MC68030 Datasheet, PDF (307/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
L/U
This bit specifies the type of limit in the LIMIT field. When the L/U bit is set, the LIMIT field
contains the unsigned lower limit; the index value for the next level of the tree must be
greater than or equal to the value in the LIMIT field. When the bit is cleared, the limit is an
unsigned upper limit, and the index value must be less than or equal to the LIMIT. An out-
of-bounds access causes the B bit in the ATC entry for the address to be set and causes
the table search to abort.
LIMIT
This 15-bit field contains a limit to which the index portion of an address is compared to
detect an out-of-bounds index. The limit check applies to the index into the table at the
next lower level of the translation tree. If the descriptor is an early termination page de-
scriptor, the limit field is still used as a check on the next index field of the logical address.
M
This bit identifies a modified page. The MC68030 sets the M bit in the corresponding page
descriptor before a write operation to a page for which the M bit is zero, except after a
descriptor with the WP bit set is encountered, or after a supervisor violation is encoun-
tered. An access is considered to be a write for updating purposes if either the R/W or
RMC signal is low. The MC68030 never clears this bit.
PAGE ADDRESS
This 24-bit field contains the physical base address of a page in memory. The low-order
bits of the address are supplied by the logical address. When the page size is larger than
256 bytes, one or more of the least significant bits of this field are not used. The number
of unused bits is equal to the PS field value in the TC register minus eight.
S
This bit identifies a pointer table or a page as a supervisor only table or page. When the
S bit is set, only programs operating at the supervisor privilege level are allowed to access
the portion of the logical address mapped by this descriptor. If this bit is clear, accesses
using this descriptor are not restricted to supervisor-only unless the access is restricted
by some other level of the translation tree.
TABLE ADDRESS
This 28-bit field contains the physical base address of a table of descriptors. The low-or-
der bits of the address are supplied by the logical address.
MOTOROLA
MC68030 USER’S MANUAL
9-19