
6-22
G2 PowerPC Core Reference Manual
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MOTOROLA
Memory Segment Model
The G2 core causes the R bit to be set for the execution of the
dcbt
or
dcbtst
instruction to
that page (by causing a TLB miss exception to load the TLB entry in the case of a TLB
miss). However, neither of these instructions causes the C bit to be set.
As defined by the PowerPC architecture, the referenced and changed bits are updated as if
address translation were disabled (real addressing mode translation). Additionally, these
updates should be performed with single-beat read and byte write transactions on the bus.
6.4.1.1
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 R bit is then set in the
page table. The OEA specifies that the referenced bit may be set immediately, or the setting
may be delayed until the memory access is determined to be successful. Because the
reference to a page is what causes a PTE to be loaded into the TLB, the referenced bit in all
G2 core TLB entries is effectively always set. The processor never automatically clears the
referenced bit.
The referenced bit is only a hint to the operating system about the activity of a page. At
times, the referenced bit may be set by software although the access was not logically
required by the program, or even if the access was prevented by memory protection.
Examples of this in these systems include the following:
Fetching of instructions not subsequently executed
Accesses generated by an
lswx
or
stswx
instruction with a zero length
Accesses generated by a
stwcx.
instruction when no store is performed because a
reservation does not exist
Accesses that cause exceptions and are not completed
6.4.1.2
Changed Bit
The changed bit of a page is located both in the PTE in the page table and in the copy of the
PTE loaded into the TLB (if a TLB is implemented, as in the G2 core). Whenever a data
store instruction is executed successfully, if the TLB search (for page address translation)
Table 6-7. Table Search Operations to Update History Bits—TLB Hit Case
R and C Bits in
TLB Entry
Processor Action
00
Combination does not occur
01
Combination does not occur
10
Read:
Write:
No special action
Table search operation required to update C.
Causes a data TLB miss on store exception.
11
No special action for read or write
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