Rainbow Electronics AT91CAP9S250A - Manual (Page 64)

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6264A–CAP–21-May-07
AT91CAP9S500A/AT91CAP9S250A
The DCache contains an eight data word entry, single address entry write-back buffer used to
hold write-back data for cache line eviction or cleaning of dirty cache lines.
The Write Buffer can hold up to 16 words of data and four separate addresses. DCache and
Write Buffer operations are closely connected as their configuration is set in each section by the
page descriptor in the MMU translation table.
12.6.2.2 Write Buffer
The ARM926EJ-S contains a write buffer that has a 16-word data buffer and a four- address
buffer. The write buffer is used for all writes to a bufferable region, write-through region and
write-back region. It also allows to avoid stalling the processor when writes to external memory
are performed. When a store occurs, data is written to the write buffer at core speed (high
speed). The write buffer then completes the store to external memory at bus speed (typically
slower than the core speed). During this time, the ARM9EJ-S processor can preform other
tasks.
DCache and Write Buffer support write-back and write-through memory regions, controlled by C
and B bits in each section and page descriptor within the MMU translation tables.
Write-though Operation
When a cache write hit occurs, the DCache line is updated. The updated data is then written to
the write buffer which transfers it to external memory.
When a cache write miss occurs, a line, chosen by round robin or another algorithm, is stored in
the write buffer which transfers it to external memory.
Write-back Operation
When a cache write hit occurs, the cache line or half line is marked as dirty, meaning that its
contents are not up-to-date with those in the external memory.
When a cache write miss occurs, a line, chosen by round robin or another algorithm, is stored in
the write buffer which transfers it to external memory.
12.7 Tightly-Coupled Memory Interface
12.7.1 TCM Description
The ARM926EJ-S processor features a Tightly-Coupled Memory (TCM) interface, which
enables separate instruction and data TCMs (ITCM and DTCM) to be directly reached by the
processor. TCMs are used to store real-time and performance critical code, they also provide a
DMA support mechanism. Unlike AHB accesses to external memories, accesses to TCMs are
fast and deterministic and do not incur bus penalties.
The user has the possibility to independently configure each TCM size with values within the fol-
lowing ranges, [0KB, 0 KB] for ITCM size and [0KB, 0 KB] for DTCM size.
TCMs can be configured by two means: HMATRIX TCM register and TCM region register (regis-
ter 9) in CP15 and both steps should be performed. HMATRIX TCM register sets TCM size
whereas TCM region register (register 9) in CP15 maps TCMs and enables them.
The data side of the ARM9EJ-S core is able to access the ITCM. This is necessary to enable
code to be loaded into the ITCM, for SWI and emulated instruction handlers, and for accesses to
PC-relative literal pools.