MC68030 Datasheet, PDF (349/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

Memory Management Unit
An operating system can use an early termination page descriptor to map a contiguous
block of memory for each task (both program and data). The tasks can be relocated by
changing the physical address portion of the descriptor. This scheme is useful when the
tasks in a system consist of one or a few sequential blocks of memory that can be swapped
as a group. The operating system memory map can treat the entire address space within
these blocks as a uniform virtual space available for all tasks. The system only requires one
translation table; by the use of limit fields and early termination page descriptors, it maps
complete segments of memory.
9.9.3.5 INDIRECT DESCRIPTORS. An indirect descriptor is a table descriptor residing in a
page table. It points to another page descriptor in the translation tree. Using an indirect
descriptor for a page makes the page common to several tasks. History information for a
common page is maintained in only one descriptor. Access to the page sets the used (U)
bit, and a write operation to the page sets the M (modified) bit for that page. When the
operating system is searching for an available page, it simply checks the page table
containing the descriptor for the common page to determine its status. With other methods
of page sharing, the system would have to check page tables for all sharing tasks to
determine the status of the common page.
9.9.3.6 USING UNUSED DESCRIPTOR BITS. In general, the bits in the unused fields of
many types of descriptors are available to the operating system for its own purposes. The
invalid descriptor, in particular, uses only two bits of the 32 (short) or 64 (long) bits available
with that format. An operating system typically uses these fields for the software flags,
indicating whether the virtual address space is allocated and whether an image resides on
the paging device. Also, these fields often contain the physical address of the image.
The operating system often maintains information in an unused field about a page resident
in memory. This information may be an aging counter or some other indication of the page's
frequency of use. This information helps the operating system to identify the pages that are
least likely to impact system performance if they are reallocated. The system should first use
physical page frames that are not allocated to a virtual page. Next it should use pages with
the longest time since the most recent access. Pages that do not have the M (modified) bit
set should be taken first, since they do not need to be copied to the paging device (the
existing image remains valid).
MOTOROLA
MC68030 USER’S MANUAL
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