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Code Optimization and Instruction Timings C-7
C.2.8 Loads and Store Misses
Cachable stores that miss in the DCU, and noncachable stores, are queued internally in the DCU so
that the store instruction appears to execute in one cycle. Under certain conditions, the DCU can
pipeline up to three store instructions. (See the Chapter 4, “Cache Operations,” for more information.)
Because the PPC405 can execute instructions that follow load misses if no load-use dependency
exists, the load and the “using” instruction should be separated by “non-using” instructions whenever
possible. The number of load miss penalty cycles incurred by a load that misses in the DCU or DCU
line fill buffer is reduced by one cycle for every non-use instruction following the load. When the
number of non-use instructions following the load is equal to or greater than the number of cycles that
it takes to obtain the load data, the load instruction appears to execute in a single cycle. The number
of cycles that it takes to obtain load data when it misses in the data cache and line fill buffer depends
on whether operand forwarding is enabled or disabled and the system memory timing.
C.2.9 Instruction Cache Misses
Refer to “Instruction Processing” on page 2-23 for detailed information about the instruction queue
and instruction fetching. Table C-3 illustrates instruction cache penalties for cachable and
noncachable fetches that miss in the ICU array and line fill buffer.
Table C-3 assumes that:
• The PPC405 and processor local bus (PLB) run at the same frequency
• The PLB returns an address acknowledge during the first cycle in which the DCU asserts the PLB
request
• The target instruction is returned in the cycle following the address acknowledge cycle
The penalty cycles shown for sequential ICU requests assume that the DCD stage and pre-fetch
queue are filled with single-cycle nonbranching instructions or BKNT branch instructions. The penalty
cycles for the remaining two rows are for taken branches from DCD and PFB0, respectively.
Table C-3. Instruction Cache Miss Penalties
Type of ICU Request Miss Penalty Cycles
Sequential
3
Branch Taken from DCD
5
Branch Taken from PFB0
4