MC68030 Datasheet, PDF (297/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
For example, a TC register in which the FCL bit is set to one, the TIA field contains five, the
TIB field contains nine, and the TIC and TID fields contain zero defines a three-level
translation tree. The top level is indexed by the function code, the next level by five logical
address bits, and the bottom level by nine logical address bits. If the TIC field contained nine
instead of zero, the translation tree would have four levels, and the two bottom levels would
each be indexed by 9-bit portions of the logical address.
The following equation for fields in the TC register must be satisfied:
IS+PS+TIA+TIB¹+TIC¹+TID¹ = 32
That is, every bit of the logical address either addresses a byte within the page, is part of the
index at some level of the address table, or is explicitly ignored by initial shift.
Table 9-1 lists the valid sizes of the table indexes at each of the levels indexed by the TIx
fields and the position of each table index within the logical address. When the function code
is also used to select a descriptor, a total of five levels can be defined by the logical address.
The function code lookup level and levels B, C, and D can be suppressed.
Field
A
B
C
D
Table 9-1. Size Restrictions
Starting Bit Position
31-IS
31-IS-TIA
31-IS-TIA-TIB
31-IS-TIA-TIB-TIC
Size Restrictions
1-15 (TIA must be greater than zero; minimum of two if TIB=0)
0-15
0-15 (ignored if TIB is zero)
31-IS-TIA-TIB-TIC 0-15 (ignored if TIB or TIC is zero)
9.1.2 Translation Table Descriptors
The address translation trees consist of tables of descriptors. These descriptors can be one
of four basic types: table descriptors, page descriptors (normal or early termination), invalid
descriptors, or indirect descriptors. Each of these descriptors has both a long-format and a
short-format representation.
A root pointer descriptor defines the root of a tree and can be a table descriptor or an early
termination page descriptor. A table descriptor points to a descriptor table in memory that
defines the next lower level in the translation tree. An early termination page descriptor
causes immediate termination of the table search and contains the physical address of an
area in memory that contains page frames corresponding to contiguous logical addresses
(Refer to 9.5.3.1 Early Termination and Contiguous Memory).
1.NOTE 1: If any of these fields are zero, the remaining fields are ignored.
MOTOROLA
MC68030 USER’S MANUAL
9-9