User's Manual

452 U3585-J-Z125-8-76
Glossary
paging in BS2000
The number of addressable virtual pages is usually greater than the number of page
frames available in main memory.
For the removal of pages which can no longer be kept in main memory, see “page fault”
on page 451.
The main memory management strategies are based on the assumption that the
programs (or tasks) will only address a limited set of the total number of virtual pages
within a specific period of time, i.e. that the programs are more or less local. This set of
pages, whose size varies dynamically, is the called the working set.
The PPC (planned page count) indicates how local a program or task is. This value is
provided by the memory management system before task activation and - because it is
a measure of the intensity of memory utilization by the task - is used as a task activation
criterion.
The PPC value for the next activation phase depends on program behavior during the
preceding activation phase.
While the task is active, the PPC value can be modified in accordance with program
behavior.
The number of pages used by a task is called UPG (USED PAGE COUNT).
There are three main memory (real memory) management methods:
1. Task-local management
This means that, when there is a shortage of free page frames, the task’s pages are
checked first and pages used by other tasks are removed from main memory only
in exceptional cases.
In determining the pages to be removed, the LRU (LEAST RECENTLY USED)
principle is used, i.e. those task pages are removed first which have not been used
longest.
With this method the UPG is always less than or equal to the PPC because the
number of main memory pages assigned to a task depends on the size of its
working set (PPC). Pages not included in the working set are always removed.
These checks are made very often to ensure high PPC computation precision.
2. System-global management
This means that, when determining the pages which should be removed because
of a shortage of free real-memory page frames, all main memory pages are
checked - in contrast to task-local management (see above).
As in task-local management, the LRU principle is used for determining the pages
to be removed from main memory:
To this end the main memory pages are sorted according to their access times.