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from inadvertent modification by a running User mode application. This
protection can be achieved by defining CPLB descriptors for protected
memory ranges that allow write access only when in Supervisor mode. If a
write to a protected memory region is attempted while in User mode, an
exception is generated before the memory is modified. Optionally, the
User mode application may be granted read access for data structures that
are useful to the application. Even Supervisor mode functions can be
blocked from writing some memory pages that contain code that is not
expected to be modified. Because CPLB entries are MMRs that can be
written only while in Supervisor mode, user programs cannot gain access
to resources protected in this way.
If either the L1 Instruction Memory or the L1 Data Memory is configured
partially or entirely as cache, the corresponding CPLBs must be enabled.
When an instruction generates a memory request and the cache is enabled,
the processor first checks the ICPLBs to determine whether the address
requested is in a cacheable address range. If no valid ICPLB entry in an
MMR pair corresponds to the requested address, an MMU exception is
generated to obtain a valid ICPLB descriptor to determine whether the
memory is cacheable or not. As a result, if the L1 Instruction Memory is
enabled as cache, then any memory region that contains instructions must
have a valid ICPLB descriptor defined for it. These descriptors must either
reside in MMRs at all times or be resident in a memory-based Page
Descriptor Table that is managed by the MMU exception handler. Like-
wise, if either or both L1 data banks are configured as cache, all potential
data memory ranges must be supported by DCPLB descriptors.
Before caches are enabled, the MMU and its supporting data struc-
tures must be set up and enabled.
ADSP-BF53x/BF56x Blackfin Processor Programming Reference
Memory
6-53
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