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MOTOROLA M68040 USER'S MANUAL 3- 17
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
SUPERVISOR
MODE
TABLE $00
TABLE $7F TABLE $1F
TABLE $00 TABLE $00
$3B
$EC $54 $04
ADDRESS OFFSET =
$00001800 $00003000
FRAME ADDRESS
SRP
$15 $01
Figure 3-13. Example Translation Table
Using the indirection capability, single entries or entire tables can be shared between
multiple tasks. Figure 3-14 illustrates two tasks sharing a page using indirect descriptors.
When the M68040 has completed a normal table search, it examines the PDT field of the
last entry fetched from the page tables. If the PDT field contains an indirect ($2) encoding,
it indicates that the address contained in the highest order 30 bits of the descriptor is a
pointer to the page descriptor that is to be used to map the logical address. The processor
then fetches the page descriptor from this address and uses the physical address field of
the page descriptor as the physical mapping for the logical address.
The page descriptor located at the address given by the indirect descriptor must not have
a PDT field with an indirect encoding (it must be either a resident descriptor or invalid).
Otherwise, the descriptor is treated as invalid, and the M68040 creates an ATC entry with
a signaled error condition (R-bit in MMUSR is clear).
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