GPU/Pitfalls

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This page collects some oddities and pitfalls of the PICA GPU which is used in the 3DS.

Internal Registers[edit]

Vertex attribute alignment[edit]

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[edit]

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[edit]

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[edit]

Configued Output components must be written exactly once[edit]

Each configured output component has to be written exactly once or the PICA freezes.

MOVA instructions can't be adjacent[edit]

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.