MC68LC040RC25A Datasheet, PDF (73/442 Pages) Freescale Semiconductor, Inc – M68040 Users Manual

Freescale Semiconductor, Inc.
FOR TASK 'A'
URP FOR TASK 'A'
USER A LEVEL TABLE
•
•
•
TRANSLATION
TABLE FOR
TASK 'A'
FOR TASK 'B'
URP FOR TASK 'B'
USER A LEVEL TABLE
•
•
•
TRANSLATION
TABLE FOR
TASK 'B'
POINTER
COMMON SRP
SUPERVISOR A LEVEL TABLE
•
•
•
TRANSLATION
TABLE FOR
ALL SUPERVISOR
ACCESSES
Figure 3-17. Translation Table Structure for Two Tasks
3.2.6.3 WRITE PROTECT. The M68040 provides write protection independent of other
protection mechanisms. All table and page descriptors contain W-bits to protect areas of
memory from write accesses of any kind, including supervisor writes. An ATC descriptor
corresponding to the logical address is created with the W-bit set after the table search is
completed when a table search encounters a W-bit set in any table or page descriptor.
The subsequent retry of the write access results in an access error exception being taken.
The W-bit can be used to protect the entire area of memory defined by a branch of the
translation table or protect only one or more pages from write accesses. Figure 3-18
illustrates a memory map of the logical address space organized to use supervisor-only
and write-protect bits for protection. Figure 3-19 illustrates an example translation table for
this technique.
SUPERVISOR AND USER SPACE
THIS AREA IS SUPERVISOR ONLY, READ-ONLY
THIS AREA IS SUPERVISOR ONLY, READ/WRITE
THIS AREA IS SUPERVISOR OR USER, READ-ONLY
THIS AREA IS SUPERVISOR OR USER, READ/WRITE
Figure 3-18. Logical Address Map with Shared
Supervisor and User Address Spaces
3-24
M68040 USER'S MANUAL
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