Memory Management Unit
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
8-11
The page size and the level-two base address are read from the level-one descriptor. If the page size is 512
Kbytes or 8Mbytes, the level-two base address is used directly as the address of the level-two descriptor.
If the page size is less than 512 Kbytes, the address of the level-two descriptor is determined by indexing
the level-two base address by EA[12–21]. For 4Kbyte or 16 Kbyte pages, this requires that multiple
identical level-two descriptors be provided. This is summarized in
Table 8-1
.
The number of replaced bits depends on the page size, as shown in
Table 8-2
. The remaining physical
address bits are taken directly from the effective address.
8.7.1
Level-One Descriptor
Table 8-3
describes the level-one descriptor format supported by the hardware to minimize the software
tablewalk routine.
Table 8-2. 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 8-3. Level-One Segment Descriptor Format
Bits
Name
Description
0–19
L2BA
Level-2 table base address. Bits 18–19 should be 0b00 unless MD_CTR[TWAM] = 1.
20–22
—
Reserved
23–26
APG
Access protection group
27
G
Guarded memory attribute for entry
0 Nonguarded memory
1 Guarded memory
28–29
PS
Page size level one. Used with the level-two (L2) descriptor’s small-page-size (SPS) field; see
Section 8.7.3, “Page Size.”
00 Small (4 Kbyte or 16 Kbyte)
01 512 Kbyte
10 Reserved
11 8 Mbyte
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 PowerQUICC MPC870
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