Registers - ps_3_0

Pixel shaders depend on registers to get vertex data, to output pixel data to the rasterizer, to hold temporary results during calculations and to identify texture sampling stages. There are several types of registers, each with a unique functionality. This section contains reference information for the input and output registers implemented by pixel shader version 3_0.

Input Register Types

Where:

• r is read, w is write, u is used.
• partial(x, y, z, w) - If only a subset of channels are updated in the register, the remaining channels will default to specified values (x, y, z, w).
• aL only - These register banks can be addressed using only the aL register. If it is out of range then the hardware and the result is undefined.
• The number of readports is the number of different registers (for each register type) that can be read in a single instruction. For example, a readport of one for a constant register means that all of these are legal since they use at most one constant register in the single instruction:

  mad r0, c0, c0, v0 
  mad r0, c2, t1, v0 
  mad r0, c3, c3, c3 
  

  But the following is illegal because it uses more than 1 constant registers:

  mad r0, c1, c2, v0
  

• Defaults for unspecified channels of sampler register lookups exist, but values depend on texture format. It is usually 1111, except 000a for a8 format.

Output Register Types

Where:

• R is read, W is write, U is used.
• partial(x, y, z, w) - If only a subset of channels are updated in the register, the remaining channels will default to specified values (x, y, z, w).
• aL only - These register banks can be addressed using only the aL register. If it is out of range then the hardware and the result is undefined except for the floating point constant bank which should return (0,0,0,0).

Input Register

The input registers are now fully floating point and the texture registers have been consolidated with them into a single register type. The register declaration uses a semantic to describe what is contained in a particular input register(v#). No clamping is performed when the input registers are defined as colors (like texture coordinates).

Face Register

The face register is a new floating-point scalar register. In ps_3_0 only, the sign of this register is valid. If the value is less than zero, the primitive is a back face (the area is negative). The face register can be used inside a pixel shader to make decisions about which lighting technique (two-sided lighting for example). This register requires a declaration (example: dcl vFace), so undeclared usage will be flagged as an error. This register is undefined for lines and point primitives. The face register can only be used with the setp and if comp instructions.

Position Register

This register contains the current pixel position (x, y) in the corresponding channels. The (z, w) channels are undefined. This register needs to be declared (example: dcl vPos.xy).

When multisampling, the position will contain the pixel coordinates and not the sample coordinates (multisampling happens once per pixel). When the driver performs super sampling and the pixel shader is run multiple times per pixel, the position will will contain the resolved coordinates, that is, they will be normalized by the render-target bloat factor.