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ATI R700 Technology

Conditional Execution 3-13

instructions that update the active mask. These instructions can be used to

permanently disable pixels that were recently activated. Valid pixel mode

normally is not used to exit whole quad mode; whole quad mode normally is

exited automatically when reaching the end of scope for the branch-loop

instruction that began in whole quad mode.

Instructions using the CF_DWORD[0,1] or the CF_ALLOC_EXPORT_DWORD[0,1]

microcode formats have VALID_PIXEL_MODE fields. ALU clause instructions

behave as if the VALID_PIXEL_MODE bit were cleared. Valid pixel mode is not the

default mode; normal programs that do not contain gradient operations clear the

VALID_PIXEL_MODE bit. The valid pixel mode is used only to deactivate pixels

invalidated by a KILL instruction and to temporarily inhibit the effects of whole

quad mode. Do not set both the WHOLE_QUAD_MODE bit and VALID_PIXEL_MODE bit.

Branch-loop instructions that pop from the stack interpret the valid pixel mode

differently. If the mode is set on an instruction that pops the stack, invalid pixels

are deactivated after the active mask is restored from the stack. This can make

the effect of the valid pixel mode permanent for a killed pixel that is executed

inside a conditional branch. By default, the per-pixel active state is overwritten

with the stack contents on each pop, without regard for the current active state;

however, when VALID_PIXEL_MODE is set, the invalid pixels are deactivated even

though they were active going into the conditional scope.

3.6.3 The Condition (COND) Field

Instructions that use the CF_DWORD[0,1] microcode formats have a COND field

that lets them be conditionally executed. The COND field can have one of the

following values:

• CF_COND_ACTIVE — Pixel currently active. Non-branch-loop instructions can

use only this setting.

• CF_COND_BOOL — Pixel currently active, and the boolean referenced by

CF_CONST is one.

• CF_COND_NOT_BOOL — Pixel currently active, and the boolean referenced by

CF_CONST is zero.

For most CF instructions, COND is used only to determine which pixels are

executing that particular instruction; the result of the test is discarded after the

instruction completes. Branch-loop instructions that manipulate the active state

can use the result of the test to update the new active mask; these cases are

described below. Non-branch-loop instructions can use only the CF_COND_ACTIVE

setting. Generally, branch-loop instructions that push pixel state onto the stack

push the original pixel state before beginning the instruction, and use the result

of COND to write the new active state. Some instructions that pop from the stack

can pop the stack first, then evaluate the condition code, and update the per-

pixel state based on the result of the pop and the condition code.

Instructions that do not have a COND field behave as if CF_COND_ACTIVE were

used. ALU clauses do not have a COND field; they execute pixels based on the

current active mask. ALU clauses can update the active mask using PRED_SET*

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Boxis R700 Manuel d'utilisateur Page 49

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