161
Chapter 6
Instruction Cache
Preliminary User’s Manual U15839EE1V0UM00
(3)
Operation on Instruction Cache Miss
(1)
On a fetch access from memory, the CPU outputs the instruction fetch request and the
concerned address to the instruction cache.
(2)
If an instruction cache miss occurs due to the address not existing in the instruction cache,
the fetch request and the address will be output from the instruction cache to the BCU.
(3)
The BCU then outputs the address to external memory via the VSB and refills the
instruction cache with one line (4 words) at the address to be read.
(4)
The instruction cache then transfers the data to be read among the 4 words of refill data to
the CPU.
Caution:
The miss penalty time when a miss occurs varies depending on such things as mem-
ory controller specifications for external memory and VSB bus cycle wait insertion
time.
Figure 6-7:
Operation on Instruction Cache Miss
Instruction Cache
Instruction
Cache Interface
BCU
CPU
NB85E
VSB
MEMC
External Memory
(4)
(1)
(3)
(2)
(3)
(3)
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