Using Access Protection Groups - Freescale Semiconductor MPC850 User Manual

Using Access Protection Groups

MSR[PR] M_CASID[CASID]
32-Entry Fully Associative TLB
Translation
Enabled
32-Bit Physical Address
Figure 8-3. Effective-to-Physical Address Translation for 4-Kbyte Pages Block
A TLB hit occurs if the incoming EA matches the EPN and M_CASID[CASID] matches
the ASID field in a valid TLB entry, and if the subpage validity flag is set for the subpage
that the incoming EA points to. If a hit is detected, the contents of the physical page number
are concatenated with the appropriate number of lsbs from the EA to form the physical
address sent to the cache and memory system.
8.4 Using Access Protection Groups
Access control is assigned on a page-by-page basis; additional control is provided on a
protection group basis. Each TLB entry holds an access protection group (APG) number.
When a match is detected, the value of the matched entry's APG is used to index a field in
the access protection register (MI_AP or MD_AP) that defines access control for the
translation. Each Mx_AP contains 16 fields. The field content is used according to the group
protection mode.
To be consistent with the PowerPC OEA, the APG value should match the four msbs of the
effective page number. To override protection using APG, use it on blocks of addresses
which are defined by the 4 msbs of the effective page number. If APG is not to be used for
a particular block, set the GP for that block to 'client' in the Mx_AP register. To ignore it
globally, set all of the Mx_AP fields to 01. In PowerPC mode, each field holds the Kp and
Ks bits for the corresponding segment defined by the level-one table descriptor. In domain
manager mode, each field holds override information over the page protection setting—no
override, no access override, and free access override.
32-Bit EA
20-Bit
Page Byte
20-Bit
20-Bit
Physical Page Number Byte
Translation
Enabled
Diagram
MPC850 Family User's Manual
32-Bit Logical
Address
12-Bit
Page Protection
Free Access
Protection
Group Number
No Access
Protection
Lookup Table
Implementation-
Specific TLB
Miss Exceptions
to Core
Implementation-
Specific
Error Exceptions
to Core
Exception
Logic