MOTOROLA
Chapter 8. Instruction and Data Caches
8-15
Part II. PowerPC Microprocessor Module
To read the copyback buffer data or the tags stored in the data cache:
1. Write the address of the copyback buffer or tag to be read to the DC_ADR,
according to the format shown in Table 8-4.
Note that it is also possible to read this register for debugging purposes.
2. Read the DC_DAT register. Note that writing to the DC_DAT register is illegal. A
write to DC_DAT results in an undeÞned data cache state.
For tag array (DC_ADR[18] = 0) read commands, the tag and state information is placed
in the target general-purpose register. Table 8-5 provides the format of the DC_DAT
register when reading a tag.
The last copyback address or data buffer can be read by using the copyback buffer read
command (DC_ADR[18] = 1). The copyback buffer select Þeld (DC_ADR[21Ð27]), shown
in Table 8-11, determines which word of the cache block in the copyback buffer is read.
8.3.2.2 DC_CST Commands
All DC_CST commands, except the load & lock cache block and ßush 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 & lock cache block and ßush cache block
commands.
Note that when the data cache is executing a command, it stops handling CPU requests,
which can result in machine stalls.
Table 8-10. DC_DAT Format when Reading a Tag
0Ð20
21
22
23
24
25
25Ð31
Tag value
Reserved
0 Invalid
1 Valid
0 Unlocked
1 Locked
LRU bit of this
set
0 UnmodiÞed
1 ModiÞed
Reserved
Table 8-11. Copyback Buffer Select Field (DC_CST[21Ð27]) Encoding
DC_CST[21Ð27]
(in hex)
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
Содержание MPC860 PowerQUICC
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