Fragment Shader ISA

Overview

The Tegra fragment shader ISA is not as straight forward as Vertex Shader ISA.

The fragment shader is separated into five different instruction streams:

• PSEQ - fetching data from the memory and feeding it to the pipeline as registers data or instructions
• MFU - multi-function unit, varying interpolate and special functions
• TEX - texture lookups
• ALU - arithmetic logic unit
• DW - writing to the output surface / buffers

The different units seems to be synchronized by separate timing streams.

Instructions flow

Sequence:	1	2	2	2	3	4	4	4	5
Stage 1:	PSEQ
Stage 2:		MFU	MFU	MFU
Stage 3:					TEX
Stage 4:						ALU	ALU	ALU
Stage 5:									DW

Program sequencer

Patent: https://www.google.com/patents/US8411096

Instructions scheduling

Patent: https://www.google.com/patents/US8856499

Instructions schedule specifies the number of MFU and ALU instructions executed by the respective stage, each from 1 to 3 per fragment pipeline instructions batch.

Bits	Meaning
2..7	Address
0..1	Number of instructions to execute

If the "number of instructions to execute" is 0, then the pipeline stage is NOP, however still takes 1 clock cycle. The "address" is the number of pushed instructions before the instruction to execute, so unit[Address] ... unit[Address + Number of instructions to execute] instructions will be executed, where unit stands for MFU or ALU.

ALU instruction word encoding

The ALU instructions comes in packets of 3 or 4 scalar instructions (the fourth instruction can be traded for embedded constants). Each ALU instruction package seems to run pipelined, and each instruction in a package can use partial results from the previous instruction.

Bits	Meaning
62..63	opcode
61	accumulate result: other
60	accumulate result: this
59	addition disable
57..58	scale result
56	saturate result
54..55	condition code
47..53	destination register
46	write high subregister
45	write low subregister
32..44	operand rA
19..31	operand rB
6..18	operand rC
0..5	operand rD

ALU buffer and pixel packet:

Patent: https://www.google.com/patents/US7710427

ALU embedded constants:

Patent: https://www.google.com/patents/US8775777

Unlike a regular ALU instruction, the ALU3 instruction words, constituting immediate constants, shouldn't be swapped. Constant is either one fp20 or two fx10.

Bits	Meaning
44..63	Immediate constant 2
24..43	Immediate constant 1
4..23	Immediate constant 0

Opcodes:

opcode	Mnemonic	Meaning	pseudo-code
0	MAD	Multiply-Add	rA * rB + rC * rD
1	MIN	Minimum	min(rA * rB, rC * rD)
2	MAX	Maximum	max(rA * rB, rC * rD)
3	CSEL	Conditional select	(rA < 0) ? rB : (rC * rD)

Scale result:

Value	Meaning
0	No scale
1	x2
2	x4
3	/ 2

Condition code:

The conditional operation is applied to the ALU's result, so the final result is either 0.0 (false) or 1.0 (true).

Value	Meaning
0	no comparison
1	zero
2	greater than zero
3	greater or equal to zero

Condition registers:

The condition register comprises two fixed10 values, 0.0 or 1.0. In order to write a value to the condition register, the destination register should be selected to the condition register 64..71. To write to the higher part of the condition register, "write high subregister" bit needs to be set; otherwise lower part will be written regardless of the "write low subregister" bit state.

Condition register stored value = !!(ALU result)

Looks like ALU can address up to 2 source CR's in one ALU[0-3] sub-instruction.

Kill register:

Non-zero value written to the KILL register discards the fragment, zero keeps it alive and doesn't resurrect the killed fragment. Destination register write mask (low/high halves) is ignored.

Position registers:

The returned fragment position X register value is accumulated by 8192.0, fragment position Y value is negative and accumulated by 8192.0 plus the render target height - 1.

pos.x = 8192 + frag.x
pos.y = 8192 + (target_height - 1) - frag.y

Operands (rA, rB, rC):

Bits	Meaning
12..6	register selector
5	sub-register selector
4	fixed10 minus one
3	fixed10
2	absolute value
1	negate
0	scale by two

Operand rD:

Bits	Meaning
5	rD selector (0 = rB, 1 = rC)
4	sub-register selector
3	fixed10 minus one
2	enable rD (scale rC by rD)
1	absolute value
0	fixed10

Registers:

Value	Meaning
0..15	row registers
16..23	general purpose registers
24..27	ALU result registers
28..30	embedded constants
31	lowp vec2(0, 1)
32..63	uniform registers
64..71	condition registers
72	fragment x-position
73	fragment y-position
75	polygon face
76	kill (discard) fragment

Result accumulation:

Patent: https://www.google.com/patents/US8521800

When "accumulate this" bit is set, operand rC is overridden with the accumulation value.

	Accumulate this	Accumulate other
ALU0:	ALU0 += ALU3	No action
ALU1:	ALU1 += ALU0	ALU0 += ALU1
ALU2:	ALU2 += ALU1	ALU0 += ALU2
ALU3:	ALU3 += ALU2	ALU2 += ALU3

Addition disable

When bit "addition disable" is set, the Multiply-Add operation turns into two multiplies. The fx10 result of each multiply goes to the low/high subregisters of the destination register.

write high subregister	write low subregister	destination low	destination high
1	0	rC*rD	rA*rB
0	1	rA*rB	rC*rD

MFU instruction word encoding

The MFU unit can fetch and interpolate 4 component vectors per instruction and/or evaluate scalar special functions. Based on this design.

Bits	Meaning
58..63	register
54..57	opcode
43..53	mul1
32..42	mul0
28..31	???
21..27	var3
14..20	var2
7..13	var1
0..6	var0

opcode	Mnemonic	Meaning	pseudo-code
0	NOP	No operation
1	RCP	Reciprocal	rD = 1.0 / rA
2	RSQ	Reciprocal square root	rD = 1.0 / sqrt(rA)
3	LG2	Logarithm base 2	rD = log2(rA)
4	EX2	Exponent base 2, second step	rD = pow(2.0, rA)
5	SQRT	Square root	rD = sqrt(rA)
6	SIN	Sine, second step	rD = sin(rA)
7	COS	Cosine, second step	rD = cos(rA)
8	FRC	Fractional value	rD = rA - floor(rA)
9	PREEX2	Exponent base 2, first step	rD = pow(2.0, rA)
10	PRESIN	Sine, first step	rD = sin(rA)
11	PRECOS	Cosine, first step	rD = cos(rA)

var0..3:

The varying id corresponds to the TRAM component: var0 is TRAM.x, var1 is TRAM.y, var2 is TRAM.z, var3 is TRAM.w.

Bits	Meaning
3..6	TRAM index to fetch from
1..2	opcode
0	saturate

opcode	Mnemonic	Meaning
0	NOP	No operation
1	VAR1	Interpolate one float20 value
2	VAR2	Interpolate two fixed10 values

mul0..1

Bits	Meaning
8..10	destination register
4..7	source register 1
0..3	source register 0

mul0..1 destination registers:

Value	Meaning
0	???
1	barycentric weight
2..3	???
4	row register 0
5	row register 1
6	row register 2
7	row register 3

mul0..1 source registers:

The SFU result is evaluated first and is available to use by MUL's via "SFU result" register.

Value	Meaning
0	row register 0
1	row register 1
2	row register 2
3	row register 3
4..7	??? (general purpose registers?)
8..9	???
10	SFU result
11	barycentric coef 0
12	barycentric coef 1
13	1.0
14..15	???

Interpolation

The barycentric interpolation weights are the MUL's results written to the "barycentric weight" destination register. The weight w0 (related to the first triangle vertex - the "barycentric coef 0" source register) is hardwired to the result of the mul0, the second vertex w1 ("barycentric coef 1" source register) to the mul1. Again, only destination "barycentric weight" registers are hardwired, "barycentric coef" sources are not. The third vertex weight is derived from the w0 and w1 as "1.0 - w0 - w1". The SFU operation should be set to "rcp r4".

barycentric weight = barycentric coef * 1.0 / w

Fragment w component

The w component is stored in the r4 and available to the first instruction of the scheduled MFU instructions sequence.

Fragment z component

The z component comes in some form via r3. The fetch operation of the r3 should be set to NOP with "saturation" being enabled for r3. After the r3 has been populated, the following expansion should be performed:

gl_FragCoord.z = 1/1000 + max(0.0, r3.low) * 1/4000 + max(0.0, r3.high) * 1/4

There is also dependency on the linker: it should perform the "magic" write to the TRAM0.w with "VEC4 select = VEC4.z".

TEX instruction word encoding

The TEX instruction takes the texture coordinates from the first row of the pixel packet (i.e. R0-R3 registers) and writes the sampled data to that first row as well.

Bits	Meaning
12..31	???
12	enable bias
11	???
10	enable
6..9	???
5	sampled data destination regs select
4	texcoord regs select
0..3	sampler index

Texture coordinate registers select:

The texture coordinate components (S, T, R) are loaded from the three registers as fp20's.

Value	Meaning (S, T, R order)
1	R2, R3, R0
0	R0, R1, R2

Sampled data destination registers select:

The sampled RGBA data is stored in the two registers as four fx10's.

Value	Meaning
1	R2-R3
0	R0-R1