Disclaimer: I do not work for Sony, despite the disturbing percentage of my shirts, jackets, and bookbags that are PlayStation dev-related. I do, however, have many friends that work at Sony, some of which I hope will call off the corporate lawyers. JayStation is in no way associated with Sony or PlayStation, and any stupid things I say represent only my own ineptitude and silliness.

This is a bittersweet post for me to write. At the end of August I will be temporarily pausing my eight year Japanese adventure and returning to the States for personal reasons, making this the last JayStation blog update from beautiful Kyoto, Japan. As part of the moving process, I am selling my only computer tomorrow, giving me just 24 hours to bang out this post before I no longer have any means of doing so. It won’t be as in depth or interesting as I had hoped, and it will probably be rushed and error-riddled with unclear wording, but it will show everything you need to know to render textured triangles. The next update probably won’t come until September or October when I settle in, unless of course I am murdered in the street for riding my bike to work. #America

There are three steps to getting textured polygons rendering. First you set up your vert data in memory, including two varyings for the normalized S and T texture coordinates. Next you set up between 1 and 4 uniforms, corresponding to texture config parameters 0 through 3. Finally you write a fragment shader that does interpolation for the ST coordinate varyings, and reads the texture data. The first step, setting up verts and varyings, was covered in the previous post so it won’t be duplicated here.

I Just Love A Config In Uniform

Texture config params are shader uniforms that are used to specify things like base address, dimensions, pixel format, mip levels, min/mag filters, and wrap mode for texture unit memory accesses. They roughly correspond to some combination of T#’s and samplers on GCN. The number of config params needed is dictated by the type of data being accessed, with one word required for 1D buffers and general memory accesses, two words for 2D textures, and three or four words for cubemaps and child images.

No matter the type of data, you must at least specify config param 0. Bits [31:12] give the base address of the LOD0 image in units of 4 KiB blocks, so all images must be at least 4 KiB aligned. Bits [11:10] give the cache swizzle mode, and will be discussed in depth in a later post. Bits [7..4] are the four LSB bits of the 5-bit pixel format value, and [3..0] give the number of mip levels minus one.

The supported pixel formats are

0 RGBA8888 32 8-bit per channel red, green, blue, alpha
1 RGBX8888 32 8-bit per channel RGA, alpha set to 1.0
2 RGBA4444 16 4-bit per channel red, green, blue, alpha
3 RGBA5551 16 5-bit per channel red, green, blue, 1-bit alpha
4 RGB565 16 Alpha channel set to 1.0
5 LUMINANCE 8 8-bit luminance (alpha channel set to 1.0)
6 ALPHA 8 8-bit alpha (RGA channels set to 0)
7 LUMALPHA 16 8-bit luminance, 8-bit alpha
8 ETC1 4 Ericsson Texture Compression format
9 S16F 16 16-bit float sample (blending supported)
10 S8 8 8-bit integer sample (blending supported)
11 S16 16 16-bit integer sample (point sampling only)
12 BW1 1 1-bit black and white
13 A4 4 4-bit alpha
14 A1 1 1-bit alpha
15 RGBA64 64 16-bit float per RGBA channel
16 RGBA32R 32 Raster format 8-bit per channel red, green, blue, alpha
17 YUYV422R 32 Raster format 8-bit per channel Y, U, Y, V

For 2D texture data, a uniform for config param 1 must also be given. This will include things useful for 2D texture reads such as width and height, wrap mode, min/mag mode, and the MSB of the 5-bit pixel format value.

As with the uniforms in previous examples, the texture config params are stored as a word-aligned list in memory, and the address of the list is encoded in the uniform data address field of the NV shader state record. The main difference here is that you won’t be manually reading these uniforms in the fragment shader. Rather, every time your shader writes to a texture unit register (TMUn_S, TMUn_T, TMUn_R, TMUn_B), the texture unit automatically fetches the next config param from the uniform FIFO and pushes it to the texture FIFO. If all four config params are needed, you have to write to all four TMU registers, writing a zero to TMUn_B if no bias is actually needed. Since the S register is the only one required by all access types, writing it also kicks off texture processing, and therefore it must always be written last.

The Shader

The high level shader flow is as follows: read and interpolate the S and T varyings, write T to TMU0_T causing the TMU to auto fetch config param 0, write S to TMU0_S causing the TMU to auto fetch config param 1 and kick off texture processing, send the GPU the texture read signal, and read back the packed pixel data from accumulator r4. Let’s start with the S and T varyings interpolation

; Tex S: ACC0 = S * W (r15a)
; add op: No operation, add cond: never
; mul pipe: Floating point multiply, ACC0, R15, VARYING_READ, cond: always
.word 0x203E3DF7, 0x110059E0

; Tex S coord: ACC0 = S * W + C, Tex T: ACC1 = T * W (r15a)
; add pipe: Floating point add, ACC0, acc r0, acc r5, cond: always
; mul pipe: Floating point multiply, ACC1, R15, VARYING_READ, cond: always
.word 0x213E3177, 0x11024821

; Tex T write reg = T * W + C, triggering first sampler param uniform read
; add pipe: Floating point add, TMU0_T, acc r1, acc r5, cond: always
; mul op: No operation, mul cond: never
.word 0x13E3377, 0x11020E67

Notice how the destination for the T coordinate’s add is TMU0_T. This not only feeds the texture unit the T coordinate, but also causes the TMU to fetch config param 0 from the uniform FIFO.

; moving S coord (in ACC0) to S register
; triggering read of second tex param uniform, and kicking it all off
; add pipe: Bitwise OR, TMU0_S_RETIRING, acc r0, acc r0, cond: always
; mul op: No operation, mul cond: never
.word 0x159E7000, 0x10020E27

Next we move the S value into TMU0_S. This feeds the texture unit the S coordinate, causes the TMU to fetch config param 1 from the uniform FIFO, and kicks off texture processing. Note that because S has to be written last, but usually comes first in the vert data, storing the texture coordinates as TS instead of ST can avoid an extra mov and be a potential optimization in some cases.

; signal TMU texture read. Can this be done with prev instruction?
; add op: No operation, add cond: never
; mul op: No operation, mul cond: never
; signal: load data from tmu0 to r4
.word 0x9E7000, 0xA00009E7

This instruction doesn’t execute any ALU operations, but it does signal the GPU to load data from the texture unit into accumulator r4. Whether or not the signal can occur in the same instruction as the write to TMU0_S is a bit unclear, and may need a bit of testing. For now, just to be safe it’s being done in the instruction after the S coordinate is written.

Also worth noting, data from the texture units is always returned as either packed 8-bit RGBA8888 or RG1616 and BA1616 values in r4, depending on the bits per channel. The special r4 unpack mode (pm=1) mentioned in the previous post exists to convert this packed texture data to normalized [0..1] data. Reading 64-bit pixel data requires two 32-bit reads to get all four channels. For general 1D buffer access, 32-bit data is always fetched.

; exporting read texture data to MRT0
; add pipe: Bitwise OR, TLB_COLOUR_ALL, acc r4, acc r4, cond: always
; mul op: No operation, mul cond: never
.word 0x159E7924, 0x10020BA7

After the signal, the data arrives in r4 and is immediately available for the next instruction to use. Here the sampled texture data in r4 is just directly copied to TLB_COLOUR_ALL for export.

Be careful not to overfill the texture receive FIFO, as it is only 8 entries deep, and each write to a TMU register constitutes one entry. For 2D textures you only write to S and T, and so you can queue up four texture requests. If you only need to write S (1D buffer case), you can queue up eight requests. Finally in the child image case where S, T, R, and B are all needed, you can only queue up two. If your shader runs in multithreaded mode and you suspend, both threads share the same FIFO and should only use half.

I Wanna See You Swizzle It Just A Little Bit

Finally we need to cover how texture data is laid out in memory. Texture reads support both linear (raster order) formats and microtile-based T and LT formats. Microtiles are 64 byte 2D blocks of pixels, whose geometry depends on the number of bits per pixel. In the common case of 32-bit pixels, a microtile would be a 4×4 block. In the 64-bit pixel case, microtiles are 2×4 blocks, and 1-bit pixels are laid out as 32×16 blocks, for example. The pixels are stored in simple raster scan order, left to right and bottom to top, with the origin in the lower left corner.

In T-format, microtiles are grouped into subtiles, where a subtile is a 1 KiB block of 16 microtiles, arranged 4×4 in simple raster scan order. In the 32bpp case, that’s 4×4 pixels per microtile times 4×4 microtiles per subtile, or 16×16 pixels per subtile. 256 pixels times 4 bytes per pixel is 1 KiB, as expected.

Next, four 1 KiB subtiles are grouped into one image tile (sometimes called a 4K tile). The ordering of subtiles within a 4K tile depends on the 4K tile row. Even rows (0, 2, 4,…) orders the four subtiles lower left, upper left, upper right, lower right. Odd rows of 4K tiles (1, 3, 5,…) orders them upper right, lower right, lower left, upper left.

4K tiles on even lines have subtiles wound clockwise from lower left
4K tiles on odd lines have subtiles wound clockwise from upper right

Finally, the 4K tiles themselves go left to right for even rows, and right to left for odd rows. Again, the origin is in the lower left corner. The image below shows a texture three 4K tiles wide and two 4K tiles high. The pattern repeats, alternating every row

This is the tiling order for T-format. Because the format is 4K tile based, the texture dimensions must always be padded out to a multiple of 4K tiles. This can be wasteful for smaller textures, so the hardware assumes any mip level less than a 4K tile in size will be stored in LT (or linear tiled) format. LT-format is also microtile based, but the image is stored as a series of microtiles in linear scan order, without any concept of subtiles and 4K tiles.

That’s about all I have time for.  I’ll have to save cache swizzling, mipmap layout, cubemaps, and hardware utilization for another post this fall.

Shoutouts as always to Peter Lemon for continuing to push low level and bare metal on RPI, Cort Danger Stratton for being recognizable even as a 32×32 texture, Graham Wihlidal (father of the never-gonna-be-released GrahamBox), and new kid in the console game Mike Nicølella who I can only assume is making the Miketendo Nii…


Shaders, Uniforms, and Varyings

Disclaimer: I do not work for Sony, despite the disturbing percentage of my shirts, jackets, and bookbags that are PlayStation dev-related. I do, however, have many friends that work at Sony, some of which I hope will call off the corporate lawyers. JayStation is in no way associated with Sony or PlayStation, and any stupid things I say represent only my own ineptitude and silliness.

The road to full GL pipeline support is fraught with peril. Quite a few things are involved, such as using a completely different pipeline, using VPM and VCD, and writing vert shaders that read attributes and write varyings and vertices in the right format. Luckily before jumping to full-on vert shaders, there is an intermediate step that allows us to try out some of the useful things like uniforms and varyings, all without leaving our familiar NV pipeline mode. If you haven’t read the previous post on how to initialize the GPU and set up basic binning and rendering command buffers, now is the time. You’re gonna need it.

Shader ISA

Before getting into how to set up uniforms and varyings, we have to go over the basics of writing shaders. This will mainly cover the QPU ISA and instruction encoding, the register file, and some basic rules and limitations. There are two register files: A and B. The first 32 registers in each regfile [r0a .. r31a] and [r0b .. r31b] are the physically backed registers, and locations above that (from r32a/b to r63a/b) are for register-space IO. There are also four general purpose accumulators and two special purpose accumulators, whose magic power is that unlike the physically backed registers, their value can be used immediately after being written.

Each register file is single ported, so instructions can’t read or write two different registers in the same file. The register map looks like this:

The address on the left is the register number, from 0 to 63, and the other columns tell you what each register means when reading and writing A and B. Sometimes A and B have the same meaning, for example reading from register r35a and r35b will both read a varying from a FIFO. Sometimes they differ, like r41a and r41b which give you the X and Y pixel coordinate respectively.

Instructions are all fixed size 64-bit, and fall into the following classes: ALU, ALU with small immediate, branch, 32-bit immediate loads, and semaphores. All of the shaders covered in this post use ALU and ALU with small immediate, so let’s focus on those two. The ALU instruction encoding looks like this

That is alot of fields, so let’s take it one by one. The ALU has two pipelines. The add pipeline handles add-like operations, integer shift, and bitwise operations, while the mul pipeline does multiplication-like things and operations on individual bytes in a word. Each ALU instruction can encode up to one add pipe operation via the op_add field, and one mul pipe operation via the op_mul field, to be executed together.

Where an instruction pulls its inputs from is determined by a multiplexer, which can select from register file A, register file B, or any of the six accumulators. The fields specifying the two input muxes for the add pipe are confusingly called add_a and add_b. Here the ‘a’ and ‘b’ suffixes refer to whether its the first or second input operand and have nothing to do with the A and B register files. Similarly, the input muxes for the mul pipe are given with mul_a and mul_b.

So for example if you wanted to add together a value in accumulator 5 with some register in the A file, and multiply together some register in the A file with accumulator 4, you might do something like this

op_add = ADD_PIPE_INST_FADD    ; add pipe instruction is a floating point add
add_a =  INPUT_MUX_ACC5        ; input 1 is operand is ACC5
add_b = INPUT_MUX_REGFILE_A    ; input 2 is *some* register in the A file
op_mul = ADD_PIPE_INST_FMUL    ; mul pipe instruction is a floating point mul
mul_a = INPUT_MUX_REGFILE_A    ; input 1 is operand is *some* register in the A file
mul_b = INPUT_MUX_ACC4         ; input 2 is operand is ACC4

Because the register file is single ported, an instruction can read from two different registers in the A file and B file, but not two different registers in the same file. This means that any input mux field set to INPUT_MUX_REG_FILE_A must necessarily refer to the same register, as will any mux field set to INPUT_MUX_REG_FILE_B. The specific register number is given with raddr_a (read address regfile A) and raddr_b (read address regfile B). In the above example we could set raddr_a = 17, then any mux using ALU_INPUT_MUX_REG_FILE_A would now mean register r17a.

There is a variation on ALU instructions that only allows you to get inputs from accumulators and register file A, but in exchange lets you can reuse the six bits in raddr_b to encode some commonly used literals. This is the “small immediate” form, and some allowed literals are [-16..15], [1.0, 2.0, 4.0, 8.0, … , 128.0 ], and [ 1.0/256.0, 1.0/128.0, 1.0/64.0, …, 1.0/2.0 ]. To use one of these literals as an input operand, the input mux should be set to ALU_INPUT_MUX_REG_FILE_B.

Outputs are a bit simpler. When the write select (ws) bit is cleared, the add pipe writes to a register in regfile A and the mul pipe to B. Setting ws to 1 reverses that so add writes to B and mul to A. The specific destination register numbers are specified with waddr_add and waddr_mul. Let’s say waddr_add = 7 and waddr_mul = 13. With the ws bit cleared, the add pipe will write to r7a and the mul pipe to r13b. Setting ws would make the add pipe write to r7b and the mul pipe to r13a. If an instruction writes out a value that is needed by the next instruction, then you need to use the accumulators, as this isn’t allowed with the physically backed registers.

The hardware has two unpack/pack modes, controlled by the pm bit. When pm is 0 the add ALU can unpack various 8 and 16-bit inputs to 32-bit types, as well as pack 32-bit outputs into 8 and 16-bit types. By setting the pm bit to 1, the mul ALU can saturate and pack 32-bit normalized floats to 8-bit, and insert the result in the R, G, B, A, or all channels of the destination word. However, when the pm bit is set unpack only does some limited conversions that are useful for things like textures, and only works with accumulator 4. Regardless of the value of pm, packing and unpacking is only supported for register file A, registers 0 through 15.

Finally, here are some miscellaneous bits you might find useful. Both add and mul pipe instructions can be conditionally executed based on the Z, N, and C flags, allowing some branchless coding. Whether or not an instruction updates the flags is controlled by the set flags (sf) bit.  Usually its the result of the add ALU that sets the flags, except if the add operation is a NOP or its condition code is set to never. In that case flags are updated based on the mul ALU result. There is also a 4-bit signal field used to signal certain conditions to the GPU. Some of the ones we’ll need for this post are software breakpoint, program end, small immediate, and scoreboard unlock.


That’s alot to take in so let’s look at some examples. Uniforms are 32-bit words that can be read from a shader, and whose value are uniform across all invocations of the shader (as opposed to varyings, which can vary across the triangle). All uniforms for a shader should be packed contiguously in a list, and the list address is then set in an NV Shader State record field.

.align 4 ; 128-Bit Align
     .byte 0                    ; Flag Bits: 0 = Fragment Shader Single Threaded
     .byte 3 * 4                ; Shaded Vertex Data Stride
     .byte 0                    ; Fragment Shader Num Uniforms (unused)
     .byte 0                    ; Fragment Shader Num Varyings
     .word FRAGMENT_SHADER_CODE ; Fragment Shader Code Address
     .word UNIFORM_DATA         ; Frag Shader Uniforms Addr, now non-zero
     .word VERTEX_DATA          ; Shaded Vertex Data Address

.align 4 ; RGBA as 4 floats
     .single 0.0               ; red
     .single 1.0               ; green
     .single 0.0               ; blue
     .single 1.0               ; alpha

This example is a bit contrived, as it could be done more efficiently with one uint uniform. However, doing it this way allows us to demonstrate mul ALU packing (pm=1).

To read uniforms from a shader, just specify the IO-space register UNIFORM_READ (r32a or r32b) as an input operand. Each successive read from UNIFORM_READ will get you the next uniform from the FIFO. If you want to reset the stream and re-read the uniforms, the uniform base pointer can be written from SIMD element 0. Lets look at an instruction to read a uniform and pack to a color channel.

; read in a uniform and pack to R
; add op: No operation, add cond: never
; mul pipe: fmul, dest: ACC5, src1: UNIFORM_READ, src2: float 1.f, cond: always
;    pack mode: 32->8a Convert mul float result to 8-bit color in range [0, 1.0] (PM1)
.word 0x20820DF7, 0xD14059E5

The instruction is fmul, the first input operand is the IO-space UNIFORM_READ register, and the second is the small immediate 1.0f. The pack mode saturates the mul ALU result, converts to a byte, and inserts the byte into the R channel. Let’s look at the bits

m_mul_b          7  (mul input 2 uses value from register file B, or small immediate)
m_mul_a          6  (mul input 1 uses value from register file A)
m_add_b          7  (add input 2 uses value from register file B, or small immediate)
m_add_a          6  (add input 1 uses value from register file A)
m_small_imm      32 (1.0f)
m_raddr_a        32 (regfile A uses register 32: uniform read)
m_op_add         0  (add ALU is NOP)
m_op_mul         1  (mul ALU is fmul)
m_waddr_mul      37 (mul ALU writes to accumulator 5)
m_waddr_add      39 (add ALU writes to NOP, no write)
m_ws             1  (write swap: add ALU writes to regfile B, mul ALU writes to regfile A)
m_sf             0  (don’t set flags)
m_cond_mul       1  (cond always)
m_cond_add       0  (cond never)
m_pack           4  (normalized float to u8, write only R channel)
m_pm             1  (use mul ALU packing)
m_unpack         0  (no unpacking of input operands)
m_1101           13 (small immediate signal)

Once R, G, B, and A are converted and packed in this way, the resulting color is written to TLB_COLOUR_ALL (r46a or r46b) to export the fragment color.


Varyings are slightly more involved. Like uniforms they are stored in lists of 32-bit words, but unlike uniforms they are specified per-vertex and get barycentrically interpolated across the triangle. This interpolation requires a bit of manual work in the shader.

Varyings are interpolated with an equation of the form (A*(x-x0)+B*(y-y0))*W+C, but the GPU is generous enough to calculate V=(A*(x-x0)+B*(y-y0)) in the hardware for us. Specifically the PSE sets up the varying coefficients and sends them to coefficients memory for each QPU slice’s VRI interpolator, but that’s just some fun trivia and not something you necessarily need to worry about right now. The frag shader is responsible for reading the partially interpolated value V from the VARYING_READ register (r35b), multiplying by W, and then adding C to get the final value. W is initialized for a particular pixel shader instance a few cycles before the shader instance launches, and arrives ready to read in r15a. Reading from VARYING_READ also automatically triggers a write of C to accumulator 5, which is available to read by the next instruction. Therefore the flow for a single varying will be

ACC0 = VARYING_READ * r15a  // ACC0 = V * W, trigger write of C to ACC5
ACC0 = ACC0 + ACC5          // V * W + C

Remember we can execute a mul and add ALU instruction together, so in the case of multiple varyings we can do the previous varying’s add and the next varying’s multiply together. Let’s take a look at how this is done

// ACC0 = R * W + C, ACC1 = G * W (r15a)
// add pipe: fadd, dest: ACC0, src1: acc r0, src2: acc r5, cond: always
// mul pipe: fmul, dest: ACC1, src1: r15a, src2: VARYING_READ, cond: always
// signal: no signal

m_mul_b          7  (mul input 2 uses value from register file B)
m_mul_a          6  (mul input 1 uses value from register file A)
m_add_b          5  (add input 2 uses accumulator ACC5)
m_add_a          0  (add input 1 uses accumulator ACC0)
m_raddr_b        35 (r35b is VARYING_READ)
m_raddr_a        15 (r15a is pre-setup with the W for varying interp)
m_op_add         1  (add ALU does fadd)
m_op_mul         1  (mul ALU does fmul)
m_waddr_mul      33 (mul ALU writes to r33b, is ACC1)
m_waddr_add      32 (add ALU writes to r32a, is ACC0)
m_ws             0  (ws off, add ALU writes regfile A, mul ALU writes regfile B)
m_sf             0  (don’t set result flags)
m_cond_mul       1  (cond always)
m_cond_add       1  (cond always)
m_pack           0  (no packing)
m_pm             1  (we aren’t packing or unpacking, so who cares what this bit is?)
m_unpack         0  (no unpacking)
m_signal         1  (no signal)

Of course this gives us an interpolated float. If you’re working with colors, you’d still need to convert and pack in the same way as the uniforms example. Also similar to uniforms, reading from VARYING_READ register internally increments so subsequent reads of the register will fetch the next varying in the FIFO.

The CPU-side setup for varyings isn’t much more complicated than uniforms. The varyings themselves are stored after each vertex’s data. In the NV Shader State Record, you must now specify the number of varyings. The stride field must also be changed to the total size of a vertex plus the per-vertex varyings.

.align 4 ; 128-Bit Align
     .byte 0                     ; Flag Bits: 0 = Single Threaded
     .byte 24                    ; Shaded Vert Stride. 2 * hword + 5 * word
     .byte 0                     ; Fragment Shader Num Uniforms (unused)
     .byte 3                     ; Fragment Shader Num Varyings
     .word FRAGMENT_SHADER_CODE  ; Fragment Shader Code Address
     .word 0                     ; Fragment Shader Uniforms Address
     .word VERTEX_DATA           ; Shaded Vertex Data Address

.align 4 ; 128-Bit Align
     ; Vertex: Top
     .hword 320 * 16             ; X In 12.4 Fixed Point
     .hword  32 * 16             ; Y In 12.4 Fixed Point
     .single 0e1.0               ; Z
     .single 0e1.0               ; 1 / W
     .single 1.0                 ; varying R
     .single 0.0                 ; varying G
     .single 0.0                 ; varying B
     ; Vertex: Right …
     ; Vertex: Left …

Finally, shaders, much like life, have some rules you need to follow if you don’t want to fail mysteriously. The full list can be gotten from the Videocore IV manual, but here are a few important ones that directly affect the examples in this blog post.

  • The last three instructions of any program (Thread End plus the following two delay-slot instructions) must not do varyings read, uniforms read or any kind of VPM, VDR, or VDW read or write.
  • The Thread End instruction must not write to either physical regfile A or B.
  • The Thread End instruction and the following two delay slot instructions must not write or read address 14 in either regfile A or B. The reason for this is kind of fun. The W value for the next pixel is set up in r14b during the current pixel’s two final delay slots. Then the LSB is flipped so it is accessed as r15b for the next pixel.
  • A scoreboard wait must not occur in the first two instructions of a fragment shader. This is either the explicit Wait for Scoreboard signal or an implicit wait with the first tile-buffer read or write instruction.
  • An instruction must not read from a location in physical regfile A or B that was written to by the previous instruction. To do this you must use the accumulators.
  • VPM cannot be accessed in Fragment shaders, because the FIFOs in the interface hardware are shared with the varyings interpolation system.
  • When a shader is started back-to-back with the preceding program, the interpolation can start as early as the Program End instruction of the previous program. For this reason, a fragment shader must finish reading varyings before issuing the Program End instruction. All of the set up varyings must be read before the shader completes.

With varyings and uniforms out of the way, you now have everything you need to do texturing.