MC68030 Datasheet, PDF (298/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
Tables at intermediate levels of a translation tree contain descriptors that are similar to the
root pointer descriptors. They can contain table descriptors or early termination page
descriptors and can also contain invalid descriptors.
The descriptor tables at the lowest level of a translation tree can only contain page
descriptors, indirect descriptors, and invalid descriptors. A page descriptor in the lowest
level of a translation tree defines the physical address of a page frame in memory that
corresponds to the logical address of a page. An indirect descriptor contains a pointer to the
actual page descriptor and can be used when a single page descriptor is accessed by two
or more logical addresses.
To enhance the flexibility of translation table design, descriptors (except for root pointer
descriptors) can be either short or long format. The short-format descriptors consist of one
long word and have no index-limiting capabilities or supervisor-only protection. The long-
format descriptors consist of two long words and contain all defined descriptor fields for the
MC68030. The pointer and page tables can each contain either short- or long-format
descriptors, but no single table can contain both sizes. Tables at different levels of the
translation tree can contain different formats of descriptors. Tables at the same level can
also contain descriptors of different formats, but all descriptors in a particular pointer table
or page table must be of the same format. Figure 9-7 shows a translation tree that uses
several different format descriptors.
All descriptors contain a descriptor type (DT) field, which identifies the descriptor or specifies
the size of the descriptors in the table to which the descriptor points. It is always the two least
significant bits of the most significant (or only) long word of a descriptor.
Invalid descriptors can be used at any level of the translation tree except the root. When a
table search for a normal translation encounters an invalid descriptor, the processor takes
a bus error exception. The invalid descriptor can be used to identify either a page or branch
of the tree that has been stored on an external device and is not resident in memory or a
portion of the translation table that has not yet been defined. In these two cases, the
exception routine can either restore the page from disk or add to the translation table.
9-10
MC68030 USER’S MANUAL
MOTOROLA