MC68030 Datasheet, PDF (306/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
$2 VALID 4 BYTE
This code specifies that the next table to be accessed contains short-format de-
scriptors. The MC68030 multiplies the index for the next table by four to access the
next descriptor. (Short-format descriptors must be long-word aligned.) When used
in a page table (bottom level of an translation tree), this code identifies an indirect
descriptor that points to a short-format page descriptor.
$3 VALID 8 BYTE
This code specifies that the next table to be accessed contains long-format de-
scriptors. The MC68030 multiplies the index for the next table by eight to access
the next descriptor. (Long-format descriptors must be quad-word aligned.) When
used in a page table (bottom level of an address translation tree), this code identi-
fies an indirect descriptor that points to a long-format page descriptor.
U
This bit is automatically set by the processor when a descriptor is accessed in which the
U bit is clear except after a supervisor violation is detected. In a page descriptor table, this
bit is set to indicate that the page corresponding to the descriptor has been accessed. In
a pointer table, this bit is set to indicate that the pointer has been accessed by the
MC68030 as part of a table search. Note that a pointer may be fetched, and its U bit set,
for an address to which access is denied at another level of the tree. Updates of the U bit
are performed before the MC68030 allows a page to be accessed. The processor never
clears this bit.
WP
This bit provides write protection. The states of all WP bits encountered during a table
search are logically ORed, and the result is copied to the ATC entry at the end of a table
search for a logical address. During a table search for a PTEST instruction, the processor
copies this result into the MMU status register (MMUSR). When WP is set, the MC68030
does not allow the logical address space mapped by that descriptor to be written by any
program (i.e., this protection is absolute). If the WP bit is clear, the MC68030 allows write
accesses using this descriptor (unless access is restricted at some other level of the trans-
lation tree).
CI
This bit is set to inhibit caching of items within this page by the on-chip instruction and data
caches and, also, to cause the assertion of the CIOUT signal by the MC68030 for bus cy-
cles accessing items within this page.
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MC68030 USER’S MANUAL
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