Instruction and Data Caches
MPC885 PowerQUICC Family Reference Manual, Rev. 2
7-14
Freescale Semiconductor
Table 7-9
describes the bits of the DC_DAT register.
7.3.2.1
Reading Data Cache Tags and Copyback Buffer
The MPC885 supports reading the tags, the state bits and the lock bits stored in the data cache as well as
the last copyback address, and data words in the copyback buffer. The data cache read command, issued
by reading DC_DAT, uses the DC_ADR register to qualify what is to be read.
Table 7-10
describes the
fields of the DC_ADR register during a data cache read command.
To read the copyback buffer data or the tags stored in the data cache, do the following:
1. Write the address of the copyback buffer or tag to be read to the DC_ADR according to the format
shown in
Table 7-10
.
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 undefined 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.
Figure 7-11
provides the format of the DC_DAT register for a tag read.
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 field (DC_ADR[20–27]), shown in
Table 7-12
,
determines which word of the cache block in the copyback buffer is read.
Table 7-9. Data Cache Data Port Register—DC_DAT
Bits
Name
Description
0–31
DAT
Data cache command data. The data received when reading information from the data cache.
See
Section 7.3.2.1, “Reading Data Cache Tags and Copyback Buffer,”
for more information.
Table 7-10. DC_ADR Fields for Cache Read Commands
0–17
18
19
20–27
28–31
Reserved
0 Tags
0 Way 0
1 Way 1
Set select
(0–255)
Reserved
1 Copyback buffer
Reserved
Copyback buffer address/
data-word select
Table 7-11. DC_DAT Format for a Tag Read (DC_ADR[18] = 0)
0–19
20–21
22
23
24
25
26–31
Tag value
Reserved
0 Invalid
1 Valid
0 Unlocked
1 Locked
LRU bit of this
set
0 Unmodified
1 Modified
Reserved
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