III/VC SCM

This article may need to be rewritten. Please help improve this article. The discussion page may contain suggestions.

Overall Format

(02 00 01)h + 32 bit int     Jump to second segment
byte                         Target game ('l' (0x6C) GTA III, 'm' (0x6D) for GTA VC, 's' for GTA SA)
  (Global vars)              Space for variable saving
(02 00 01)h + 32 bit int     Jump to third segment
byte                         Padding (always 0)
  32 bit int                 Number of models
  (model names)              24 byte model names * number of models (model 0 name is empty and therefore unused)
(02 00 01)h + 32 bit int     Jump to fourth segment
byte                         Padding (always 0)
  32 bit int                 MAIN size
  32 bit int                 Largest mission size
  32 bit int                 Number of missions
  (mission addresses)        32 bit addresses * number of missions
(MAIN code)                  MAIN section, equal to size defined earlier
(mission code)               The mission data, missions stored at offsets defined earlier

Main Section

Made up of many opcodes one after the other. Opcodes are of the format:

TOpcode {
  Opcode[2]: Word; //Opocode number in word format
  Parameters: Array Of TParameter;
}

The number of parameters is specific for each opcode. The opcode database has all of the opcodes in VC, with their parameter numbers, and for most, a description of what they do. Parameters are of the format:

TParameter {
  ParameterType[1]: Byte; //Says which type of parameter will follow
  ActualParameter: varies; //The actual parameter
}

The parameter types are as follows (but there are more in SA):

01: 32-bit int   (DWord)
02: global var   (Word)
03: local var    (Word)
04: 8-bit int    (Byte)
05: 16-bit int   (Word)
06: 4-byte float (Single)

Labels use the type 01 (although if they address is small enough using data types 04 and 05 will also work). Global and local vars can also be used for labels. Global vars reference a position in the file much like a label. There is a space at the top of the scm file filled with 00's which is space reserved for global vars. This is divided into blocks of 4 bytes, each var taking up one of these blocks. In compiled form, the global vars each point to somewhere in that block (or should do as otherwise writing to it will modify the actual code).

Example

This code:

0001: wait 8 ms
0002: jump $var1
0050: gosub 1@
0051: return

...compiles as:

01 00 04 08 //wait opcode (First 2 bytes = opcode, next byte = 04 the parameter type for 8-bit int, and 08 is the actual value)
02 00 02 18 00 // jump opcode. The 18 00 would mean that the global var is stored at address 0x0018
50 00 03 01 00 //the gosub opcode
51 00 //the return opcode (it has no parameters)