Tlb Sync Instruction (Tlbsync); Page Reference And Change Status Management; Figure 5-5. Tlb Entry Word Definitions - IBM PPC440X5 CPU Core User Manual

User's Manual
PPC440x5 CPU Core
TLB Word 0 (WS=0)
0
TLB Word 1 (WS=1)
0
TLB Word 2 (WS=2)
0 1 2
14
PAR2
Figure 5-5. TLB Entry Word Definitions

5.9.3 TLB Sync Instruction (tlbsync)

The tlbsync instruction is used to synchronize software TLB management operations in a multiprocessor
environment with hardware-enforced coherency, which is not supported by the PPC440x5. Consequently,
this instruction is treated as a no-op. It is provided in support of software compatibility between PowerPC-
based systems.

5.10 Page Reference and Change Status Management

When performing page management, it is useful to know whether a given memory page has been refer-
enced, and whether its contents have been modified. Note that this may be more involved than determining
whether a given TLB entry has been used to reference or change memory, since multiple TLB entries may
translate to the same memory page. If it is necessary to replace the contents of some memory page with
other contents, a page which has been referenced (accessed for any purpose) is more likely to be maintained
than a page which has never been referenced. If the contents of a given memory page are to be replaced and
the contents of that page have been changed, then the current contents of that page must be written to
backup physical storage (such as a hard disk) before replacement.
Similarly, when performing TLB management, it is useful to know whether a given TLB entry has been refer-
enced. When making a decision about which entry of the TLB to replace in order to make room for a new
entry, an entry which has never been referenced is a more likely candidate to be replaced.
The PPC440x5 does not automatically record references or changes to a page or TLB entry. Instead, the
interrupt mechanism may be used by system software to maintain reference and change information for TLB
entries and their associated pages, respectively.
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September 12, 2002
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