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Temporary Register (HLSL VS reference)

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A vertex shader temporary register is used to hold intermediate results.

A temporary register must be initialized before it is used. Each temporary register has single-write and triple-read access. This means that a single shader instruction can use as many as three temporary registers as inputs.

Values in a temporary register that remain from preceding invocations of the vertex shader cannot be used.

A register consists of properties that determine how each register behaves.

Property Description
Name r[n]. n is an optional register number. The default is 0, and is the value used if no value is specified.
Count A maximum of 12 registers.
I/O permissions Read/write. This register can be read or written by the API or by the shader.
Read ports The number of times a register can be read within a single instruction is 3. A temporary register is the only register that can be read and written more than once in a single instruction.

 

Each temporary register has single-write and triple-read access. Therefore, an instruction can have as many as three temporary registers in its set of input source operands.

No values in a temporary register that remain from preceding invocations of the vertex shader can be used. Vertex shaders that read a value from a temporary register before writing to it will fail the Direct3D API call to create the vertex shader.

Example

Here is an example using a temporary register:

def c4, 0,0,0,1
...
// Decompress position
mov r0.x, v0.x
mov r0.y, c4.w       // 1
mov r0.z, v0.y
mov r0.w, c4.w       // 1

// Compute theta from distance and time
mov r4.xz, r0        // xz
Vertex shader versions 1_1 2_0 2_sw 2_x 3_0 3_sw
Temporary Register x x x x x x

 

Vertex Shader Registers