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User’s Manual
IBM PowerPC 750GX and 750GL RISC Microprocessor
Memory Management
Page 186 of 377
gx_05.fm.(1.2)
March 27, 2006
5.1.3 Address-Translation Mechanisms
PowerPC processors support the following three types of address translation:
Figure 5-4, Address-Translation Types shows the three 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 implemen-
tations, the appropriate segment descriptor is selected from the 16 on-chip segment registers 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. Note that the direct-store interface was present in the architec-
ture only for compatibility with existing I/O devices that use this interface. However, it is being removed from
the architecture, and the 750GX does not support it. When an access is determined to be to the direct-store
interface space, the 750GX takes a data-storage interrupt (DSI) exception if it is a data access (see
Section 4.5.3, DSI Exception (0x00300), on page 169), and takes an instruction storage interrupt (ISI) excep-
tion if it is an instruction access (see Section 4.5.4, ISI Exception (0x00400), on page 169).
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 trans-
lation misses in the 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.
Because blocks are larger than pages, there are fewer upper-order effective address bits to be translated into
physical address bits (more low-order address bits (at least 17) are untranslated to form the offset into a
block) for block-address translation. Also, instead of segment descriptors and a TLB, block-address transla-
tions use the on-chip BAT registers as a BAT array. If an effective address matches the corresponding field of
a BAT register, the information in the BAT register is used to generate the physical address. In this case, the
results of the page translation (occurring in parallel) are ignored.
Page address Translates the page frame address for a 4-KB page size.
Block address Translates the block number for blocks that range in size from 128 KB to 256 MB.
Real-addressing
mode address
When address translation is disabled, the physical address is identical to the effec-
tive address.