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Instruction Cache
MOTOROLA
MPC801 USER’S MANUAL
9-11
9
When read from the data array, the 32 bits of the word selected by the IC_ADR register is
placed in the targeted general-purpose register. When read from the tag array, the 21 bits
of the tag and related information that is selected by the IC_ADR are all placed in the
targeted general-purpose register. The following table provides the bit layout of the
instruction cache data register when reading a tag.
9.4.8 Instruction Cache Write
Instruction cache write is only enabled when the MPC801 is in test mode.
9.5 RESTRICTIONS
Zero wait state devices that are placed on the internal bus are considered to be in the
cache-inhibited memory region and the hardware correct operation depends on the software
to follow the exact steps mentioned in
Section 9.7 Updating Code And Memory Region
Attributes
. It is not advisable to perform load & lock from zero wait state devices that are
placed on the internal bus, especially since it is not guaranteed that the data will be fetched
from the instruction cache. In most cases, it is fetched from the device, but found in the
instruction cache.
9.6 INSTRUCTION CACHE COHERENCY
Cache coherency in a multiprocessor environment is maintained by the software and
supported by the invalidation mechanism that is described above. All instruction storage is
considered to be in Memory Coherence Not Required mode.
9.7 UPDATING CODE AND MEMORY REGION ATTRIBUTES
To update the code or change the programming of the memory regions in the chip-select
logic, follow these steps:
1. Update the code and change the memory region programming in the chip-select logic.
2. Execute the
sync
instruction to ensure that the update/change operation has finished.
3. Unlock all locked lines that contain updated code.
4. Invalidate all lines that contain updated code.
5. Execute the
isync
instruction.
Table 9-2. IC_DAT Bit Layout When Reading a Tag
0-21
22
23
24
25-31
Tag Value
0 - Not Valid
1 - Valid
0 - Not Locked
1 - Locked
LRU Bit
Reserved