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 aligned.

• 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.

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 mapping in GPUREG_SH_OUTMAP_MASK configures how the registers starting at GPUREG_SH_OUTMAP_Oi will map the 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

Write output components 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.