Boxis R700 User Manual download pdf (Page 19)

ATI R700 Technology

ATI R700-Family Instruction Set Architecture 2-1

Chapter 2

Program Organization and State

R700 programs consist of control-flow (CF), ALU, texture-fetch, and vertex-fetch

instructions, which are described in this manual. ALU instructions can have up

to three source operands and one destination operand. The instructions operate

on 32-bit or 64-bit IEEE floating-point values and signed or unsigned integers.

The execution of some instructions cause predicate bits to be written that affect

subsequent instructions. Graphics programs typically use vertex-fetch and

texture-fetch instructions for data loads, whereas general-computing applications

typically use texture-fetch instructions for data loads.

2.1 Program Types

The following program types are commonly run on the R700 (see Figure 1.2, on

page 1-3):

• Vertex Shader (VS)—Reads vertices, processes them. Depending on

whether a geometry shader (GS) is active, it outputs the results to either a

VS ring buffer, or the parameter cache and position buffer. It does not

introduce new primitives. A vertex shader can invoke a Fetch Subroutine

(FS), which is a special global program for fetching vertex data that is

treated, for execution purposes, as part of the vertex program. The FS

provides driver independence between the process of fetching data required

by a VS, and the VS itself.

• Geometry Shader (GS)—Reads primitives from the VS ring buffer, and, for

each input primitive, writes one or more primitives as output to the GS ring

buffer. This program type is optional; when active, it requires a DMA copy

(DC) program to be active. The GS simultaneously reads up to six vertices

from an off-chip memory buffer created by the VS; it outputs a variable

number of primitives to a second memory buffer.

• DMA Copy (DC)—Transfers data from the GS ring buffer into the parameter

cache and position buffer. It is required for systems running a geometry

shader.

• Pixel Shader (PS) or Fragment Shader—This type of program:

– receives pixel data from the rasterizer to be shaded.

– processes sets of pixel quads (four pixel-data elements arranged in a 2-

by-2 array), and

– writes output to up to eight local-memory buffers, called multiple render

targets (MRTs), which can include one or more frame buffers.

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Boxis R700 User Manual Page 19