- global jump (data type 01)<- mission jump (data type 01 - negative addressing)
! - integer (data type 06)
$ - global variable (data type 02)
@ - local variable (data type 03)
?? - DMA global variable (like a global variable, only its memory position is its name, not assigned to it - data type 02)
: - label (text directly after used to reference this label in jumps)
"" - string (no data type, first 8 bytes after opcode when compiled)
# - model identifier (means you can enter the id name of a model rather than the number - data type 05 for the compiled number)

Later versions of the builder got rid of number type definitions, assigning types based on the size of the number. Integers were made integers by being not a whole number (e.g. 10.5 or 10.0 if you want a whole number defined as an integer). They also replaced DMA variables with global variables where the name was the hex address in decimal divided by 4 (each variable uses 4 bytes of memory).

Advantages

Widely used.
Commands related to the parameters.
Macros and program execution facilities inbuilt.

Disadvantages

Creator retired (no future updates / bug fixes).
Decompilation bugs (especially in certain advanced jumps).
Many unofficially usable SCM features uncatered for (although these are mostly advanced problems never experienced by the average coder).
Inconvenient syntax.

Other notes

GUI.
Compiler inbuilt.

Point

This is still very much in the development stages, it is the first user made high-level scripting tool (in fact, first high-level at all, even Rockstar's compiler is only an advanced parser) made for coding GTA. Originally developed for VC/III, this tool has been expanded to work for all three 3D games. One major disadvantage to this is that the file headers it writes, while readable by the game, are unrecognized by any line by line decompilers. So once a file has been compiled in Point, it cannot be decompiled again by another tool to see the exact generated code.

Sanny Builder

Main article: Sanny Builder.

Another new tool by Seemann. This is being developed for SA initially (although porting to other games may be an option) and is loosely based on BWME code. It was originally created using the SAscm.ini file from BWME 0.33 SA but has since been expanded to include many different features, some taken from Gtama, some new (such as a basic class system and direct HEX input (as requested by Y_Less)). This tool is still being developed and has now introduced a basic class system so you can do things such as "player.health += 5", this system is also being adapted for point.

The code format is based on a combination of both Gtama and BWME formats, although you can't force data types as you could in early Mission Builders (e.g. 0004: $var = 0&& which would normally be assigned one byte, not four).

Code format (note these are for SA, not VC as the others listed are)

$ - global variable
s$ - global string variable
v$ - global long string variable
@ - label (text directly AFTER used to reference this label in jumps)
@ - local variable (number BEFORE denotes variable)
@s - local string variable (number BEFORE denotes variable)
@v - local long string variable (number BEFORE denotes variable)
'...' - string (first 8 bytes after opcode when compiled)
"..." - debug string text
# - model identifier (means you can enter the id name of a model rather than the number)

Disassembler/Assembler

(GTA Mission Assembler - gtaMa, Vice City Disassembler - DisAsm):

These tools use one word commands, although they may consist of multiple words concatenated by an underscore ("_"), e.g. is_player_defined. They still compile each line as-is (i.e. no interpretation or code generation) so the game will execute exactly the commands you enter. This is similar to programming in ASM mnemonics, whereas BWME is more similar to machine code. The decompiled file is split up into a number of .gsr files, each one containing the code to one mission. This reduces file sizes considerably as BWME generated files are huge (around 6 MB .txt files), containing the whole code. The code is in the format of a command, followed by a list of parameters, separated by spaces - this can make named variables easy to distinguish from commands.

The disassembler (DisAsm) is written by CyQ and the assembler (gtaMa) is written by Dan Strandberg. These two tools work together to de- and re-compile the code.

Code format

$ - global variable (data type 02)
! - local variable (data type 03)
@ - label (text directly after used to reference this label in jumps)
"" - string (no data type, first 8 bytes after opcode when compiled)
% - model identifier (means you can enter the id name of a model rather than the number - data type 05 for the compiled number)

Advantages

Small files sizes.
Clearer code - data and commands separated.
Active creator (although no longer developing).
More support for advanced features (supports memory hacking methods not widely used).
Open source.
Online updateable ini.
Format used on custom error handler for VC.

Disadvantages

Not widely used.
Code spread across multiple files - harder for searching.
Data not easily related to code.

Other notes

Command line based.