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 global variables container which contains enough space to hold the scripts global variables. The last is the public variables container, it is unclear what this segment actually does.

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 - Global Var Count
4b - UINT32 - Public Var Count
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. To decrypt an encrypted version you must decrypt each segment (except the header) separately using GTA IV's AES Cryptography. The Code Size refers to the amount of bytes the code section takes up. The Global Var Count refers to the amount of global variables the SCO file contains. The segment for global variables starts at the end of Header + Code Size, and continues for 4x the global variable count (due to the global variables being stored in 4 byte's each). The Public Var Size refers to the amount of public variables the SCO file contains. The segment for public variables starts at the end of Header + Code Size + Global Var Count * 4, and continues for 4x the public variable count (due to the public variables being stored in 4 byte's each). 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

Opcodes can have varying sizes, but all opcodes are identified by their first byte. There are 79 opcodes which can occur and any opcode above 80 is a Push opcode which pushes it's own number - 96 onto the stack. For example, opcode 100 will push 4 onto the stack. Opcodes 76,77,78 deal with XLive protected buffers and are available on the PC platform only. Undefined opcodes (i.e. opcode 79), will cause a forceful abort of the script execution.

ID	Name	Description	Length
0	nop	No operation	1 byte
1	iadd	Adds the top 2 items on the stack	1 byte
2	isub	Subtracts the top 2 items on the stack	1 byte
3	imul	Multiplies the top 2 items on the stack	1 byte
4	idiv	Divides the top 2 items on the stack	1 byte
5	imod	Mods the top 2 items on the stack	1 byte
6	iszero	Checks the first item on the stack to see if it equals 0	1 byte
7	ineg	Reverses the sign on the item on the top of the stack	1 byte
8	icmpeq	Compares the top 2 integers on the stack to see if they are equal	1 byte
9	icmpne	Compares the top 2 integers on the stack to see if they are not equal	1 byte
10	icmpgt	Compares the top 2 integers on the stack to see if the first one is greater than the second one	1 byte
11	icmpge	Compares the top 2 integers on the stack to see if the first one is greater than or equal to the second one	1 byte
12	icmplt	Compares the top 2 integers on the stack to see if the first one is less than the second one	1 byte
13	icmple	Compares the top 2 integers on the stack to see if the first one is less than or equal to the second one	1 byte
14	fadd	Adds the top 2 floats on the stack	1 byte
15	fsub	Subtracts the top 2 floats on the stack	1 byte
16	fmul	Multiplies the top 2 floats on the stack	1 byte
17	fdiv	Divides the top 2 floats on the stack	1 byte
18	fmod	Mods the top 2 floats on the stack	1 byte
19	fneg	Reverses the sign on the first float on the stack	1 byte
20	fcmpeq	Compares the top 2 floats on the stack to see if they are equal	1 byte
21	fcmpne	Compares the top 2 floats on the stack to see if they are not equal	1 byte
22	fcmpgt	Compares the top 2 floats on the stack to see if the first one is greater than the second one	1 byte
23	fcmpge	Compares the top 2 floats on the stack to see if the first one is greater than or equal to the second one	1 byte
24	fcmplt	Compares the top 2 floats on the stack to see if the first one is less than the second one	1 byte
25	fcmple	Compares the top 2 floats on the stack to see if the first one is less than or equal to the second one	1 byte
26	vadd	Adds the top 2 Vectors[1] on the stack	1 byte
27	vsub	Subtracts the top 2 Vectors[1] on the stack	1 byte
28	vmul	Multiplies the top 2 Vectors[1] on the stack	1 byte
29	vdiv	Divides the top 2 Vectors[1] on the stack	1 byte
30	vneg	Reverses the sign on the first vector[1] on the stack	1 byte
31	iand	Performs an And operation to the first 2 integers on the stack	1 byte
32	ior	Performs an Or operation to the first 2 integers on the stack	1 byte
33	ixor	Performs a Xor operation to the first 2 integers on the stack	1 byte
34	jmp	Jumps to a section of code, using the next 4 bytes after the opcode as a placement	5 bytes
35	jmpf	Jumps to a section of code if the top of the stack is 0, using the next 4 bytes after the opcode as a placement	5 bytes
36	jmpt	Jumps to a section of code if the top of the stack is 1, using the next 4 bytes after the opcode as a placement	5 bytes
37	itof	Converts the top integer on the stack to a float, and puts that float on the stack	1 byte
38	ftoi	Converts the top float on the stack to an integer, and puts that integer on the stack	1 byte
39	ftov	Converts the top float into a Vector[1] containing 3 instances of the same float, and pushes the pointer to that Vector[1] onto the top of the stack	1 byte
40	ipush2	Pushes a short onto the stack, the short is defined in the next 2 bytes after the opcode	3 bytes
41	ipush	Pushes an int onto the stack, the integer is defined in the next 4 bytes after the opcode	5 bytes
42	fpush	Pushes a float onto the stack, the float is defined in the next 4 bytes after the opcode. Performs exactly the same as Push	5 bytes
43	dup	Duplicates the first item on the stack, and pushes it back onto the stack	1 byte
44	pop	Pops the top item off the stack	1 byte
45	native	Calls a native function. The number of arguments for the native to take is defined in the next byte after the opcode. The number of return values is defined in the byte after that (it is always 1 or 0). The next 4 bytes are the hash of the native's name.	7 bytes
46	call	Calls a function within the script, and puts the return address on top of the stack. The location of the function is defined in the next 4 bytes after the opcode	5 bytes
47	enter	Indicates the beginning of an internal function. The byte after the opcode indicates the amount of arguments the function takes off the stack, and the next 2 bytes after that indicate the number of variables the function will have to generate on the stack.	4 bytes
48	ret	Indicates the end of an internal function. The byte after the opcode indicates the amount of arguments that will have to be popped off the stack, and the next byte after that indicates the stack number of the return address	3 bytes
49	pget	Pops a pointer off the stack and pushes the value stored in that pointer back onto the stack	1 byte
50	pset	Pops 2 items off the stack and stores the second item at the location of the first item (the first item being a pointer)	1 byte
51	ppeekset	Pops the first item off the stack and peeks at the second item on the stack, then stores the first item at the location pointed to by the second item on the stack	1 byte
52	explode	Pops 2 items off the stack, the first and second items being the beginning of the memory of the array and the end of the memory of the array. It then divides the difference of these 2 locations by 4 to get the number of items in the array, after which is pushes these items one by one onto the stack in 4 byte segments	1 byte
53	implode	Pops the first item off the stack to get the address of the array to write to, and then pops the stack off onto that array	1 byte
54	flvar0	Pushes the pointer to the first function local variable onto the stack.	1 byte
55	flvar1	Pushes the pointer to the second function local variable onto the stack.	1 byte
56	flvar2	Pushes the pointer to the third function local variable onto the stack.	1 byte
57	flvar3	Pushes the pointer to the fourth function local variable onto the stack.	1 byte
58	flvar4	Pushes the pointer to the fifth function local variable onto the stack.	1 byte
59	flvar5	Pushes the pointer to the sixth function local variable onto the stack.	1 byte
60	flvar6	Pushes the pointer to the seventh function local variable onto the stack.	1 byte
61	flvar7	Pushes the pointer to the eigth function local variable onto the stack.	1 byte
62	flvar	Pushes the pointer to a function local variable onto the stack where the index is above or equal to 8	1 byte
63	global	Pops the index of the global variable off the stack, and pushes a pointer to the script global variable onto the stack	1 byte
64	public	Pops the index of the public variable off the stack, and pushes a pointer to the public variable onto the stack	1 byte
65	array	Pops the array location, element size and index off the stack, the pushes a pointer to the index of that array onto the stack	1 byte
66	switch	Pops the item to compare off the stack, and then jumps to location corresponding to that item. After the opcode byte it contains a byte defining the number of possible entries, and after that the number of possible entries times 8 are taken up with repeating instances of 4 bytes of the index identifier, and 4 bytes of the location to jump to if that index is correct	(Byte after opcode * 8) + 2
67	spush	Pushes a string onto the stack. The byte after the opcode signals the string length, and for the amount of string length after that byte contains each character of the string	(Byte after opcode)+2
68	null	Pushes a pointer to an empty memory container onto the stack	1 byte
69	scpy	Pops 2 pointers off the stack, and copies the second item to the first item	1 byte
70	itos	Pops an integer off the stack and pushes an array to a string representation of that integer onto the stack	1 byte
71	sadd	Pops 2 pointers off the stack, and appends the second item to the first item	1 byte
72	saddi	Pops 2 items off the stack, and performs a IntToStr on the second item (the integer), then appends that string representation to the first item	1 byte
73	catch	Sets up a safe area that has the ability to catch errors	1 byte
74	throw	Indicates an area that handles a script error relative to the catch opcode	1 byte
75	memcpy	Pops 3 items off the stack. Copies the third item's memory into the first item's memory with repeating defined by the second item. It then appends a null terminator to the first item's memory	1 byte
76	getxprotect	Pops a memory location off the stack and pushes its XLive unprotected value onto the stack (PC Only)	1 byte
77	setxprotect	Pops a memory location off the stack, pops another value off the stack. Protects the second value using XLive and stores the value in the memory location. (PC Only)	1 byte
78	refxprotect	Pops a memory location off the stack, pops another value off the stack to determine protection flags, and finally pops another value which indicates the number of items to operate on. If the 1st bit of the flags is set, the number of items at the memory location will be XLive unprotected. If the 2nd bit is set, they will be XLive protected instead. (PC Only)	1 byte
79	exit	Terminates the script under an error condition	1 byte
80 -> 255	ipush1	Pushes an integer onto the stack. The integer is defined by the opcode number - 96. For example opcode 95 would push -1 onto the stack, and opcode 97 would push 1	1 byte

^ The Vectors on the stack are pointers to the memory containing the full vector. A Vector is characterised by this structure:

4b - FLOAT32 - X
4b - FLOAT32 - Y
4b - FLOAT32 - Z

Global Variables

This contains the Global Variables in the script. Each global variable is 4 bytes long, and can contain static information in the script file itself.

Public Variables

It is uncertain what this section actually does.

High Level Representation

To turn the assembly of a SCO file into a high level representation some factors have to be considered. Arrays and structures can be defined, so it is reasonable to assume there is some kind of typecasting, even if it is only between the following types; int, float, string or a predefined structure. It is interesting to note that opcodes exist to typecast integers and concatenate them directly to strings, this suggests the SCO language has a java like way of handling strings (using + and += for concatenation). The SCO file does not seem to have native support for the notion of classes, however this is not to say they could not be implemented within SCO itself similar to how they are done at a low level in C++.

Tools

• SCO Toolbox – by flitskikker
• Scone – SCO (Dis-)assembler by Sacky
• OpenIV – contains a built-in decompiler
• SparkIV – contains a built-in decompiler

See also

• Mission Scripting (Overview) – introduction to mission scripting in the previous games

External link

•  GTAForums: SCO editor development – by Shadow-Link