Memory Management Unit
MPC885 PowerQUICC Family Reference Manual, Rev. 2
8-10
Freescale Semiconductor
The page size and the level-two base address are read from the level-one descriptor. If the page size is 512
Kbytes or 8 Mbytes, 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[10–19]. For 16-Kbyte pages, this requires that multiple identical
level-two descriptors be provided. This is summarized in
Table 8-1
.
Figure 8-5
shows the two-level translation table when MD_CTR[TWAM] = 0 (1-Kbyte resolution of
protection).
Figure 8-5. Two-Level Translation Table (MD_CTR[TWAM] = 0)
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 page size determines the number of replaced bits as
shown in
Table 8-2
. The remaining physical address bits come directly from the effective address. When
MD_CTR[TWAM] = 0, the tablewalk begins at the level-one base address placed in M_TWB. The
level-one table is indexed by EA[0–11] to get the level-one page descriptor. As shown in
Table 8-1
,
8-Mbyte pages must have eight identical entries in the level-one table for EA[9–11].
20 for 1 Kbyte
20 for 4 Kbyte
18 for 16 Kbyte
13 for 512 Kbyte
9 for 8 Mbyte
Level-1 Table Base
Level-1 Index
00
Level-1 Index
0
21
31
11
Level-2 Index
Page Offset
0
Level-1 Table Pointer (M_TWB)
Level-1 Descriptor 0
Level-1 Descriptor 1
Level-1 Descriptor N
Level-1 Descriptor 4095
Level-2 Table Base
Level-2 Index
18-Bit
12-Bit
00
10-Bit
Level-2 Descriptor 0
Level-2 Descriptor 1
Level-2 Descriptor N
Level-2 Descriptor 1023
Physical Page Address
Page Offset
18-Bit
20-Bit
12 for 1 Kbyte
12 for 4 Kbyte
14 for 16 Kbyte
19 for 512 Kbyte
20-Bit
10-Bit
22
12
17
23 for 8 Mbyte
12-Bit
Effective Address
Physical Address
Level-1 Table
Level-2 Table
Summary of Contents for PowerQUICC MPC870
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