IBM PPC440X5 CPU Core, User Manual

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User’s Manual
Preliminary PPC440x5 CPU Core
cache.fm.
September 12, 2002
Page 121 of 589
Table 4-5 summarizes how the various storage attributes and other circumstances affect the DCC behavior
on store accesses.
4.3.1.4 Line Flush Operations
When a store operation (or the dcbz instruction) writes data into the data cache without also writing the data
to main memory, the cache line is said to become dirty, meaning that the data in the cache is the current
value, whereas the value in memory is obsolete. Of course, when such a dirty cache line is replaced (due, for
example, to a new cache line fill overwriting the existing line in the cache), the data in the cache line must be
copied to memory. Otherwise, the results of the previous store operation[s] that caused the cache line to be
marked as dirty would be lost. The operation of copying a dirty cache line to memory is referred to as a cache
line flush. Cache lines are flushed either due to being replaced when a new cache line is filled, or in response
to an explicit software flush request associated with the execution of a dcbst or dcbf instruction.
The DCC implements four dirty bits per cache line, one for each aligned doubleword within the cache line.
Whenever any byte of a given doubleword is stored into a data cache line without also writing that same byte
to memory, the corresponding dirty bit for that cache line is set (if CCR1[FFF] is set, then all four dirty bits are
Table 4-5. Data Cache Behavior on Store Accesses
Store Access Attributes DCC Actions
Caching
Inhibited
(I)
Hit/Miss SWOA
Write
through
(W)
Guarded
(G)
Write
Cache?
Write
Memory?
Gather?
1
0 Hit — 0 — Yes No N/A
0 Hit — 1 0 Yes Yes Yes
0 Hit — 1 1 Yes Yes No
0 Miss 0 0 — Yes No N/A
0 Miss 0 1 0 Yes Yes Yes
0 Miss 0 1 1 Yes Yes No
0 Miss 1 — 0 No Yes Yes
0 Miss 1 — 1 No Yes No
1 —
2
— —
3
0 No Yes Yes
4
1 —
2
— —
3
1 No Yes No
Note 1: If store gathering is disabled altogether (by setting CCR0[DSTG] to 1), then
such gathering will not occur, regardless of the indication in this table.
Furthermore, where this table indicates that store gathering may occur it is
presumed that the operations being gathered are targeting the same aligned
quadword of memory, and are contiguous with respect to each other.
Note 2: It is a programming error for a data cache hit to occur on a store access to a
caching inhibited page. The result of such an access is undefined.
Note 3: It is programming error for the write-through storage attribute to be set for a
page which also has the caching inhibited storage attribute set. The result of an
access to such a page is undefined.
Note 4: Stores to caching inhibited memory locations may only be gathered with
previous store operations if none of the targeted bytes overlap with the bytes
targeted by the previous store operations.