GPU/Shader Instruction Set

Overview

A compiled shader binary is comprised of two parts : the main instruction sequence and the operand descriptor table. These are both sent to the GPU around the same time but using separate GPU Commands. Instructions (such as format 1 instruction) may reference operand descriptors. When such is the case, the operand descriptor ID is the offset, in words, of the descriptor within the table. Both instructions and descriptors are coded in little endian. Basic implementations of the following specification can be found at [1] and [2] Please note that this page is being written as the instruction set is reverse engineered; as such it may very well contain mistakes.

Instruction formats

Format 1 : (used for register instructions)

Offset	Size (bits)	Description
0x0	0x7	Operand descriptor ID (DESC)
0x7	0x5	Source 2 register (SRC2)
0xC	0x7	Source 1 register (SRC1)
0x13	0x2	Address register index (IDX)
0x15	0x5	Destination register (DST)
0x1A	0x6	Opcode

Format 1i : (used for register instructions)

Offset	Size (bits)	Description
0x0	0x7	Operand descriptor ID (DESC)
0x7	0x7	Source 1 register (SRC1)
0xE	0x5	Source 2 register (SRC2)
0x13	0x2	Address register index (IDX)
0x15	0x5	Destination register (DST)
0x1A	0x6	Opcode

Format 1c : (used for comparison instructions)

Offset	Size (bits)	Description
0x0	0x7	Operand descriptor ID (DESC)
0x7	0x7	Source 1 register (SRC1)
0xE	0x5	Source 2 register (SRC2)
0x13	0x2	Address register index (IDX)
0x15	0x3	Comparison operator for Y (CMPY)
0x18	0x3	Comparison operator for X (CMPX)
0x1B	0x5	Opcode

Format 2 : (used for flow control instructions)

Offset	Size (bits)	Description
0x0	0x8	Number of instructions (NUM)
0xA	0xC	Destination offset (in words) (DST)
0x16	0x2	Condition boolean operator (CONDOP)
0x18	0x1	Y negation bit (NEGY)
0x19	0x1	X negation bit (NEGX)
0x1A	0x6	Opcode

Format 3 : (used for uniform-based conditional flow control instructions)

Offset	Size (bits)	Description
0x0	0x8	Number of instructions ? (NUM)
0xA	0xC	Destination offset (in words) (DST)
0x16	0x4	Uniform ID (BOOL/INT)
0x1A	0x6	Opcode

Format 4 : (used for SETEMIT)

Offset	Size (bits)	Description
0x16	0x2	Primitive ID (PRIMID)
0x18	0x2	Vertex ID (VTXID)
0x1A	0x6	Opcode

Format 5 : (used for MAD)

Offset	Size (bits)	Description
0x0	0x5	Operand descriptor ID (DESC)
0x5	0x5	Source 3 register (SRC3)
0xA	0x7	Source 2 register (SRC2)
0x11	0x7	Source 1 register (SRC1)
0x18	0x5	Destination register (DST)
0x1D	0x3	Opcode

Instructions

Opcode	Format	Name	Description
0x00	1	ADD	Adds two vectors component by component; DST[i] = SRC1[i]+SRC2[i] for all i (modulo destination component masking)
0x01	1	DP3	Computes dot product on 3-component vectors; DST = SRC1.SRC2
0x02	1	DP4	Computes dot product on 4-component vectors; DST = SRC1.SRC2
0x08	1	MUL	Multiplies two vectors component by component; DST[i] = SRC1[i].SRC2[i] for all i (modulo destination component masking)
0x0C	1	MAX	Takes the max of two vectors, component by component; DST[i] = MAX(SRC1[i], SRC2[i]) for all i (modulo destination component masking)
0x0D	1	MIN	Takes the min of two vectors, component by component; DST[i] = MIN(SRC1[i], SRC2[i]) for all i (modulo destination component masking)
0x0E	1	RCP	Computes the reciprocal of the vector, component by component; DST[i] = 1/SRC1[i] for all i (modulo destination component masking)
0x0F	1	RSQ	Computes the reciprocal of the square root of the vector, component by component; DST[i] = 1/sqrt(SRC1[i]) for all i (modulo destination component masking)
0x12	1	ARL	Address Register Load; sets (a0, a1, _, _) to SRC1 (cast to integer).
0x13	1	MOV	Moves value from one register to another; DST = SRC1.
0x18	1i	DP4I?	Computes dot product on 4-component vectors; DST = SRC1.SRC2 ?
0x21	1	END2	?
0x22	1	END1	?
0x23	1	BREAK	Conditionally breaks out of LOOP block. Conditions for this are assumed to work the same was as for IF.
0x24	2	CALL	Jumps to DST and executes instructions until it reaches DST+NUM instructions
0x26	3	CALLC	Jumps to DST and executes instructions until it reaches DST+NUM instructions if BOOL is true
0x27	3	IFU	If condition BOOL is true, then executes instructions until DST, then jumps to DST+NUM; else, jumps to DST.
0x28	2	IF	If condition (see below for details) is true, then executes instructions until DST, then jumps to DST+NUM; else, jumps to DST
0x29	3	FORLOOP	Loops over the code between itself and DST. Increments lcnt after each loop. Stops looping once lcnt reaches the value contained by the integer uniform specified by INT. (i0-i7)
0x2A	0 (no param)	EMIT	(geometry shader only) Emits a vertex (and primitive if PRIMID is non-zero). SETEMIT must be called before this.
0x2B	4	SETEMIT	(geometry shader only) Sets VTXID and PRIMID for the next EMIT instruction. VTXID is the ID of the vertex about to be emitted within the primitive, while PRIMID is zero if we are just emitting a single vertex and non-zero if are emitting a vertex and primitive simultaneously. Note that the output vertex buffer (which holds 4 vertices) is not cleared when the primitive is emitted, meaning that vertices from the previous primitive can be reused for the current one. (this is still a working hypothesis and unconfirmed)
0x2C	2	JMPC?	Jumps to DST if condition (don't know how condition flags work yet) is true, else does nothing.
0x2E-0x2F	1c	CMP	Sets booleans cmp.x and cmp.y based on the operand's x and y components and the CMPX and CMPY comparison operators respectively. See below for details about operators.
0x38-0x3F	5	MAD	Multiplies two vectors and adds a third one component by component; DST[i] = SRC3[i] + SRC2[i].SRC1[i] for all i (modulo destination component masking)

Operand descriptors

Sizes below are in bits, not bytes.

Offset	Size	Description
0x0	0x4	Destination component mask. Bit 3 = x, 2 = y, 1 = z, 0 = w.
0x4	0x1	Source 1 negation bit
0x5	0x8	Source 1 component selector
0xD	0x1	Source 2 negation bit
0xE	0x8	Source 2 component selector
0x16	0x1	Source 3 negation bit
0x17	0x8	Source 3 component selector

Component selector :

Offset	Size	Description
0x0	0x2	Component 3 value
0x2	0x2	Component 2 value
0x4	0x2	Component 1 value
0x6	0x2	Component 0 value

Value	Component
0x0	x
0x1	y
0x2	z
0x3	w

The component selector enables swizzling. For example, component selector 0x1B is equivalent to .xyzw, while 0x55 is equivalent to .yyyy.

Relative addressing

There are 3 global address registers : a0, a1 and a2 = lcnt (loop counter). For format 1 instructions, when IDX != 0, the value of the corresponding address register is added to SRC1's value.

For example, if IDX = 2, a1 = 3 and SRC1 = c8, then instead SRC1+a1 = c11 will be used for the instruction.

a0 and a1 can be set manually through the ARL instruction. lcnt is set automatically by the LOOP instruction. Note that lcnt is still accessible and valid after exiting a LOOP block.

Comparison operator

CMPX/CMPY raw value	Operator name	Expression
0x0	EQ	src1 == src2
0x1	NE	src1 != src2
0x2	LT	src1 < src2
0x3	LE	src1 <= src2
0x4	GT	src1 > src2
0x5	GE	src1 >= src2
0x6	??	true ?
0x7	??	true ?

6 and 7 seem to always return true.

Conditions

A number of format 2 instructions are executed conditionally. These conditions are based on two boolean registers which can be set with CMP : cmp.x and cmp.y.

Conditional instructions include 3 parameters : CONDOP, NEGX and NEGY. NEGX and NEGY determine whether the conditional expression will use cmp.x or !cmp.x, and cmp.y or !cmp.y respectively (NEGX set means we use cmp.x, and NEGX not set means we use !cmp.x). CONDOP describes the actual expression. There are four conditional expression formats :

CONDOP raw value	Expression	Description
0x0	[!]cmp.x || [!]cmp.y	OR
0x1	[!]cmp.x && [!]cmp.y	AND
0x2	[!]cmp.X	X
0x3	[!]cmp.y	Y

For example, with CONDOP=1, NEGX=1 and NEGY=0, the resulting expression would be (cmp.x && !cmp.y).

Registers

Most registers (all the ones within the 0x00-0x7F range) are float[4] vectors. There are also boolean registers (b0-b7) and integer registers (i0-i7). How the latter ones are set is as of yet unknown.

Attribute (input, RO) registers are located within the 0x0-0xF range. What data they are fed is specified by the CPU.

Output (WO) registers are also located within the 0x0-0xF range. What type of data they are contain is specified by the CPU.

Temporary (RW) register are located within the 0x10-0x1F range. They can contain any type of data.

Uniform (RO) registers are located within the 0x20-0x7F range. Their content is set by the CPU.

SRC2 being only 5 bits long rather than 7 bits like its friend SRC1, it can only access v (input attribute) and r (temporary) registers.

Registers in the 0x88-0x97 range are uniform booleans.

It appears that writing twice to the same output register can cause problems, such as the GPU hanging.

DST mapping :

DST raw value	Register name	Description
0x0-0x7	o0-o7	Output registers.
0x10-0x1F	r0-r15	Temporary registers.

SRC mapping :

SRC1 raw value	Register name	Description
0x0-0x7	v0-v7	Input attribute registers.
0x10-0x1F	r0-r15	Temporary registers.
0x20-0x7F	c0-c95	Vector uniform registers.

Note that 5bit SRC registers (SRC2 in format 1 for example) can't access c0-c95 because they don't have enough bits.