SCM Instruction

This article deals with the technical information on the opcode format. For the opcodes documentation see list of opcodes

A SCM file itself is a bytecode containing instructions telling to the game what to do. An instruction consist of an opcode and its parameters (if there are any). Sometimes the whole script instruction is called opcode.

Each script instruction is represented by a number called operation code which is implemented using an 16 bit unsigned integer. By this number the game engine identifies an action to perform. Say, opcode 0001 tells to wait for amount of time, 0003 shakes the camera, 0053 creates a player, etc.

This is how an opcode 0001 looks in a scm file:

0100 04 00

• First part is the opcode number in a little-endian format.
• Second part is the data type
• Third part is a parameter value

When a mission script is disassembled, opcodes are written in a human-readable format. The example above will look something like this:

wait 0

This is made for the end-user convenience only. The game does not know what the word wait means, but it knows what the opcode 0001 is, so when a mission script is assembled the commands are written back in raw byte form.

As it has been said, an opcode is UINT16 number. It means the minimum opcode is 0000 and maximum opcode is 0xFFFF. However due to a specific of the SCM language, any numbers above 0x7FFF denote negative conditional opcodes. More on this read there. The original unmodded game supports a way smaller amount of opcodes (maximum 0A4E for San Andreas), but there are tools adding new ones, most notably CLEO library.

After an opcode number the data types and parameter values follow^[*].

Data types

A parameter's data type is determined with a single byte written before it^[*]. The purpose of it is to tell to the game how much bytes to read next and what kind of data it is.

Data type (hex)	Arg. length	Target game	Description
Typified
00	0		End of argument list (EOAL, 004F or 0913 and similar)[*]
01	4		Immediate 32-bit signed int scriptParam.m_iIntValue = *(int *)m_pScriptPC; m_pScriptPC += 4;
02	2		Global integer/floating-point variable scriptParam.m_usGlobalOffset = *(unsigned short *)m_pScriptPC; m_pScriptPC += 2;
03	2		Local integer/floating-point variable scriptParam.m_sLocalVar = *(short *)m_pScriptPC; m_pScriptPC += 2;
04	1		Immediate 8-bit signed int scriptParam.m_iIntValue = *(char *)m_pScriptPC++;
05	2		Immediate 16-bit signed int scriptParam.m_iIntValue = *(short *)m_pScriptPC; m_pScriptPC += 2;
06	2		Immediate 16-bit fixed-point (see remark) scriptParam.m_fFloatValue = (float)(*(short *)m_pScriptPC) / 16.0f; m_pScriptPC += 2;
06	4		Immediate 32-bit floating-point scriptParam.m_fFloatValue = *(float *)m_pScriptPC; m_pScriptPC += 4;
07	6		Global integer/floating-point array[*] scriptParam.m_usGlobalOffset = *(unsigned short *)m_pScriptPC; scriptParam.m_sArrayIndexVar = *(short *)(m_pScriptPC + 2); scriptParam.m_ucArraySize = *(unsigned char *)(m_pScriptPC + 4); scriptParam.m_arrayProperties = *(ArrayProperties *)(m_pScriptPC + 5); m_pScriptPC += 6;
08	6		Local integer/floating-point array[*] scriptParam.m_sLocalVar = *(short *)m_pScriptPC; scriptParam.m_sArrayIndexVar = *(short *)(m_pScriptPC + 2); scriptParam.m_ucArraySize = *(unsigned char *)(m_pScriptPC + 4); scriptParam.m_arrayProperties = *(ArrayProperties *)(m_pScriptPC + 5); m_pScriptPC += 6;
09	8		Immediate 8-byte string[*] strcpy(scriptParam.m_szTextLabel, (char *)m_pScriptPC); m_pScriptPC += 8;
0A	2		Global 8-byte string variable scriptParam.m_usGlobalOffset = *(unsigned short *)m_pScriptPC; m_pScriptPC += 2;
0B	2		Local 8-byte string variable scriptParam.m_sLocalVar = *(short *)m_pScriptPC; m_pScriptPC += 2;
0C	6		Global 8-byte string array[*] scriptParam.m_usGlobalOffset = *(unsigned short *)m_pScriptPC; scriptParam.m_sArrayIndexVar = *(short *)(m_pScriptPC + 2); scriptParam.m_ucArraySize = *(unsigned char *)(m_pScriptPC + 4); scriptParam.m_arrayProperties = *(ArrayProperties *)(m_pScriptPC + 5); m_pScriptPC += 6;
0D	6		Local 8-byte string array[*] scriptParam.m_sLocalVar = *(short *)m_pScriptPC; scriptParam.m_sArrayIndexVar = *(short *)(m_pScriptPC + 2); scriptParam.m_ucArraySize = *(unsigned char *)(m_pScriptPC + 4); scriptParam.m_arrayProperties = *(ArrayProperties *)(m_pScriptPC + 5); m_pScriptPC += 6;
0E	1 + (n - 1)		Immediate variable-length string[*] (non null-terminated) char cStrLength = *(char *)m_pScriptPC++; strncpy(scriptParam.m_szTextLabel, (char *)m_pScriptPC, cStrLength); memset(&scriptParam.m_szTextLabel[cStrLength], '\0', ucMaxLength - cStrLength); m_pScriptPC += cStrLength;
0F	16		Immediate 16-byte string[*] strcpy(scriptParam.m_szTextLabel, (char *)m_pScriptPC); m_pScriptPC += 16;
10	2		Global 16-byte string variable scriptParam.m_usGlobalOffset = *(unsigned short *)m_pScriptPC; m_pScriptPC += 2;
11	2		Local 16-byte string variable scriptParam.m_sLocalVar = *(short *)m_pScriptPC; m_pScriptPC += 2;
12	6		Global 16-byte string array[*] scriptParam.m_usGlobalOffset = *(unsigned short *)m_pScriptPC; scriptParam.m_sArrayIndexVar = *(short *)(m_pScriptPC + 2); scriptParam.m_ucArraySize = *(unsigned char *)(m_pScriptPC + 4); scriptParam.m_arrayProperties = *(ArrayProperties *)(m_pScriptPC + 5); m_pScriptPC += 6;
13	6		Local 16-byte string array[*] scriptParam.m_sLocalVar = *(short *)m_pScriptPC; scriptParam.m_sArrayIndexVar = *(short *)(m_pScriptPC + 2); scriptParam.m_ucArraySize = *(unsigned char *)(m_pScriptPC + 4); scriptParam.m_arrayProperties = *(ArrayProperties *)(m_pScriptPC + 5); m_pScriptPC += 6;
Untypified
N/A	8		Immediate 8-byte string[*] strcpy(scriptParam.m_szTextLabel, (char *)m_pScriptPC); m_pScriptPC += 8;
N/A	128		Immediate 128-byte string strcpy(scriptParam.m_szString, (char *)m_pScriptPC); m_pScriptPC += 128;

As it might be seen from the table two bytes 02 00 could have 3 different meanings as a parameter: if it's preceeded by a data type of 02 it is a global variable ($2), data type of 03 - local variable (2@), data type of 05 - 16-bit number (2), so only the data type allows the game to determine the correct parameter meaning.

Data types for Liberty City Stories and Vice City Stories are much different. First of all, many data types itself denote an immediate value. For example, data type 01 is a value of 0, data type 02 the value 0.0, etc. Floating-point values are packed (1, 2 or 3 bytes of length instead of the common 4). Some data types itself are somewhat the identifier of a variable.

Data type (hex)	Arg. length	Target game	Description
Typified
00	0		End of argument list (EOAL)
01	0		Immediate 8-bit signed integer constant 0 scriptParam.m_iIntValue = 0;
02	0		Immediate 8-bit floating-point constant 0.0 scriptParam.m_fFloatValue = 0.0f;
03	1		Immediate 8-bit packed floating-point unsigned int uiUnpackedFloat = *(unsigned char *)m_pScriptPC++ << 24; scriptParam.m_fFloatValue = *(float *)&uiUnpackedFloat;
04	2		Immediate 16-bit packed floating-point unsigned int uiUnpackedFloat = *(unsigned short *)m_pScriptPC << 16; scriptParam.m_fFloatValue = *(float *)&uiUnpackedFloat; m_pScriptPC += 2;
05	3		Immediate 24-bit packed floating-point unsigned int uiUnpackedFloat = (*(unsigned short *)m_pScriptPC << 16) | (*(unsigned char *)(m_pScriptPC + 2) << 8); scriptParam.m_fFloatValue = *(float *)&uiUnpackedFloat; m_pScriptPC += 3;
06	4		Immediate 32-bit signed integer scriptParam.m_iIntValue = *(int *)m_pScriptPC; m_pScriptPC += 4;
07	1		Immediate 8-bit signed integer scriptParam.m_iIntValue = *(char *)m_pScriptPC++;
08	2		Immediate 16-bit signed integer scriptParam.m_iIntValue = *(short *)m_pScriptPC; m_pScriptPC += 2;
09	4		Immediate 32-bit floating-point scriptParam.m_fFloatValue = *(float *)m_pScriptPC; m_pScriptPC += 4;
0A	n + NUL		Immediate null-terminated string[*] strcpy(scriptParam.m_szTextLabel, (char *)m_pScriptPC); m_pScriptPC += strlen((char *)m_pScriptPC) + 1;
Untypified (script variables)
T<0C	1		Local timers (TIMERA, TIMERB) scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC++ + 0x5E;
T<0D	1		Local timers (TIMERA, TIMERB) scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC++ + 0x5D;
T<6C	1		Local integer/floating-point variable scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC++ - 0x0C;
T<6D	1		Local integer/floating-point variable scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC++ - 0x0D;
T<CC	3		Local integer/floating-point array scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC - 0x6C; scriptParam.m_sArrayIndex = *(unsigned char *)(m_pScriptPC + 2); scriptParam.m_ucArraySize = *(unsigned char *)(m_pScriptPC + 3); m_pScriptPC += 3;
T<CD	3		Local integer/floating-point array scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC - 0x6D; scriptParam.m_sArrayIndex = *(unsigned char *)(m_pScriptPC + 2); scriptParam.m_ucArraySize = *(unsigned char *)(m_pScriptPC + 3); m_pScriptPC += 3;
T<E6	2		Global integer/floating-point variable unsigned short usBigEndianWord = *(unsigned short *)m_pScriptPC - 0x00CC; scriptParam.m_sGlobalVar = (short)((usBigEndianWord << 8) | (usBigEndianWord >> 8)); m_pScriptPC += 2;
			Global integer/floating-point variable unsigned short usBigEndianWord = *(unsigned short *)m_pScriptPC - 0x00CD; scriptParam.m_sGlobalVar = (short)((usBigEndianWord << 8) | (usBigEndianWord >> 8)); m_pScriptPC += 2;
T>=E6	4		Global integer/floating-point array unsigned short usBigEndianWord = *(unsigned short *)m_pScriptPC - 0x00E6; scriptParam.m_sGlobalVar = (short)((usBigEndianWord << 8) | (usBigEndianWord >> 8)); scriptParam.m_sArrayIndex = *(unsigned char *)(m_pScriptPC + 2); scriptParam.m_ucArraySize = *(unsigned char *)(m_pScriptPC + 3); m_pScriptPC += 4;
N/A	8		Immediate 8-byte string strcpy(scriptParam.m_szTextLabel, (char *)m_pScriptPC); m_pScriptPC += 8;

^ This type was introduced in VCS due to the presence of string variables.

All the data types above haven't been tested in a decompiling process yet, hence they still need a practical confirmation.

This section is incomplete. You can help by fixing and expanding it.

Parameters

The game engine knows amount of parameters for each opcode (1 for 0001, 2 for 0004, 13 for 014B, etc). If the script contains another number of parameter it causes a crash.

The parameters could be one of following kinds:

• Immediate values
  • integer numbers
  • floating-point numbers
  • short strings
  • long strings
• Variables
  • global variables
  • local variables
• arrays

This section is incomplete. You can help by fixing and expanding it.

Strings

Strings are sequences of characters. Those include letters, numbers, and other symbols like _ or @. Unlike other programming languages a string could start with any character, even a space.

There are two kinds of strings.

^ Short string or a null-terminated string. This is the most common type been used since GTA 3. The term short means that the string length is strongly limited. It may contain up to 7 characters and the last one (8th) is a null terminator byte. When compiled these strings occupy 8 bytes of a SCM file even if they are actually shorter (the rest of bytes is filled with zero bytes).

^ San Andreas introduces data type 15 for strings containing up to 15 symbols. They behave same as 8 bytes strings, but always occupy 16 bytes in a SCM file. These strings are only supported by Sanny Builder.

String	Equivalent in SCM
'MAIN'	09   4D 41 49 4E 00 00 00 00
'MODDING'	09   4D 4F 44 44 49 4E 47 00
'SAVE_OUR_SOULS!'	0F   53 41 56 45 5F 4F 55 52 5F 53 4F 55 4C 53 21 00

^ Long string. This type was first introduced in San Andreas. Maximum length depends on the opcode^[*] (the longest parameter ever read has got 40 characters).

This section is incomplete. You can help by fixing and expanding it.

Arrays

^ Native arrays support was introduced in GTA SA, however there were different implementations of arrays in Vice City. In SA SCM arrays are assembled as 2 UINT16s, 1 INT8 and a UINT8:

2b - UINT16 - array offset[*]
2b - UINT16 - array index[*]
1b - INT8   - array size
1b - UINT8  - array properties

^ An array offset is basically a variable number. If it's a global array, the offset is a global variable index from which the array begins. For example, if the global array offset is 150 (96 00) it means that the first element of the array is $150, the second one is $151, etc. Same valid for the local arrays (offset is a local variable index).

^ An array index is a variable number (global or local one) that holds the value of array index. For example, if array index is 3 (03 00), the game will read either global variable $3 or local variable 3@ depending on the array properties (see below). This variable holds the number which is array element ID to work with. For example, if the array index is $3, and $3 holds number 5, the game will read 5th element of the array.

Properties

Array properties describe the data type of each array element, held by the first 7 bits of the reference field, plus a flag which signals if the array was declared in a global scope, as the most significant bit indicates:

enum eArrayElementType
{
	ELEMTYPE_INT,
	ELEMTYPE_FLOAT,
	ELEMTYPE_TEXT_LABEL,
	ELEMTYPE_TEXT_LABEL16
};

struct ArrayProperties
{
	unsigned char m_nElementType : 7;
	unsigned char m_bIsIndexGlobalVar : 1;
};

Array	Equivalent in SCM
$150(3@,6f)	07   96 00 03 00 06 01
10@(9@,5s)	0D   0A 00 09 00 05 02

Notes

^ In GTA 3, Vice City and Liberty City Stories short strings (8 bytes) have no data type preceeding it. If the byte does not fit data type range (00-06 for GTA 3 and VC), it's recognized as a beginning of a string and next 8 bytes are read.

^ Some opcodes have variable amount of parameters. Most known opcode is 004F that creates a new thread and passes arguments to it. The number of such parameters could vary, so the special data type denotes the end of parameters.

The maximum amount of parameters for any opcode is 16 for GTA 3 and VC, 32 for SA, LCS and VCS. However, those that admit an undefined amount of arguments can pass 18 parameters for GTA 3 and VC, 34 for SA, 106 for LCS and VCS (this information still needs confirmation).

^ Opcode 05B6 is a special opcode that defines a table. Immediately after opcode number the stream of data (128 bytes) follows, without a data type.

^  GTAForums: Post by Seemann describing limits for the long strings in SA

See also

• Mission Scripting (Overview)