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Memory Management Unit
MOTOROLA
M68060 USER’S MANUAL
4-19
4.2.5 Table Search Accesses
Table search accesses bypass the data cache. No allocation is done and no cache search
is performed. Translation tables must not be placed in copyback space, since the normal
accesses which build the translation tables would be cached and not written to external
memory, but the processor only uses tables in external memory.
During a table search, the U- and M-bits of the table descriptors are examined. For any
access, if the U-bit is not set, the processor sets it using a complete read-modify-write
sequence with the LOCK pin asserted. LOCK is asserted in this case to avoid loss of the
status in certain multiprocessor applications which share translation tables. For a write
access, if the M-bit in the page descriptor is not set, and if the page is not write-protected
(W = 0) and the access is not a supervisor violation (for user accesses, the S-bit of the page
descriptor must be clear), then the M-bit is set using a simple write. The U- and M-bits are
Figure 4-15. Translation Table with Nonresident Tables
$15 $01
TABLE $3B
$3B
SUPERVISOR
TABLE $00 TABLE $00
UDT = INVALID
UDT = INVALID
UDT = INVALID
UDT = INVALID
UDT = RESIDENT
POINTER-LEVEL
TABLES
PAGE-LEVEL
TABLES
TABLE $7F
TABLE $15
TABLE $00
TABLE $1F
UDT = INVALID
UDT = INVALID
UDT = INVALID
UDT = INVALID
UDT = RESIDENT
SRP
0111011001010100001XXXXXXXXXXXXX
ROOT INDEX POINTER INDEX PAGE INDEX PAGE OFFSET
LOGICAL ADDRESS
$76543210 =
$3B $15 $01
TABLE ENTRY # =
$EC $54 $04
ADDRESS OFFSET =
FRAME ADDRESS
NONRESIDENT
(PAGED OR
UNALLOCATED)
ROOT-LEVEL
TABLES
NONRESIDENT
(PAGED OR
UNALLOCATED)
NONRESIDENT
(PAGED OR
UNALLOCATED)
NONRESIDENT
(PAGED OR
UNALLOCATED)
NONRESIDENT
(PAGED OR
UNALLOCATED)
NONRESIDENT
(PAGED OR
UNALLOCATED)
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