Memory Management Unit
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
8-7
8.5
Protection Resolution Modes
The MMUs can be programmed in three different modes that have different methods of defining the
protection resolution of the address space. These are as follows:
•
Mode 1—Protection resolution to 4-Kbyte minimum page size. This is the simplest mode with the
most efficient memory size (that is, MMU tables are smaller). Use this mode if 1-Kbyte protection
resolution is not required.
In this mode, program the following:
— MD_CTR[TWAM] = 1
— Mx_CTR[PPM] = 0
— Bits 20–27 of the level-two descriptor take on the meaning described in the right side of
Table 8-4
.
•
Mode 2—Protection resolution to 1-Kbyte minimum subpage size, where all 4-Kbyte logical
address pages map to the same 4-Kbyte physical page, but the four 1-Kbyte subpages may have
different protection attributes.
In this mode, program the following:
— MD_CTR[TWAM]=1
— Mx_CTR[PPM]=1
For 4-Kbyte pages, program the four PP pairs (bits 20–27) to the subpage protection attributes for
the 1-Kbyte subpages.
For pages larger than 4 Kbytes, the four PP pairs (bits 20–27) must all be programmed to the same
protection attributes, which are applied to the full page.
This mode is just as efficient in memory size as mode 1, but has the memory protection resolution
of mode 3.
•
Mode 3—Protection resolution to 1-Kbyte minimum subpage size, with no restriction on subpage
mapping. In this mode, program:
— MD_CTR[TWAM] = 0
— Mx_CTR[PPM] = 0
— Mx_CTR[PPCS] = 0
For pages larger than 4-Kbyte, program subpage validity flags (bits 24-27) of the level-two
descriptor (and thus Mx_RPN) to 0b1111.
For 4-Kbyte pages, there are four separate entries with different encodings of subpage validity flags
(bits 24–27) of the level-two descriptor (and thus Mx_RPN) allowable for each entry.
For 4-Kbyte pages, the subpage validity flags (bits 24–27) of the level-two descriptor (and thus
Mx_RPN) can be different for each of the four separate entries.
In this mode, the MMU page tables defined for the software tablewalk resolve to a single level-two
descriptor entry for a 1-Kbyte page. This is done by allowing manipulation of the subpage validity
flags of a 4-Kbyte page. For example:
— To define a 4-Kbyte page with uniform protection, create four level-two descriptors for the
4-Kbyte page, each with subpage validity flags set to 0b1111. All other fields of the level-two
descriptors must also be the same for each of these entries.
Summary of Contents for PowerQUICC MPC870
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Page 118: ...MPC885 Overview MPC885 PowerQUICC Family Reference Manual Rev 2 1 20 Freescale Semiconductor ...
Page 158: ...The MPC8xx Core MPC885 PowerQUICC Family Reference Manual Rev 2 3 18 Freescale Semiconductor ...
Page 288: ...MPC885 PowerQUICC Family Reference Manual Rev 2 III 4 Freescale Semiconductor ...
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Page 1090: ...UTOPIA Interface MPC885 PowerQUICC Family Reference Manual Rev 2 43 8 Freescale Semiconductor ...
Page 1312: ...Byte Ordering MPC885 PowerQUICC Family Reference Manual Rev 2 A 8 Freescale Semiconductor ...
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Page 1370: ...MPC880 MPC885 PowerQUICC Family Reference Manual Rev 2 E 4 Freescale Semiconductor ...
Page 1386: ...Revision History MPC885 PowerQUICC Family Reference Manual Rev 2 I 2 Freescale Semiconductor ...