Instruction and Data Caches
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
7-15
7.3.2.2
DC_CST Commands
All DC_CST commands, except the load-and-lock cache block and flush cache block commands, are
executed immediately after writing to the DC_CST register and do not generate any errors. Therefore,
there is no need to check the error type bits in the DC_CST register except when executing the
load-and-lock cache block and flush cache block commands.
Note that when the data cache is executing a command, it stops handling CPU requests, which can result
in machine stalls.
7.3.2.2.1
Data Cache Enable/Disable Commands
The data cache enable command (DC_CST[CMD] = 0b0010) is used to enable the data cache; the data
cache disable command (DC_CST[CMD] = 0b0100) is used to disable the data cache. Neither of these
commands has any error cases. The current state of the data cache is available by reading the data cache
enable status bit (DC_CST[DEN]).
When disabled, the MPC885 ignores the data cache state bits and operates as if all accesses have
caching-inhibited access attributes (that is, all accesses are propagated to the bus as single-beat
transactions). Disabling the data cache does not affect the data address translation logic; MSR[DR]
controls data address translation.
Note that the data cache is disabled at hard reset. Also, the data cache is automatically disabled when a
type 1 data cache error (see
Table 7-7
for DC_CST[CCER1] conditions) generates a machine check
exception.
7.3.2.2.2
Data Cache Load-and-Lock Cache Block Command
The data cache load-and-lock cache block command (DC_CST[CMD] = 0b0110) is used to lock critical
data in the data cache. Locked cache blocks are not replaced during misses and are not affected by
invalidate commands.
To load and lock one or more cache blocks:
1. Read the DC_CST error type bits to clear them.
2. Write the address of the cache block to be locked to the DC_ADR register.
3. Write the load-and-lock cache block command (DC_CST[CMD] = 0b0110) to the DC_CST
register.
Table 7-12. Copyback Buffer Select Field (DC_ADR[20–27]) Encoding
DC_ADR[20–27]
Buffer Selected
0x00
Copyback buffer data word 0
0x01
Copyback buffer data word 1
0x02
Copyback buffer data word 2
0x03
Copyback buffer data word 3
0x04
Copyback address
Summary of Contents for PowerQUICC MPC870
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