
6-8
G2 PowerPC Core Reference Manual
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MOTOROLA
MMU Features
6.1.3
Address Translation Mechanisms
Processors that implement the PowerPC architecture support the following four types of
address translation:
Page address translation—translates the page frame address for a 4-Kbyte page size.
Block address translation—translates the block number for blocks that range in size
from 128 Kbytes to 256 Mbytes.
Direct-store interface address translation—used to generate direct-store interface
accesses on the external bus; not implemented in the G2 core.
Real addressing mode translation—when address translation is disabled, the
physical address is identical to the effective address.
Figure 6-4 shows the three implemented address translation mechanisms provided by the
MMUs. The segment descriptors shown in the figure, control the page address translation
mechanism. When an access uses page address translation, the appropriate segment
descriptor is required. In 32-bit implementations, one of the 16 on-chip segment registers
(which contain segment descriptors) is selected by the 4 highest-order effective address
bits.
A control bit in the corresponding segment descriptor then determines if the access is to
memory (memory-mapped) or to the direct-store interface space (selected when the
direct-store translation control bit (T bit) in the corresponding segment descriptor is set).
Note that the direct-store interface existed in previous processors only for compatibility
with I/O devices that use this interface. When an access is determined to be to the
direct-store interface space, the G2 core takes a DSI exception as described in
Section 5.5.3, “DSI Exception (0x00300),” if it is a data access. The G2 core takes an ISI
exception as described in Section 5.5.4, “ISI Exception (0x00400),” if it is an instruction
access.
For memory accesses translated by a segment descriptor, the interim virtual address is
generated using the information in the segment descriptor. Page address translation
corresponds to the conversion of this virtual address into the 32-bit physical address used
by the memory subsystem. In most cases, the physical address for the page resides in an
on-chip TLB and is available for quick access. However, if the page address translation
misses in an on-chip TLB, the MMU causes a search of the page tables in memory (using
the virtual address information and a hashing function) to locate the required physical
address. When this occurs, the G2 core vectors to the exception handlers that search the
page tables with software.
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