Motorola MPC823e - Instruction Cache Data Port Register; Instruction Cache Operation; Instruction Cache Hit

Instruction Cache
MOTOROLA MPC823e REFERENCE MANUAL 9-7
INSTRUCTION CACHE
9
9.2.3 Instruction Cache Data Port Register
The instruction cache data port register (IC_DAT) contains data that is received from the
instruction cache.
DAT—Data
This field represents the data received when reading information from the instruction cache.
The format may vary depending on the selected cache.
9.3 INSTRUCTION CACHE OPERATION
On an instruction fetch, bits 20-27 of the instruction’s address point into the cache to retrieve
the tags and data of one set. The tags from all four accessed ways are then compared
against bits 0-19 of the instruction’s address. If a match is found and the matched entry is
valid, then it is a cache hit. If none of the tags match or the matched tag is not valid, it is a
cache miss. The instruction cache includes one burst buffer that holds the last line received
from the bus and one line buffer that holds the last line retrieved from the cache array. If the
requested data is found in one of these buffers, it can also be considered a cache hit. Refer
to Figure 9-2 for more information. To minimize power consumption, the instruction cache
attempts to make use of data stored in one of its internal buffers. Using a special indication
from the core, it is possible to make sure that the requested data is in one of the buffers early
enough that the cache array is not activated.
9.3.1 Instruction Cache Hit
When a cache hit occurs, bits 28-29 of the instruction address are used to select one word
from the cache line whose tag matches the instruction pointer. The instruction is then
immediately transferred to the instruction unit of the core.
IC_DAT
BIT
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
FIELD DAT
RESET —
R/W R/W
SPR 562
BIT
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
FIELD DAT
RESET —
R/W R/W
SPR 562
NOTE: — = Undefined.