Instruction Latencies; Instruction Class Latency In Cycles - Compaq 21264 Hardware Reference Manual

Instruction Issue Rules

2.3.3 Instruction Latencies

After an instruction is placed in the IQ or FQ, its issue point is determined by the avail-
ability of its register operands, functional unit(s), and relationship to other instructions
in the queue. There are register producer-consumer dependencies and dynamic func-
tional unit availability dependencies that affect instruction issue. The mapper removes
register producer-producer dependencies.
The latency to produce a register result is generally fixed. The one exception is for load
instructions that miss the Dcache. Table 2–4 lists the latency, in cycles, for each
instruction class.
Table 2–4 Instruction Class Latency in Cycles
Class Latency Comments
ild 3 Dcache hit.
13+ Dcache miss, latency with 6-cycle Bcache. Add additional Bcache loop latency if
Bcache latency is greater than 6 cycles.
fld 4 Dcache hit.
14+ Dcache miss, latency with 6-cycle Bcache. Add additional Bcache loop latency if
Bcache latency is greater than 6 cycles.
ist — Does not produce register value.
fst — Does not produce register value.
rpcc 1 Possible 1-cycle cross-cluster delay.
rx 1 —
mxpr 1 or 3 HW_MFPR:
HW_MTPR does not produce a register value.
icbr — Conditional branch. Does not produce register value.
ubr 3 Unconditional branch. Does not produce register value.
jsr 3 —
iadd 1 Possible 1-cycle Ebox cross-cluster delay.
ilog 1 Possible 1-cycle Ebox cross-cluster delay.
ishf 1 Possible 1-cycle Ebox cross-cluster delay.
cmov1 1 Only consumer is cmov2. Possible 1-cycle Ebox cross-cluster delay.
cmov2 1 Possible 1-cycle Ebox cross-cluster delay.
imul 7 Possible 1-cycle Ebox cross-cluster delay.
imisc 3 Possible 1-cycle Ebox cross-cluster delay.
fcbr — Does not produce register value.
fadd 4 Consumer other than fst or ftoi.
6 Consumer fst or ftoi.
Measured from when an fadd is issued from the FQ to when an fst or ftoi is issued
from the IQ.
Internal Architecture
2–20
Ebox IPRs = 1.
Ibox and Mbox IPRs = 3.
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