
MOTOROLA
MPC801 USER’S MANUAL
9-1
9
SECTION 9
INSTRUCTION CACHE
The MPC801 instruction cache is a 2K two-way, set-associative cache. It is organized into
64 sets at two lines per set and four words per line. Cache lines are aligned on 4-word
boundaries in memory. The cache access cycle begins with an instruction request from the
instruction unit in the core. If a cache hit occurs, the instruction is delivered to the instruction
unit and if a cache miss occurs, the cache initiates a burst read cycle on the internal bus with
the address of the requested instruction. The first word received from the bus is considered
the requested instruction. The cache forwards this instruction to the instruction unit of the
core as soon as it is received from the internal bus. A cache line is then selected to receive
the data that will be coming from the bus. A least recently used (LRU) replacement algorithm
is used to select a line when no empty lines are available.
Each cache line can be used as an SRAM, thus allowing the application to lock critical code
segments that need fast and deterministic execution time. Instruction cache coherency in a
multiprocessor environment is maintained by the software and supported by a fast hardware
invalidation capability. Figure 9-1 illustrates a block diagram view of the cache organization
and Figure 9-2 a view of the cache’s data path.
9.1 FEATURES
The following is a list of the instruction cache’s main features:
2K Two-Way, Set-Associative at Four Words Per Line
Adheres to the LRU Replacement Policy
Parked on the Internal Bus
Lockable SRAM
“Critical word first”, Burst Access
Responds to Stream Hit, Which Allows Fetching from the Burst Buffer and of the Word
Currently on the Internal Bus.
Serves the Core Request in Parallel to Bringing the Tail of the Previously Missed Line