Motorola MC68030 User Manual page 342

III
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 ca'se 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 op-
erating system performs no action for a task until a demand is made re-
questing 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
I
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 tech-
nique allows the operating system to construct a minimal transla,tion tree
for each task, conserving physical memory utilization and minimizing op-
erating system overhead.
9.5.4 Detail of Table Search Operations
9-40
The table search operations described in this section are shown in detail in
Figures 9-25-9-29.
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