MC68030 Datasheet, PDF (314/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
(UNABLE TO LOCATE ART)
Figure 9-20. Five-Level Table Search
The MC68030 enforces a limit on the index value for the next level of a table search when
long-format descriptors are used.
The root pointer includes a limit field that applies when the function code lookup is not used
(the FCL bit of the TC register is zero). The index used to access the next level table is
compared to the contents of the limit field. The limit field effectively reduces the portion of
the address space to which a descriptor applies and also reduces the size of the translation
table. The index must reside within the range defined by the limit field. The limit can be a
lower limit or an upper limit, according to the L/U bit value. When the L/U bit is set, the limit
is a lower limit, and an index less than the limit is out of bounds. When the L/U bit is zero,
the limit is an upper limit, and an index greater than the limit is out of bounds. The limit field
is effectively disabled if L/U is set and the limit field contains zero or if L/U is clear and the
limit field contains $7FFF.
During a table search for an normal translation or a PLOAD instruction, if a limit violation is
detected, the ATC is loaded with an entry having the bus error (B) bit set. If a limit violation
is detected during a table search for a PTEST instruction, the invalid (I) and limit (L) bits are
set in the MMUSR.
During a table search, the U bit in each descriptor that is encountered is checked and set if
it is not already set. Similarly, when the table search is for a write access and the M bit of
the page descriptor is clear, the processor sets the bit if the table search does not encounter
a set WP bit or a supervisor violation. Since the read-modify-write (RMC) signal is asserted
throughout the entire table search operation, the read and write operations to update the
history bits are guaranteed to be uninterrupted.
A table search terminates successfully when a page descriptor is encountered. The
occurrence of an invalid descriptor, a limit violation, or a bus error also terminates a table
search, and the MC68030 takes an exception on the retry of the cycle because of these
conditions. The exception routine should distinguish between anticipated conditions and
true error conditions. The routine can correct an invalid descriptor that indicates a
nonresident page or one that identifies a portion of the translation table yet to be allocated.
A limit violation or a bus error due to a system malfunction may result in an error message
and termination of the task.
9-26
MC68030 USER’S MANUAL
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