GPU/Pitfalls
This page collects some oddities and pitfalls of the PICA GPU which is used in the 3DS.
Internal Registers
Vertex attribute alignment
Vertex components which are defined through GPUREG_ATTRIBBUFFERi_CONFIG1 will be accessed aligned by the GPU.
- Vertex attributes will be aligned to their component element size.
- Padding attributes (Component type > 11) will always aligned to 4 byte offets into the buffer.
- The stride which is passed to the GPU should be passed unaligned.
Vertex stride in GPUREG_ATTRIBBUFFERi_CONFIG2
The vertex stride set in GPUREG_ATTRIBBUFFERi_CONFIG2 must match the actual size of the vertex contained in the buffer or the PICA will freeze or it won't draw anything.
If you want to use a different stride you have to pad the data accordingly with padding attributes.
Output mapping in GPUREG_SH_OUTMAP_MASK
The output masking in GPUREG_SH_OUTMAP_MASK influences how the registers starting at GPUREG_SH_OUTMAP_Oi map to outputs in the shader.
If an output is disabled in GPUREG_SH_OUTMAP_MASK it means that no slot in the GPUREG_SH_OUTMAP_Oi registers is consumed. GPUREG_SH_OUTMAP_TOTAL configures the number of used consecutive slots in the outmap.
Example:
| Register | Value | Meaning |
|---|---|---|
| GPUREG_SH_OUTMAP_TOTAL | 0x00000002 | 2 outputs enabled |
| GPUREG_SH_OUTMAP_MASK | 0x00000011 | o0 enabled, o4 enabled |
| GPUREG_SH_OUTMAP_O0 | 0x03020100 | o0 = pos.xyzw |
| GPUREG_SH_OUTMAP_O1 | 0x0B0A0908 | o4 = color.rgba |
| GPUREG_SH_OUTMAP_O2 | ... | (unused) |
Shaders
Configued Output components must be written exactly once
Each configured output component has to be written exactly once or the PICA freezes.
MOVA instructions can't be adjacent
Having 2 consecutive MOVA instructions will freeze the PICA. This can be relaxed by placing a NOP between 2 MOVAs or by rearranging the code.