SCO

SCO files contain GTA 4's game scripts. Its new format replaces old scm one.

File Format

A SCO file is layed out into 4 segments. First the Header containing information about the SCO file. Then the Code Segment which contains the opcode's which govern how the script behaves. The next segment is the Local Variable container which contains enough space to hold the local variables. The last is the Global Variable container, which contains enough space to house the global variables, as well as setting them by default.

Header

There are 2 types of SCO files, an encrypted and unencrypted one. Each file however shares the same unencrypted header structure, and you can use this to determine which type of SCO file you are dealing with. The size of this header is 24 bytes.

4b - CHAR[4]/UINT32 - SCO Identifier
4b - UINT32 - Code Size
4b - UINT32 - Local Var Size
4b - UINT32 - Global Var Size
4b - UINT32 - Script Flags
4b - UINT32 - Signature

The SCO Identifier will be "SCR\r" (or 0xD524353) in an unencrypted version, and "scr"+0xE (or 0xE726373) in an encrypted version. The Code Size refers to the amount of bytes the code section takes up. The Local Var Size refers to the amount of local variables the SCO file contains. The segment for local variables starts at the end of Header + Code Size, and continues for 4x the local variable size (due to the local variables being stored in 4 byte segments). The Global Var Size refers to the amount of global variables the SCO file contains. The segment for global variables starts at the end of Header + Code Size + Local Var Size * 4, and continues for 4x the global variable size (due to the global variables being stored in 4 byte segments). The Script Flags are boolean bits which are currently unexplained. The Signature only differs in navgen_main, but could possibly set the script priority.

Code Segment

The Code Segment contains the opcodes which govern the scripts behaviour.

Opcodes

Opcode's can have varying size's, but all opcodes are identified by their first byte. There are 80 opcodes which can occur, any opcode above 80 is a Push opcode which pushes it's own number - 96 onto the stack.

Add

1. Adds the top 2 items on the stack. 1 byte in length.

Sub

2.Subtracts the top 2 items on the stack. 1 byte in length.