Freescale Semiconductor 7.8.1 Cache Line States: Invalid, Valid-Unmodified, and Valid-Modified, 7.8.2 The Cache at Start-Up

MCF548x Reference Manual, Rev. 3
7-8 Freescale Semiconductor
Figure 7-3. Data Cache Organization and Line Format
A set is a group of four lines (one from each level, or way), corresponding to the same index into the cache
array.
7.8.1 Cache Line States: Invalid, Valid-Unmodified, and Valid-Modified
As shown in Table 7-3, a data cache line can be invalid, valid-unmodified (often called exclusive), or
valid-modified. An instruction cache line can be valid or invalid.
A valid line can be explicitly invalidated by executing a CPUSHL instruction.
7.8.2 The Cache at Start-Up
As Figure 7-4 (A) shows, after power-up, cache contents are undefined; V and M may be set on some lines
even though the cache may not contain the appropriate data for start up. Because reset and power-up do
not invalidate cache lines automatically, the cache should be cleared explicitly by setting
CACR[DCINVA,ICINVA] before the cache is enabled (B).
After the entire cache is flushed, cacheable entries are loaded first in way 0. If way 0 is occupied, the
cacheable entry is loaded into the same set in way 1, as shown in Figure 7-4 (D). This process is described
in detail in Section 7.9, “Cache Operation.”
Table 7-3. Valid and Modified Bit Settings
VM Description
0 x Invalid. Invalid lines are ignored during lookups.
1 0 Valid, unmodified. Cache line has valid data that matches system memory.
1 1 Valid, modified. Cache line contains most recent data, data at system memory location is
stale.
Way 0 Way 1 Way 2 Way 3
Line
Set 0
Set 1
Set 510
Set 511
•
•
•
•
•
•
•
•
•
•
•
•
TAG V M Longword 0 Longword 1 Longword 2 Longword 3
Where:
TAG—21-bit address tag
V—Valid bit for line
M—Modified bit for line (data cache only)
Cache Line Format