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MPC7400 RISC Microprocessor Users Manual
MMU Overview
5.1.5 Page History Information
The MMUs of PowerPC processors also deTne referenced (R) and changed (C) bits in the
page address translation mechanism that can be used as history information relevant to the
page. The operating system can use these bits to determine which 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 speciTes
that they can be maintained either by the processor hardware (automatically) or by some
software-assist mechanism.
When loading the TLB, the MPC7400 checks the state of the changed and referenced bits
for the matched PTE. If the referenced bit is not set and the table search operation is initially
caused by a load operation or by an instruction fetch, the MPC7400 automatically sets the
referenced bit in the translation table. Similarly, if the table search operation is caused by a
store operation and either the referenced bit or the changed bit is not set, the hardware
automatically sets both bits in the translation table. In addition, when the address translation
of a store operation hits in the DTLB, the MPC7400 checks the state of the changed bit. If
the bit is not already set, the hardware automatically updates the DTLB and the translation
table in memory to set the changed bit. For more information, see Section 5.4.1, òPage
History Recording.ó
5.1.6 General Flow of MMU Address Translation
The following sections describe the general ow used by PowerPC processors to translate
effective addresses to virtual and then physical addresses.
5.1.6.1 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 is used
(physical address equals effective address) and the access continues to the memory
subsystem as described in Section 5.2, òReal Addressing Mode.ó
Figure 5-5 shows the ow the MMUs use in determining whether to select real addressing
mode, block address translation, or the segment descriptor to select page address
translation.