RM5231A-350-H PMC-Sierra Inc, RM5231A-350-H Datasheet - Page 22

• Manufacturer Part Number: RM5231A-350-H
• Description: RM5231A Microprocessor with 32-Bit System Bus Data Sheet Preliminary
• Manufacturer: PMC-Sierra Inc

Proprietary and Confidential to PMC-Sierra, Inc and for its Customer’s Internal Use
Document ID: PMC-2002174, Issue 2
3.20 Write Buffer
4. Write-through without write allocate
The most commonly used write policy is write-back, where a store to a cache line does not
immediately cause main memory to be updated. This increases system performance by reducing
bus traffic and eliminating the bottleneck of waiting for each store operation to finish before
issuing a subsequent memory operation. Software can, however, select write-through on a per-
page basis when appropriate, such as for frame buffers.
Associated with the data cache is the store buffer. When the RM5231A executes a store
instruction, this single-entry buffer gets written with the store data while the tag comparison is
performed. If the tag matches, then the data is written into the data cache in the next cycle that the
data cache is not accessed (the next non-load cycle). The store buffer allows the RM5231A to
execute a store every processor cycle and to perform back-to-back stores without penalty. In the
event of a store immediately followed by a load to the same address, a combined merge and cache
write occurs such that no penalty is incurred.
The RM5231A cache attributes for both the instruction and data caches are summarized in Table 3.
Table 3 Cache Attributes
Writes to external memory, whether cache miss write-backs or stores to uncached or write-through
addresses, use the on-chip write buffer. The write buffer holds up to four 64-bit address and data
pairs. The entire buffer is used for a data cache write-back and allows the processor to proceed in
parallel with the memory update. For uncached and write-through stores, the write buffer
Characteristics
Size
Organization
Line size
Index
Tag
Write policy
Read order
write order
miss restart after transfer of
Parity
Cache locking
memory is written, leaving the write-back bit of the cache line unchanged. If the cache lookup
misses, the target line is first brought into the cache and then the write is performed as above.
Loads and instruction fetches first search the cache, reading main memory only if the desired
data is not cache resident. On data store operations, the cache is first searched to determine if
the target address is cache resident. If it is resident, the cache contents are updated and main
memory is written, leaving the write-back bit of the cache line unchanged. If the cache lookup
misses, then only main memory is written.
Instruction
32KB
32B
vAddr
pAddr
n.a.
sub-block
sequential
entire line
per-word
set A
2-way set associative
RM5231A™ Microprocessor with 32-Bit System Bus Data Sheet
11..0
31..12
Data
32KB
2-way set associative
32B
vAddr
pAddr
write-back/write-through
sub-block
sequential
first double
per-byte
set A
11..0
31..12
Preliminary
21