MC68030 Datasheet, PDF (313/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
(UNABLE TO LOCATE ART)
Figure 9-19. Simplified Table Search Flowchart
Table 9-2. Translation Tree Selection
FC2
SRE
0
0
0
1
1
0
1
1
Translation Table Root Pointer
CRP
CRP
CRP
SRP
The table search procedure uses physical addresses to access the translation tables. The
read-modify-write (RMC) signal is asserted on the first bus cycle of the search and remains
asserted throughout, ensuring that the entire table search completes without interruption.
The first bus cycle of the search uses the table address field of the appropriate root pointer
as the base address of the first table. The low-order bits of the address are supplied by the
logical address. The table is indexed by either the function code or the set of logical address
bits defined by the TIA field of the TC register. The FCL field of the TC register determines
whether or not the function code is used. In either case, the descriptor-type field of the root
pointer selects the scale factor (or multiplier) for the index.
The first access obtains a descriptor. If the descriptor is a table descriptor, the MC68030
again accesses memory. The next access uses the table address in the descriptor as the
base address for the next table. The low-order bits of the address are supplied by the logical
address. The table is indexed by a set of bits from the logical address using a scale factor
determined by the descriptor type code in the descriptor. If the first table access used the
function code, the second access uses the bits selected by the TIA field of the TC register.
Otherwise, the second access uses the bits selected by the TIB field.
Additional accesses are performed, using the logical address bits specified in TIB, TIC, or
TID in order, until an access obtains a page descriptor or an invalid descriptor or until a limit
violation occurs. At this point, whether or not all levels of the address table have been
accessed, the table search is over. The page descriptor contains the physical address and
other information needed for the ATC entry; the MC68030 creates the ATC entry and retries
the original bus access.
Figure 9-20 shows a table search using the function code and all four TIx fields.
MOTOROLA
MC68030 USER’S MANUAL
9-25