Motorola MC68020 - Cache Address Register (CAAR)

4–4 M68020 USER’S MANUAL MOTOROLA
F—Freeze Cache
The F-bit is set to freeze the instruction cache. When the F-bit is set and a cache miss
occurs, the entry (or line) is not replaced. When the F-bit is clear, a cache miss causes
the entry (or line) to be filled. A reset operation clears the F-bit.
E—Enable Cache
The E-bit is set to enable the instruction cache. When it is clear, the instruction cache is
disabled. A reset operation clears the E-bit. The supervisor normally enables the
instruction cache, but it can clear the E-bit for system debugging or emulation, as
required. Disabling the instruction cache does not flush the entries. If the cache is
reenabled, the previously valid entries remain valid and may be used.
4.3.2 Cache Address Register (CAAR)
The format of the 32-bit CAAR is shown in Figure 4-3.
0
31
RESERVED
1
2
INDEX
7
8
R
ESERVE
D
Figure 4-3. Cache Address Register
Bits 31–8, 1, and 0—Reserved
These bits are reserved for use by Motorola.
Index Field
The index field contains the address for the “clear cache entry” operations. The bits of
this field, which correspond to A7–A2, specify the index and a long word of a cache line.