Pick

Pick records whether drawing passed visibility tests. spr Pick writes PickValid at bit 16 and PickHit at bit 17. The current bits are readable in Flag. Both are unspecified after reset.

PickHit becomes set when a pixel passes scissor, pattern, Z and alpha tests. This happens before PMask, so a zero plane mask does not prevent a visibility hit.

The specification's sequence sets PickValid, draws the test primitives, waits for PixelBusy to clear, reads PickHit, then clears PickValid. Clear PickHit when starting a new test. Read the final result after the drawing engines are idle.

spr Pick, if1_0       ; PickValid = 1, PickHit = 0
... draw test primitives ...
... wait until Flag.PixelBusy = 0 ...
... read Flag.PickHit ...
spr Pick, zero

The notation above follows the operation sequence; it is not a complete dispatch-loop routine.

Source: Technical Spec, p.36