5-22
MPC7400 RISC Microprocessor Users Manual
Memory Segment Model
Referenced and changed recording is performed only for accesses made with page address
translation and not for translations made with the BAT mechanism or for accesses that
correspond to direct-store (T = 1) segments. Furthermore, R and C bits are maintained only
for accesses made while address translation is enabled (MSR[IR] = 1 or MSR[DR] = 1).
In the MPC7400, the referenced and changed bits are updated as follows:
¥
For TLB hits, the C bit is updated according to Table 5-7.
¥
For TLB misses, when a table search operation is in progress to locate a PTE. The
R and C bits are updated (set, if required) to reect the status of the page based on
this access.
Table 5-7 shows that the status of the C bit in the TLB entry (in the case of a TLB hit) is
what causes the processor to update the C bit in the PTE (the R bit is assumed to be set in
the page tables if there is a TLB hit). Therefore, when software clears the R and C bits in
the page tables in memory, it must invalidate the TLB entries associated with the pages
whose referenced and changed bits were cleared.
In some previous implementations, the
dcbt
and
dcbtst
instructions would execute only if
there was a TLB/BAT hit or if the processor is in real addressing mode. In case of a TLB or
BAT miss, these instructions would be treated as no-ops and did not initiate a table search
operation and did not set either the R or C bits. In the MPC7400, the
dcbt
,
dcbtst
, and data
stream touch instructions (
dst
[
t
] and
dstst
[
t
]) do cause a table search operation in the case
of a TLB miss. However, they never cause the C bit to be set.
As deTned by the PowerPC architecture, the referenced and changed bits are updated as if
address translation were disabled (real addressing mode). If these update accesses hit in the
data cache, they are not seen on the external bus. If they miss in the data cache, they are
performed as typical cache line Tll accesses on the bus (if the data cache is enabled), or as
discrete read and write accesses (if the data cache is disabled).
5.4.1.1 Referenced Bit
The referenced (R) bit of a page is located in the PTE in the page table. Every time a page
is referenced (with a read or write access) and the R bit is zero, the MPC7400 sets the R bit
Table 5-7. Table Search Operations to Update History BitsTLB Hit Case
R and C bits
in TLB Entry
Processor Action
00
Combination doesnt occur
01
Combination doesnt occur
10
Read: No special action
Write: The MPC7400 initiates a table search operation to update C.
11
No special action for read or write