NV2A/Vertex Shader
The Xbox implements the 2 GL extensions NV_vertex_program and NV_vertex_program1_1 (with some modifications). This article will mainly focus on actual encoding on hardware as the behaviour is mostly outlined in those GL extensions already.
Contents
Operating modes
- Fixed / Programmable
- Writeable / Read-Only constants
- Low-constants only / all constants
- Vertex processing / State program
Registers
Input registers
There are 16 input registers v[0] to v[15].
They normally map to the vertex attributes. However, in the case of vertex state programs, v[0] is fed from LAUNCH_DATA (PGRAPH Methods 0x1E80, 0x1E84, 0x1E88, 0x1E8C for XYZW respectively) instead.
Output registers
11 output registers o[<RegName>] (initialized to XYZ=0x00000000 W=0x3F800000).
| Index | GL Name | D3D Name | Meaning |
| 0 | HPOS | oPos | Homogeneous clip space position |
| 3 | oD0 | COL0 | Primary color (front-facing) |
| 4 | oD1 | COL1 | Secondary color (front-facing) |
| 5 | FOGC | oFog | Fog coordinate |
| 6 | PSIZ | oPts | Point size |
| 7 | BFC0 | oB0 | Back-facing primary color |
| 8 | BFC1 | oB1 | Back-facing secondary color |
| 9 | TEX0 | oT0 | Texture coordinate set 0 |
| 10 | TEX1 | oT1 | Texture coordinate set 1 |
| 11 | TEX2 | oT2 | Texture coordinate set 2 |
| 12 | TEX3 | oT3 | Texture coordinate set 3 |
Address register
A0.x exists as documented in the GL extension.
Temporary registers
There are 12 temporary registers: R0 to R11 (initialized to XYZW=0x00000000), as documented in the GL extension. Additionally, o[POS] is mirrored as R12 and can be used as source operand; so effectively you have 13 temporaries
Constant space
There are 192 constant registers in two seperate blocks with 96 constants each. They can be accessed through the PGRAPH RDI: select=0x17. Each constant slot is 4x DWORD, ordered as WZYX. Alternatively they can be uploaded through PGRAPH method [FIXME], with 4x DWORD, ordered XYZW.
In nvidia vertex programs only 96 constants are normally accessible. Microsoft exposed the 96 additional constant registers in D3D shaders through c[-96] to c[-1]. This documentation uses the GL terminology instead and expose the new registers as c[96] to c[191]. This means c[0] to c[191] valid.
Instructions
In total, there are 136 instruction slots.
Each slot consists of 16 bytes, we consider those as 4 seperate little-endian DWORDS describing the operation. Word 0 is inused.
| Meaning | Word | Offset (bits) | Size (bits) |
| ILU Operation | 1 | 25 | 3 |
| MAC Operation | 1 | 21 | 4 |
| Constant index | 1 | 13 | 8 |
| Input index | 1 | 9 | 4 |
| Source 1 negate | 1 | 8 | 1 |
| Source 1 swizzle X | 1 | 6 | 2 |
| Source 1 swizzle Y | 1 | 4 | 2 |
| Source 1 swizzle Z | 1 | 2 | 2 |
| Source 1 swizzle W | 1 | 0 | 2 |
| Source 1 register | 2 | 28 | 4 |
| Source 1 mux | 2 | 26 | 2 |
| Source 2 negate | 2 | 25 | 1 |
| Source 2 swizzle X | 2 | 23 | 2 |
| Source 2 swizzle Y | 2 | 21 | 2 |
| Source 2 swizzle Z | 2 | 19 | 2 |
| Source 2 swizzle W | 2 | 17 | 2 |
| Source 2 register | 2 | 13 | 4 |
| Source 2 mux | 2 | 11 | 2 |
| Source 3 negate | 2 | 10 | 1 |
| Source 3 swizzle X | 2 | 8 | 2 |
| Source 3 swizzle Y | 2 | 6 | 2 |
| Source 3 swizzle Z | 2 | 4 | 2 |
| Source 3 swizzle W | 2 | 2 | 2 |
| Source 3 register (Hi) | 2 | 0 | 2 |
| Source 3 register (Lo) | 3 | 30 | 2 |
| Source 3 mux | 3 | 28 | 2 |
| Destination MAC mask | 3 | 24 | 4 |
| Destination temporary register | 3 | 20 | 4 |
| Destination ILU mask | 3 | 16 | 4 |
| Destination overall mask | 3 | 12 | 4 |
| Destination select | 3 | 11 | 1 |
| Destination output register | 3 | 3 | 8 |
| Destination mux | 3 | 2 | 1 |
| Relative constant addressing | 3 | 1 | 1 |
| Final instruction marker (EOF) | 3 | 0 | 1 |
| Value | Meaning |
| 0 | X |
| 1 | Y |
| 2 | Z |
| 3 | W |
