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ATI R700 Technology

3-18 Branch and Loop Instructions

it performs POP_COUNT number of pops (may be zero), then jumps to a

specified address if all pixels fail the test.

• POP - pops per-pixel state from the stack to hardware-maintained registers; it

pops the POP_COUNT number of entries (can be zero). POP can apply the

condition test to the result of the POP, this is useful for disabling pixels that

are killed within a conditional block. To disable such pixels, set the POP

instruction’s VALID_PIXEL_MODE bit, and set the condition to

CF_COND_ACTIVE. If POP_COUNT is zero, the POP instruction simply modifies

the current per-pixel state based on the result of the condition test. Pop

instructions never jump.

• ELSE - performs a conceptual else operation. It starts by popping

POP_COUNT entries (can be zero) from the stack. Then, it inverts the sense

of active and branch-inactive pixels for pixels that are both active (as of the

last surviving PUSH operation) and pass the condition test. The ELSE

operation will then jump to the specified address if all pixels are inactive.

• JUMP - is used to jump over blocks of code that no pixel wants to execute.

JUMP first pops POP_COUNT entries (may be zero) from the stack. It then

applies the condition test to all pixels. If all pixels fail the test, it jumps to the

specified address; otherwise, it continues execution on the next instruction.

3.7.3 DirectX9 Loops

DirectX9-style loops are implemented with the LOOP_START and LOOP_END

instructions. Both instructions specify the DirectX9 integer constant using the

CF_CONST microcode field. This field specifies the integer constant to use for the

loop’s trip count (maximum number of loops), beginning value (loop index

initializer), and increment (step). The constant is a host-written vector, and the

three loop parameters are stored as three elements of the vector. The COND field

also can refer to the CF_CONST field for its boolean value. It is not be possible to

conditionally enter a loop based on a boolean constant unless the boolean

constant and integer constant have the same numerical address.

The LOOP_START instruction jumps to the address specified in the instruction’s

ADDR field if the initial loop count is zero. Software normally sets the ADDR field to

the CF instruction following the matching LOOP_END instruction. If LOOP_START

does not jump, hardware sets up the internal loop state. Loop-index-relative

addressing (as specified by the INDEX_MODE field of the ALU_DWORD0 microcode

format) is well-defined only within the loop. If multiple loops are nested, relative

addressing refers to the loop register of the innermost loop. The loop register of

the next-outer loop is automatically restored when the innermost loop exits.

The LOOP_END instruction jumps to the address specified in the instruction’s ADDR

field if the loop count is nonzero after it is decremented, and at least one pixel

has not been deactivated by a LOOP_BREAK instruction. Normally, software sets

the ADDR field to the CF instruction following the matching LOOP_START. The

LOOP_END instruction continues to the next CF instruction when the processor

exits the loop.

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Boxis R700 Manuel d'utilisateur Page 54

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