IBM 750GL, 750GX 5.4.1.2 Changed Bit, 5.4.1.3 Scenarios for Referenced and Changed Bit Recording

User’s Manual

IBM PowerPC 750GX and 750GL RISC Microprocessor

• Accesses that cause exceptions and are not completed.

5.4.1.2 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 750GX). Whenever a data store instruction is executed success- fully, if the TLB search (for page-address translation) results in a hit, then the changed bit in the matching TLB entry is checked. If it is already set, it is not updated. If the TLB changed bit is 0, the 750GX initiates the table- search operation to set the C bit in the corresponding PTE in the page table. The 750GX then reloads the TLB (with the C bit set).

The changed bit (in both the TLB and the PTE in the page tables) is set only when a store operation is allowed by the page memory-protection mechanism and the store is guaranteed to be in the execution path (unless an exception, other than those caused by the sc, rfi, or trap instructions, occurs). Furthermore, the following conditions can cause the C bit to be set:

•The execution of an stwcx. instruction is allowed by the memory-protection mechanism but a store oper- ation is not performed.

•The execution of an stswx instruction is allowed by the memory-protection mechanism but a store opera- tion is not performed because the specified length is zero.

•The store operation is not performed because an exception occurs before the store is performed.

Again, note that although the execution of the dcbt and dcbtst instructions might cause the R bit to be set, they never cause the C bit to be set.

5.4.1.3 Scenarios for Referenced and Changed Bit Recording

This section provides a summary of the model (defined by the OEA) that is used by PowerPC processors for maintaining the referenced and changed bits. In some scenarios, the bits are guaranteed to be set by the processor; in some scenarios, the architecture allows the bits to be set (not absolutely required); and in some scenarios, the bits are guaranteed to not be set. Note that when the 750GX updates the R and C bits in memory, the accesses are performed as if MSR[DR] = 0 and G = 0 (that is, as nonguarded cacheable operations in which coherency is required).

Table 5-8on page 198 defines a prioritized list of the R and C bit settings for all scenarios. The entries in the table are prioritized from top to bottom, so that a scenario near the top of the table takes precedence over a scenario near the bottom of the table. For example, if an stwcx. instruction causes a page-protection violation and there is no reservation, the C bit is not altered. Note that in the table, load operations include those generated by load instructions, by the eciwx instruction, and by the cache-management instructions that are treated as a load with respect to address translation. Similarly, store operations include those operations generated by store instructions, by the ecowx instruction, and by the cache-management instructions that are treated as a store with respect to address translation.

Table 5-8. Model for Guaranteed R and C Bit Settings (Page 1 of 2)

Priority	Scenario	Causes Setting of R Bit		Causes Setting of C Bit
		OEA	750GX	OEA		750GX
1	No-execute protection violation	No	No	No		No
2	Page-protection violation	Maybe	Yes	No		No
3	Out-of-order instruction fetch or load operation	Maybe	No	No		No
Memory Management						gx_05.fm.(1.2)
Page 198 of 377					March 27, 2006