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	Description
src:2	source select (0=VPE)
undefined_bit_2:1
vc_row:4	VPE row
undefined_bits_7_8:2
tram_row:6	TRAM row
undefined_bit_15:1
x_const_accross_width:1
x_const_accross_length:1
x_point:2
y_const_accross_width:1
y_const_accross_length:1
y_point:2
z_const_accross_width:1
z_const_accross_length:1
z_point:2
w_const_accross_width:1
w_const_accross_length:1
w_point:2

Latter word

Bits	Description
x_tram_col:2
x_tram_fmt:2
y_tram_col:2
y_tram_fmt:2
z_tram_col:2
z_tram_fmt:2
w_tram_col:2
w_tram_fmt:2
x_tri_shade_mode:1
y_tri_shade_mode:1
z_tri_shade_mode:1
w_tri_shade_mode:1
undefined_bits_20_31:12

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

0x58000000