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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
gx_09.fm.(1.2)
March 27, 2006
L2 Cache
Page 325 of 377
Whenever a way in the set is referenced, the LRU bits are updated. The new value of the LRU bits depends
on the old value, which way is currently being accessed, and whether the operation is an invalidation or a
load/store. Table 9-2 shows the new value of the LRU bits for the various combinations of these variables. An
‘x’ indicates don’t care, while a ‘-’ indicates no change from previous value.
The 4-way set-associative L2 cache can be locked by way as described below. The determination of the new
LRU value does not depend on the locked status of the ways. However, the interpretation of the LRU bits
shown in Table 9-2 does change when one or more ways of the cache are locked.
Any combination of ways can be locked. The effect of locking on the replacement algorithm is that the least
recently used of the unlocked ways is chosen for replacement. Table 9-3 shows the interpretation of the LRU
bits in the presence of one or two locked ways. If three ways are locked, the unlocked way is always replaced,
and if all four ways are locked, no replacement takes place. In Table 9-3, bit zero of the lock bits controls
whether way 0 is locked, bit one controls whether way 1 is locked, and so forth.
Table 9-2. Modification of LRU Bits
Old LRU Hit Way Invalidate New LRU
00x none x 11-
01x none x 10-
1x0 none x 0-1
1x1 none x 0-0
xxx 0 0 11-
xxx 1 0 10-
xxx 2 0 0-1
xxx 3 0 0-0
xxx 0 1 00-
xxx 1 1 01-
xxx 2 1 1-0
xxx 3 1 1-1
Table 9-3. Effect of Locked Ways on LRU Interpretation (Page 1 of 2)
LRU Bits Lock Bits LRU Way
00x 0xxx 0
00x 10xx 1
000 110x 2
001 11x0 3
01x x0xx 1
01x 01xx 0
010 110x 2
011 11x0 3
1x0 xx0x 2
1x0 xx10 3
100 0x11 0