MC68030 Datasheet, PDF (350/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
An aging counter can be set up in an unused field of a page descriptor. The system can
periodically check the U (used) bit for the page and increment the count when the page has
not been used since the previous check. The system can identify the least recently used
page from the counts in the aging counter. When the counter for a page overflows, the
system can list the page in a queue of least recently used pages from which it chooses the
next page to be reallocated.
Many schemes afford the operating system designer a variety in selecting a page to be
taken. One operating system scans page tables, starting at the lowest priority task, looking
for aged pages to steal. Another system maintains a system-wide list of all page frames as
they are used and scans the list, starting at the oldest, to find a page to steal. A sophisticated
system keeps a working set model of active pages for each individual task. From this
information, it can swap a complete block of pages in and out with a single I/O operation.
The method chosen can have a dramatic impact on limiting page fault overhead in a heavily
used system.
9.10 AN EXAMPLE OF PAGING IMPLEMENTATION IN AN
OPERATING SYSTEM
This section describes an example operating system design that illustrates some of the
MMU features. The description suggests alternatives to provide variations of the design.
Memory management algorithms that can be implemented to derive the actual code are
shown. A bus error handler routine is shown also. Implementing the algorithms develops the
basic code for the memory management services of an operating system.
9.10.1 System Description
The example system has the ability to map a large virtual memory task space, which is
required for execution of predominantly numerically intensive processing tasks. Most of
these tasks do not need more than 16 Mbytes of memory, but the system can supply a larger
virtual memory space (as large as 496 Mbytes) to the occasional task that requires more.
The system uses the relatively large page size of 8K bytes to minimize thrashing and
translation table searches. With a larger page size, fewer descriptors can map a large area
of virtual memory. Also, in a given period of time, the MC68030 experiences fewer ATC
misses and performs fewer table searches. The larger page size requires the paging I/O
operations to transfer larger blocks of data, and sometimes only a small part of the page is
actually used. However, preliminary software model simulations show that 8K-byte pages
provide optimum performance for this type of processing.
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MC68030 USER’S MANUAL
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