MOTOROLA
Chapter 9. Memory Management Unit (MMU)
9-13
Part II. PowerPC Microprocessor Module
The level-two base address from the level-one descriptor is indexed by EA[12Ð21] to Þnd
the level-two page descriptor. For pages larger than 1-Kbyte, the entry in the level-two table
must be duplicated according to the page size. During address translation, the msbs of the
missed effective address are replaced by the physical page address bits from the level-two
page descriptor. The number of replaced bits depends on the page size. The remaining
physical address bits are taken directly from the effective address, as shown in Table 9-2.
9.7.1 Level-One Descriptor
Table 9-3 describes the level-one descriptor format supported by the hardware to minimize
the software tablewalk routine.
Table 9-2. Number of Replaced EA Bits per Page Size
Page Size
Number of Replaced EA Bits
1 Kbyte
20
4 Kbyte
20
16 Kbyte
18
512 Kbyte
13
8 Mbyte
9
Table 9-3. Level-One Segment Descriptor Format
Bits
Name
Description
0Ð19
L2BA
Level-2 table base address. Bits 18Ð19 should be 0b00 unless MD_CTR[TWAM] = 1.
20Ð22
Ñ
Reserved
23Ð26
APG
Access protection group
27
G
Guarded memory attribute for entry
0 Nonguarded memory
1 Guarded memory
28Ð29
PS
Page size level one
00 Small (4 Kbyte or 16 Kbyte)
01 512 Kbyte
11 8 Kbyte
10 Reserved
30
WT
Writethrough attribute for entry
0 Copyback cache policy region (default)
1 Writethrough cache policy region
31
V
Level-one segment valid bit
0 Segment is not valid
1 Segment is valid
Содержание MPC860 PowerQUICC
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