Boxis R700 User Manual download pdf (Page 47)

ATI R700 Technology

Conditional Execution 3-11

These instructions use the CF_DWORD[0,1] microcode formats and can be

executed only by a GS program; they are invalid in other programs.

3.6 Conditional Execution

The remaining CF instructions include conditional execution and manipulation of

the branch-loop states. The following subsections describes how conditional

executions operate and describe the specific instructions.

3.6.1 Valid and Active Masks

Every element in the three bits that specify its state associated can be

manipulated by a program.

• a one-bit valid mask and a 2-bit per-pixel state. The valid mask is set for any

pixel that is covered by the original primitive and has not been killed by an

ALU KILL operation.

• a two-bit per-pixel state that reflects the pixel’s active status as conditional

instructions are executed; it can take on the following states:

– Active: The pixel is currently executing.

– Inactive-branch: The pixel is inactive due to a branch (ALU PRED_SET*)

instruction.

– Inactive-continue: The pixel is inactive due to a ALU_CONTINUE instruction

inside a loop.

– Inactive-break: The pixel is inactive due to a ALU_BREAK instruction inside

a loop.

Once the valid mask is cleared, it can not be restored. The per-pixel state can

change during the lifetime of the program in response to conditional-execution

instructions. Pixels that are invalid at the beginning of the program are put in one

of the inactive states and do not normally execute (but they can be explicitly

enabled, see below). Pixels that are killed during the program maintain their

current active state (but they can be explicitly disabled, see below).

Branch-loop instructions can push the current pixel state onto the stack. This

information is used to restore the pixel state when leaving a loop or conditional

instruction block. CF instructions allow conditional execution in one of the

following ways:

• Perform a condition test for each pixel based on current processor state:

– The condition test determines which pixels execute the current

instruction, and per-pixel state is unmodified, or

– The per-pixel state is modified; pixels that pass the condition test are put

into the active state, and pixels that fail the condition test are put into one

of the inactive states, or

– If at least one pixel passes, push the current per-pixel state onto the

stack, then modify the per-pixel state based on the results of the test. If

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Boxis R700 User Manual Page 47