Freescale Semiconductor MPC5553 - Page 138

MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor 3-23
Figure 3-15 illustrates the general flow of cache operation.
Figure 3-15. Cache Lookup Flow
To determine if the address is already allocated in the cache the following steps are taken:
1. The cache set index, virtual address bits A[20:26] are used to select one cache set. A set is defined
as the grouping of eight lines (one from each way), corresponding to the same index into the cache
array.
2. The higher order physical address bits A[0:19] are used as a tag reference or used to update the
cache line tag field.
3. The eight tags from the selected cache set are compared with the tag reference. If any one of the
tags matches the tag reference and the tag status is valid, a cache hit has occurred.
4. Virtual address bits A[27:28] are used to select one of the four doublewords in each line. A cache
hit indicates that the selected doubleword in that cache line contain valid data (for a read access),
or can be written with new data depending on the status of the W access control bit from the
MMU (for a write access).
31
27
26
20
190
Index
Tag Data / Tag Reference
MUX
Comparator
0
1
2
7
Logical OR
HIT 7
HIT 2
HIT 1
HIT 0
Hit
Select
Set 0
Set 1
Set 127
•
•
•
Tag
Reference
A[0:19]
Way 0
Way 1
Way 2
Way 7
Data or
Instruction
Status DW0 DW1 DW2 DW3
Tag
Status DW0 DW1
DW2
DW3
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Virtual Address
Set
Select
A[20:26])
Tag
Physical Address
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
*Ways 2-7 available
only in MPC5554