MC68030 Datasheet, PDF (320/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
(UNABLE TO LOCATE ART)
Figure 9-24. Example Translation Tree with Nonresident Tables
9.5.3.5 DYNAMIC ALLOCATION OF TABLES. Similar to the case of paged tables, it is not
required that a complete translation tree exist for an active task. The translation tree can be
dynamically allocated by the operating system based on requests for access to particular
areas.
As in the case of demand paging, it is difficult, if not impossible, to predict the areas of
memory that are used by a task over any extended period of time. Instead of attempting to
predict the requirements of the task, the operating system performs no action for a task until
a demand is made requesting access to a previously unused area or an area that is no
longer resident in memory. This same technique can be used to efficiently create a
translation tree for a task.
For example, consider an operating system that is preparing the system to execute a
previously unexecuted task that has no translation tree. Rather than guessing what the
memory usage requirements of the task are, the operating system creates a translation tree
for the task that maps one page corresponding to the initial value of the program counter for
that task, and possibly, one page corresponding to the initial stack pointer of the task. All
other branches of the translation tree for this task remain unallocated until the task requests
access to the areas mapped by these branches. This technique allows the operating system
to construct a minimal translation tree for each task, conserving physical memory utilization
and minimizing operating system overhead.
9.5.4 Detail of Table Search Operations
The table search operations described in this section are shown in detail in Figures 9-25-9-
29.
9-32
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