Shader Linking

In order to pass data between the vertex and fragment shader, a sort of link-program seems to be copying from the output of the vertex shader into the TRAM (the RAM used as input to rasterization and shading).

This program is located at offset 0x300 and upwards, and seems to have 64-bit words. Here's an example:

0x00000008
0x0000fecd

This copies four floats from each vertex.

The first word seems to contain the location to copy from in the vertex shader, the latter word contains the location in TRAM.

The latter word contains the following for single varyings (for a simple dot product-shader):

datatype	value	VAR instruction	0	1	2	3
lowp float	00b7	00800000	0	0	0	4
mediump float	000f	00400000	0	0	0	2
lowp vec2	00b7	00800000	0	0	0	4
mediump vec2	00fe	00408000	0	0	2	2
lowp vec3	b7a6	00810000	0	0	4	4
mediump vec3	0dfe	00408100	0	2	2	2
lowp vec4	a6b7	00810000	0	0	4	4
mediump vec4	cdfe	00408102	2	2	2	2

Those 0, 1, 2, 3-values are the bits from n * 7 to n * 7 + 2 in the VAR instruction.

splitting 0xa6b7 into 4 fields of 4-bit each:

1010
0110
1011
0111

First word

Bits	Meaning
0..1	VEC4 select: 0 = full VEC4 1 = VEC4.z (VEC4.x = VEC4.z)
3..6	VPE output row number
9..14	TRAM row number
16	VEC4.x is constant across width
17	VEC4.x is constant across length
18..19	VEC4.x across point
20	VEC4.y is constant across width
21	VEC4.y is constant across length
22..23	VEC4.y across point
24	VEC4.z is constant across width
25	VEC4.z is constant across length
26..27	VEC4.z across point
28	VEC4.w is constant across width
29	VEC4.w is constant across length
30..31	VEC4.w across point

Latter word

Bits	Meaning
0..1	VEC4.x TRAM column number
2..3	VEC4.x TRAM write mask (column halve)
4..5	VEC4.y TRAM column number
6..7	VEC4.y TRAM write mask (column halve)
8..9	VEC4.z TRAM column number
10..11	VEC4.z TRAM write mask (column halve)
12..13	VEC4.w TRAM column number
14..15	VEC4.w TRAM write mask (column halve)
16	VEC4.x triangle interpolation disable
17	VEC4.y triangle interpolation disable
18	VEC4.z triangle interpolation disable
19	VEC4.w triangle interpolation disable

Disabling triangle interpolation picks the value from the third vertex in if drawing triangles. This is what we want for triangle strips and triangle fans, but not normal triangles. Which means we'll probably need to use the "constant across length"-flag in that case.

In addition to this, something is going on at offset 0xe20 and upwards:

0x58000000