
MOTOROLA
Chapter 6. Memory Management
6-11
MMU Features
6.1.5
Page History Information
The MMUs of these processors also define referenced (R) and changed (C) bits in the page
address translation mechanism that can be used as history information relevant to the page.
This information can then be used by the operating system to determine the areas of
memory to write back to disk when new pages must be allocated in main memory. While
these bits are initially programmed by the operating system into the page table, the
architecture specifies that the R and C bits may be maintained either by the processor
hardware (automatically) or by some software-assist mechanism that updates these bits
when required as needed by the G2 core. The software table search routines used by the G2
core set the R bit when a PTE is accessed; the core causes an exception (to vector to the
software table search routines) when the C bit in the corresponding TLB entry requires
updating. See Section 6.4.1.3, “Scenarios for Referenced and Changed Bit Recording,” for
more details.
6.1.6
General Flow of MMU Address Translation
The following sections describe the general flow used by processors that implement the
PowerPC architecture to translate effective addresses to virtual and then physical addresses.
6.1.6.1
Real Addressing Mode and Block Address Translation
Selection
When an instruction or data access is generated and the corresponding instruction or data
translation is disabled (MSR[IR] = 0 or MSR[DR] = 0), real addressing mode translation is
used (physical address equals effective address) and the access continues to the memory
subsystem as described in Section 6.2, “Real Addressing Mode.”
Figure 6-5 shows the flow used by the MMUs in determining whether to select real
addressing mode, block address translation, or to use the segment descriptor to select page
address translation.
Note that if the BAT array search results in a hit, the access is qualified with the appropriate
protection bits. If the access violates the protection mechanism, an exception (ISI or DSI
exception) is generated.
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