delayed control-transfer

SPARC, programming A technique used on the SPARC to reduce

the effect of pipeline breaks by executing the instruction

after a branch instruction (the "delay instruction" in the

"delay slot"). If there is no useful instruction which can be

placed in the delay slot then the "annul bit" on the control

transfer instruction can be set, preventing execution of the

delay instruction (unless the control transfer is conditional

and is taken).

Annulled branches are indicated in SPARC assembler language by

appending ",A" to the operation code. For example,

LOOP: ...

CMP %L0,10

BLE,A LOOP

ADD %L2, %L3, #L4

If the delay instruction is also a control transfer

instruction then it gets more complicated. Both control

transfer instructions are executed (but not the following

instruction) and, assuming they are both taken, control is

transferred briefly to the destination of the first and then

immediately to the destination of the second.

(2001-06-26)