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Computing (FOLDOC) dictionary
delayed control-transfer
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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
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.