
6-2
G2 PowerPC Core Reference Manual
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MOTOROLA
MMU Features
(SPRs). There are separate instruction and data BAT mechanisms, and in the G2 core, they
reside in the instruction and data MMUs, respectively.
The MMUs, together with the exception processing mechanism, provide the necessary
support for the operating system to implement a paged virtual memory environment and for
enforcing protection of designated memory areas. Exception processing is described in
Chapter 5, “Exceptions.” Section 5.2, “Exception Processing,” describes the MSR which
controls some of the critical functionality of the MMUs.
6.1
MMU Features
The G2 core completely implements all features required by the memory management
specification of the OEA for 32-bit implementations. Thus, it provides 4 Gbytes of effective
address space accessible to supervisor and user programs with a 4-Kbyte page size and
256-Mbyte segment size. In addition, the MMUs of 32-bit processors use an interim virtual
address (52 bits) and hashed page tables in the generation of 32-bit physical addresses.
These processors also have a BAT mechanism for mapping large blocks of memory. Block
sizes range from 128 Kbytes to 256 Mbytes and are software-programmable.
Table 6-1 summarizes all G2 core MMU features including the architectural features of
PowerPC MMUs (defined by the OEA) for 32-bit processors and the
implementation-specific features provided by the core.
Table 6-1. MMU Features Summary
Feature Category
Architecturally Defined/
G2 Core-Specific
Feature
Address ranges
Architecturally defined
2
32
bytes of effective address
2
52
bytes of virtual address
2
32
bytes of physical address
Page size
Architecturally defined
4 Kbytes
Segment size
Architecturally defined
256 Mbytes
Block address
translation
Architecturally defined
Range of 128 Kbytes–256 Mbytes sizes
Implemented with IBAT and DBAT registers in BAT array
Memory protection
Architecturally defined
Segments selectable as no-execute
Pages selectable as user/supervisor and read-only
Blocks selectable as user/supervisor and read-only
Page history
Architecturally defined
Referenced and changed bits defined and maintained
Page address
translation
Architecturally defined
Translations stored as PTEs in hashed page tables in memory
Page table size determined by mask in SDR1 register
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