Virtual Memory And Virtual Machine Concepts; Virtual Memory - Motorola MC68030 User Manual

Introduction

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

A system that supports virtual memory has a limited amount of high-speed physical memory
that can be accessed directly by the processor and maintains 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 suspended
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.
MOTOROLA MC68030 USER'S MANUAL 1-11