View source for GPU/Pitfalls

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

[[Category:GPU]]

== Internal Registers ==

=== Vertex attribute alignment ===

Vertex components which are defined through [[GPU/Internal_Registers#GPUREG_ATTRIBBUFFERi_CONFIG1|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 [[GPU/Internal_Registers#GPUREG_ATTRIBBUFFERi_CONFIG2|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 [[GPU/Internal_Registers#GPUREG_SH_OUTMAP_MASK|GPUREG_SH_OUTMAP_MASK]] influences how the registers starting at [[GPU/Internal_Registers#GPUREG_SH_OUTMAP_Oi|GPUREG_SH_OUTMAP_Oi]] map to outputs in the shader.

If an output is disabled in [[GPU/Internal_Registers#GPUREG_SH_OUTMAP_MASK|GPUREG_SH_OUTMAP_MASK]] it means that no slot in the [[GPU/Internal_Registers#GPUREG_SH_OUTMAP_Oi|GPUREG_SH_OUTMAP_Oi]] registers is consumed.
[[GPU/Internal_Registers#GPUREG_SH_OUTMAP_TOTAL|GPUREG_SH_OUTMAP_TOTAL]] configures the number of used consecutive slots in the outmap.

Example:

{| class="wikitable" border="1"
! 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.