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1.6
VIRTUAL MEMORY AND VIRTUAL MACHINE CONCEPTS
The full addressing range of the MC68030 is 4 Gbytes (4,294,967,296 bytes)
in each of eight address spaces. Even though most systems implement a
smaller physical memory, the system can be made to appear to have a full
4 Gbytes of memory available to each user program by using virtual memory
techniques.
In a virtual memory system, a user program can be written as if it has a large
amount of memory available, when the physical memory actually present is
much smaller. Similarly, a system can be designed to allow user programs
to access devices that are not physically present in the system, such as tape
drives, disk drives, printers, terminals, and so forth. With proper software
emulation, a physical system can appear to be any other M68000 computer
system to a user program, and the program can be given full access to all
of the resources of that emulated system. Such an emulated system is called
a virtual machine.
1.6.1
Virtual Memory
1-12
A system that supports virtual memory has a limited amount of high-speed
physical memory that can be accessed directly by the processor and main-
'tains an image of a much larger virtual memory on a secondary storage
device such as a large-capacity disk drive. When the processor attempts to
access a location in the virtual memory map that is not resident in physical
memory, a page fault occurs. The access to that location is temporarily sus-
pended while the necessary data is fetched from secondary storage and
placed in physical memory. The suspended access is then either restarted
or continued.
The MC68030 uses instruction continuation to support virtual memory. When
a bus cycle is terminated with a bus error, the microprocessor suspends the
current instruction and executes the virtual memory bus error handler. When
the bus error handler has completed execution, it returns control to the
program that was executing when the error was detected, reruns the faulted
bus cycle (when required), and continues the suspended instruction.
MC68030 USER'S MANUAL
MOTOROLA
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