SCM Instruction

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

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

Format

Initially the instructions are written in a human-readable form. For example, a wait instruction could look like this:

wait 0

where the word wait is a compiler-dependent representation of the opcode 0001 and the number 0 is a single parameter for this particular instruction. When a mission script is assembled, the instructions are written back in raw byte form:

0100 04 00

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

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. 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.

There could be zero or more instruction parameters following the opcode number^[*].

Parameters

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

The value of a parameter could be one of following types:

• Immediate values
  • integer numbers
  • floating-point numbers
  • fixed-length strings
  • variable-length strings
• Variables
  • global variables
  • local variables
• arrays

A concrete type of the value is determined by a single byte written before it^[*]. This byte is called a data type. The purpose of it is to tell to the game how much bytes to read and how to treat it.

Data types

A data type is a classification of identifying the type of a value. Types commonly used in GTA include integer numbers, floating-point numbers, and strings. The amount of data allowed to be stored is limited by the type and size of the data. For integers there are two ways to represent the data, signed and unsigned, whereas floating-point values are always signed. A signed data range includes negative numbers while unsigned do not include negatives. The following list shows the types and sizes of data in bytes.

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;

Depending of the preceeding data type, the parameter value compiled in two bytes 02 00, could be treated either as the global variable $2, or a local variable 2@ or a number of 2. 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)
0A..0B	1		Local timers (TIMERA, TIMERB) scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC++ + 0x5E;
0B..0C	1		Local timers (TIMERA, TIMERB) scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC++ + 0x5D;
0C..6B	1		Local integer/floating-point variable scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC++ - 0x0C;
0D..6C	1		Local integer/floating-point variable scriptParam.m_sLocalVar = *(unsigned char *)m_pScriptPC++ - 0x0D;
6C..CB	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;
6D..CC	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;
CC..E5	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;
CD..E5	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;
E6..FF	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.

^ Given the data type range limit the largest global variable in LCS is $6656, in VCS $6400.

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.

Integer numbers

An integer is a number without a decimal or fractional component.

Size (bytes)	Range
		Signed	Name		Unsigned	Name
1		-128 to 127	INT8, CHAR		0 to 255	UINT8, BYTE
2		-32,768 to 32,767	INT16, SHORT		0 to 65,535	UINT16, WORD, USHORT
4		-2,147,483,648 to 2,147,483,647	INT32, LONG		0 to 4,294,967,295	UINT32, DWORD, ULONG

Floating-point numbers

A floating point is a number with a decimal component and can store extremely large or small numbers while sacrificing significant digits. This is achieved by internally using exponents in scientific notation.

Size (bytes)	Range	Name
4	±1.1754944×10-38 to ±3.4028234×1038	SINGLE, FLOAT

Strings

A string is a sequence of characters not treated as numbers. 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.

^ A fixed-length string or a null-terminated string. This is the most common type been used since GTA 3. The string length is fixed. When compiled these strings occupy 8 bytes of a SCM file even if they are actually shorter (the rest 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

^ A variable-length string. This type was first introduced in San Andreas. Maximum length depends on the instruction^[*] (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 instructions have variable amount of parameters. The most known one 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 instruction 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 instruction 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)