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MOTOROLA M68040 USER'S MANUAL 3- 19
ROOT-LEVEL
TABLES
POINTER-LEVEL
TABLES
PAGE-LEVEL
TABLES
0111011001010100001XXXXXXXXXXXXX
ROOT INDEX POINTER INDEX PAGE INDEX PAGE OFFSET
LOGICAL ADDRESS
$76543210 =
$3B $15 $01
TABLE ENTRY # =
TABLE $3B TABLE $15
TABLE $00 TABLE $00 TABLE $00
$3B
$EC $54 $04
ADDRESS OFFSET =
FRAME ADDRESS*
ROOT POINTER
$15 $01
TASK A
TASK B
ROOT POINTER
W-BIT CLEAR
* Page frame address shared by task A and B; write protected from task A.
W-BIT SET
$00003000
Figure 3-15. Translation Table Using Shared Tables
3.2.4.3 TABLE PAGING. The entire translation table for an active task need not be
resident in main memory. In the same way that only the working set of pages must be
allocated in main memory, only the tables that describe the resident set of pages need be
available. Placing the invalid code ($0 or $1) in the UDT field of the table descriptor that
points to the absent table(s) implements this paging of tables. When a task attempts to
use an address that an absent table would translate, the M68040 is unable to locate a
translation and takes access error exception when the execution unit retries the bus
access that caused the table search to be initiated.
The operating system determines that the invalid code in the descriptor corresponds to
nonresident tables. This determination can be facilitated by using he unused bits in the
descriptor to store status information concerning the invalid encoding. The M68040 does
not interpret or modify an invalid descriptor’s fields except for the UDT field. This
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