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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
gx_05.fm.(1.2)
March 27, 2006
Memory Management
Page 189 of 377
5.1.6 General Flow of MMU Address Translation
The following sections describe the general flow 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), then the real-addressing mode is used (physical address equals
effective address), and the access continues to the memory subsystem as described in Section 5.2, Real-
Addressing Mode, on page 195.
Figure 5-5 shows the flow the MMUs use in determining whether to select real-addressing mode, block-
address translation, or the segment descriptor to select page-address translation.
Note: If the BAT array search results in a hit, then the access is qualified with the appropriate protection bits.
If the access violates the protection mechanism, then an exception (either ISI or DSI) is generated.
Figure 5-5. General Flow of Address Translation (Real-Addressing Mode and Block)
Perform Address
Translation with
Segment Descriptor
Access Faulted
Compare Address with
Instruction or Data BAT
Array (As Appropriate)
Translate Address
Perform Real
Addressing Mode
Translation
Effective Address
Generated
Continue Access
to Memory
Subsystem
(See The Programming
Environments Manual)
(See Figure 5-6 on
page 191)
Instruction
Translation Disabled
(MSR[IR] = 0)
BAT
Array
Miss
Instruction Access
Access
Protected
Access
Permitted
Instruction
Translation Enabled
(MSR[IR] = 1)
Data
Translation Enabled
(MSR[DR] = 1)
Data
Translation Disabled
(MSR[DR] = 0)
Data Access
BAT
Array
Hit
Perform Real
Addressing Mode
Translation