3 cache and write buffer, 3 cache and write buffer -5 – Cirrus Logic EP93xx User Manual

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DS785UM1

2-5

ARM920T Core and Advanced High-Speed Bus (AHB)

EP93xx User’s Guide

2.2.3.2.3 MMU Enable

Enabling the MMU allows system memory control, but is also required if the Data Cache and
the Write Buffer are to be used. Features are enabled for specific memory regions, as defined
in the system page table. MMU enablement is done via CP15 register 1. The procedure is as
follows:

1. Program the Translation Table Base (TTB) and domain access control registers

2. Create level 1 and level 2 pages for the system, and enable the Data Cache and the

Write Buffer

3. Enable the MMU via bit 0 of CP15 register 1.

2.2.3.3 Cache and Write Buffer

Cache configuration is 64-way set associative. There is a 16 kbyte instruction cache and a 16
kbyte data cache. The caches have the following characteristics:

•

8 words per line, with 1 valid bit and 2 dirty bits per line to allow half-line write-backs

•

Write-through or write-back capability, selectable per memory region defined by the

MMU

•

Pseudo random or round robin replacement algorithms for cache misses. This is

determined by the RR bit (bit 14) in CP15 register 1. On a cache miss (instruction or data
not in the respective cache), an 8-word line is fetched from memory and loaded into the
cache

•

Independent cache lock-down with granularity of 1/64th of total cache size or 256 bytes

for both instructions and data. Lock-down of the cache will prevent an eight-word cache
line fill into that region of the cache

•

For compatibility with Windows CE and to reduce latency, physical addresses for data

cache entries are stored in the PA TAG RAM, which is used for cache line write-back
operations without need of the MMU. This prevents a possible TLB miss that would
degrade performance

•

The Write Buffer has a depth of 16 data words. If enabled, writes are sent to the Write

Buffer directly from the Data Cache or from the CPU (in the event of a cache miss or if
the cache is not enabled).

2.2.3.3.1 Instruction Cache Enable

•

At reset, the Instruction Cache is disabled

•

A write to bit 12 of CP15 register 1 will enable or disable the Instruction Cache. If the

Instruction Cache (I-Cache) is enabled without the MMU enabled, all accesses are
treated as cacheable

•

If the I-Cache is disabled, current contents are ignored. If re-enabled before a reset,

contents will be unchanged, but may not be coherent with eternal memory. If so,
contents must be flushed before re-enabling.