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66 SA-1110 Developer’s Manual
Memory Management Unit (MMU)
7.3.1 Cacheable Reads (Linefetches)
A linefetch can be safely aborted on any word in the transfer. If an abort occurs during the
linefetch, the cache is purged so it will not contain invalid data. If the abort happens before the
word that was requested by the access is returned, the load is aborted. If the abort happens after the
word that was requested by the access is returned, the load completes and the fill is aborted (but no
exception is generated).
7.3.2 Buffered Writes
Buffered writes cannot be externally aborted. Therefore, the system should be configured such that it
does not perform buffered writes to areas of memory that are capable of flagging an external abort.
7.4 Interaction of the MMU, Icache, Dcache, and Write
Buffer
The MMU, Icache, Dcache, and WB can be enabled or disabled independently. The Icache can be
enabled with the MMU enabled or disabled. However, the Dcache and WB can only be enabled
when the MMU is enabled. Because the write buffer is used to hold dirty copy-back cached lines
from the Dcache, it must be enabled along with the Dcache. Therefore, only four of the eight
combinations of the MMU, Dcache, and WB enables are valid. There are no hardware interlocks
on these restrictions, so invalid combinations will cause undefined results.
The following procedures must be observed.
To enable the MMU:
1. Program the translation table base and domain access control registers.
2. Program level 1 and level 2 page tables as required.
3. Enable the MMU by setting bit 0 in the control register.
Table 7-1. Valid MMU, Dcache, and Write Buffer Combinations
MMU Dcache Write Buffer
Off Off Off
On Off Off
On Off On
On On On