Translation Table Using Indirect Descriptors - Motorola M68060 User Manual

Memory Management Unit
4.2.4 Variations in Translation Table Structure
Several aspects of the MMU translation table structure are software configurable, allowing
the system designer flexibility to optimize the performance of the MMUs for a particular sys-
tem. The following paragraphs discuss the variations of the translation table structure.
4.2.4.1 INDIRECT ACTION. The MC68060 provides the ability to replace an entry in a page
table with a pointer to an alternate entry. The indirection capability allows multiple tasks to
share a physical page while maintaining only a single set of history information for the page
(i.e., the modified indication is maintained only in the single descriptor). The indirection
capability also allows the page frame to appear at arbitrarily different addresses in the logical
address spaces of each task.
Using the indirection capability, single entries or entire tables can be shared between multi-
ple tasks. Figure 4-13 illustrates two tasks sharing a page using indirect descriptors.
$76543210 =
0 1 1 1 0 1 1 0 0 1 0 1 0 1 0 0 0 0 1 X X X X X X X X X X X X X
TABLE ENTRY # =
ADDRESS OFFSET =
ROOT POINTER
TASK A
ROOT POINTER
TASK B
Figure 4-13. Translation Table Using Indirect Descriptors
4-16
ROOT INDEX POINTER INDEX
$15
$3B
$EC $54
TABLE $00
$3B $00001800
ROOT-LEVEL
TABLES
M68060 USER'S MANUAL
LOGICAL ADDRESS
PAGE INDEX
$01
$04
TABLE $00
TABLE $3B
$15 $00003000
TABLE $7F
POINTER-LEVEL
TABLES
PAGE OFFSET
TABLE $00
TABLE $15
$01
$80000010
TABLE $1F
FRAME ADDRESS
PAGE-LEVEL
TABLES
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