Boxis R700 User Manual download pdf (Page 37)

ATI R700 Technology

ATI R700-Family Instruction Set Architecture 3-1

Chapter 3

Control Flow (CF) Programs

A control flow (CF) program is a main program. It directs the flow of program

clauses by using control-flow instructions (conditional jumps, loops, and

subroutines), and it can include memory-allocation instructions and other

instructions that specify when vertex and geometry programs have completed

their operations. The R700 hardware maintains a single, multi-entry stack for

saving and restoring active masks, loop counters, and returning addresses for

subroutines.

CF instructions can:

• Execute an ALU, texture-fetch, or vertex-fetch clause, local data share

clause, or memory read clause. These operations take the address of the

clause to execute, and a count indicating the size of the clause. A program

can specify that a clause must wait until previously executed clauses

complete, or that a clause must execute conditionally (only active pixels

execute the clause, and the clause is skipped entirely if no pixels are active).

• Execute a DirectX9-style loop. There are two instructions marking the

beginning and end of the loop. Each instruction takes the address of its

paired LOOP_START and LOOP_END instructions. A loop reads from one of 32

constants to get the loop count, initial index value, and index increment

value. Loops can be nested.

• Execute a DirectX10-style loop. There are two instructions marking the

beginning and end of the loop. Each instruction takes an address of its paired

LOOP_START and LOOP_END instructions. Loops can be nested.

• Execute a repeat loop (one that does not maintain a loop index). Repeat

loops are implemented with the LOOP_START_NO_AL and LOOP_END

instructions. These loops can be nested.

• Break out of the innermost loop. LOOP_BREAK instructions take an address to

the corresponding LOOP_END instruction. LOOP_BREAK instructions can be

conditional (executing only for pixels that satisfy a break condition).

• Continue a loop, starting with the next iteration of the innermost loop.

LOOP_CONTINUE instructions take an address to the corresponding LOOP_END

instruction. LOOP_CONTINUE instructions can be conditional.

• Execute a subroutine CALL or RETURN. A CALL takes a jump address. A

RETURN never takes an address; it returns to the address at the top of the

stack. Calls can be conditional (only pixels satisfying a condition perform the

instruction). Calls can be nested.

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Boxis R700 User Manual Page 37