MC68030 Datasheet, PDF (331/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
Limit
Specifies a maximum or minimum value for the index to be used at the next level of table
search (the function code level cannot be limited). To suppress the limit function, the L/U
bit is cleared and the limit field is set to ones ($7FFF in the word containing both fields),
or the L/U bit is set and the limit field is cleared ($8000 in that word).
Descriptor Type (DT)
Specifies the type of descriptor contained in either the root pointer or in the first level of
the translation table identified by the root pointer. The values are:
$0 INVALID
This value is not allowed at the root pointer level. When a root pointer register is
loaded with an invalid root pointer descriptor, an MMU configuration exception is
taken.
$1 PAGE DESCRIPTOR
A translation table for this root pointer does not exist. The MC68030 internally cal-
culates an ATC entry (page descriptor) for accesses using this root pointer within
the current page by adding (unsigned) the value in the table address field to the
incoming logical address. This results in direct mapping with a constant offset (the
table address). For this case, the processor performs a limit check, regardless of
the state of the FCL bit in the TC register.
$2 VALID 4 BYTE
The translation table at the root of the translation tree contains short-format de-
scriptors. The MC68030 must scale the table index for this level of the table
search by 4 bytes to access the next descriptor.
$3 VALID 8 BYTE
The translation table at the root of the translation tree contains long-format de-
scriptors. The MC68030 must scale the table index for this level of the table
search by 8 bytes to access the next descriptor.
Table Address
Contains the physical base address (in bits 31-4) of the translation table at the root
pointer level. When the DT field contains $1, the value in the table address field is the
offset used to calculate the physical address for the page descriptor. The table address
field can contain zero (for zero offset).
Unused
Bits 3-0 of the root pointer are not used and are ignored when written. All other unused
bits must always be zeros.
MOTOROLA
MC68030 USER’S MANUAL
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