Motorola MC68030 User Manual page 313

nation of the table search and contains the physical address of an area in
memory that contains page frames corresponding to contiguous logical ad-
dresses (Refer to 9.5.3.1 EARLY TERMINATION AND CONTIGUOUS MEM-
ORY).
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 de-
scriptor 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 descrip-
tors 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 de-
scriptors.
All descriptors contain a descriptor type (DT) field, which identifies the de-
scriptor 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 sig-
nificant (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 trans-
lation table.
MOTOROLA MC68030 USER'S MANUAL
9-11