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.