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9.5 Protection Resolution Modes
The MMUs can be programmed in three different modes that different methods of deÞning
the protection resolution of the address space. These are as follows:
¥ Mode 1ÑProtection resolution to 4-Kbyte minimum page size. This is the simplest
mode with the most efÞcient memory size (that is, MMU tables are smaller). Use
this mode if 1-Kbyte protection resolution is not required.
In this mode, set the following:
Ñ MD_CTR[TWAM] = 1
Ñ Mx_CTR[PPM] = 0
Ñ Bits 20Ð27 of the level-two descriptor take on the meaning described in the right
side of Table 9-4.
¥ Mode 2ÑProtection resolution to 1-Kbyte minimum subpage size, where all
4-Kbyte subpages map to the same physical page but may have different protection
attributes.
In this mode, set the following:
Ñ MD_CTR[TWAM]=1
Ñ Mx_CTR[PPM]=1
For pages larger than 4 Kbytes, bits 20Ð27 of the level-two descriptor take on the
meaning described in the right side of Table 9-4
For 4-Kbyte pages, set the subpage validity ßags (bits 20Ð27) to subpage protection
mode for 4 subpages; see Table 9-4. In this mode, the MMU page tables deÞned for
the software tablewalk resolve to a single level-two descriptor entry for a 4-Kbyte
page.
Page protection (and thus level-two descriptor entries) for pages larger than 4-Kbyte
is the same as for mode 1, described above. However, for 4-Kbyte pages, the
level-two descriptor encoding has special meaning. The level-two descriptor deÞnes
a 4-Kbyte page in physical memory, but each 1-Kbyte of that page can be deÞned
with different protection attributes.
This mode is just as efÞcient in memory size as mode 1, but the protection encoding
and capabilities for 4-Kbyte pages differ.
¥ Mode 3ÑProtection resolution to 1-Kbyte minimum subpage size, with no
restriction on subpage mapping. In this mode, set:
Ñ MD_CTR[TWAM] = 0
Ñ Mx_CTR[PPM] = 0
Ñ Mx_CTR[PPC5] = 0
MOTOROLA
Chapter 9. Memory Management Unit (MMU)
Part II. PowerPC Microprocessor Module
9-7
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