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Revision 1.0
Execution Pipeline
43
Mary Jo’s Rules
1
Avoiding pipeline stalls in software can be accomplished by understanding
the following rules.
1. VU register destination writes 4 cycles later (need 3 cycles between
load and use). This applies to vector computational instructions,
vector loads, and coprocessor 2 moves (
mtc2
).
2. SU register load takes 3 cycles (need 2 cycles between load and
use). This applies to SU loads and coprocessor moves (
mfc0,
cfc2
,
mfc2
). SU computational results are available in the next
cycle (see “SU is Bypassed” on page 44).
3.
Any
load followed by
any
store 2 cycles later, causes a one cycle
bubble. Coprocessor moves (
mtc0, mfc0, mtc2, mfc2,
ctc2, cfc2
) count as both loads
and
stores.
4. A branch target not 64-bit aligned always single issues.
5. Branches:
a. Can dual issue (with preceding instruction).
b. No branch instruction permitted in a delay slot.
c.
Delay slot always single issues.
d. Taken branch causes a 1 cycle bubble.
Register Hazards
The RSP hardware implements register hazard locking for SU and VU
registers. Once an instruction is fetched and decoded, its destination register
is marked as a “hazard”; if this register is used as an input to a subsequent
instruction, the pipeline will stall.
1
Named after Mary Jo Doherty, the designer of the RSP.
Содержание Ultra64
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