Page 136 - A Practical Guide from Design Planning to Manufacturing

Computer Architecture 109

always be located in the exact same addresses every time it ran. This
created problems when running the same program on computers with
varying amounts of memory. A program compiled assuming a certain
amount of memory might try to access more memory than the user’s
computer had. If instead, the program had been compiled assuming a
very small amount of memory, it would be unable to make use of extra
memory when running on machines that did have it.
Even more problems occurred when trying to run more than one pro-
gram simultaneously. Two different programs might both be compiled
to use the same memory addresses. When running together they could
end up overwriting each other’s data or instructions. The data from one
program read as instructions by another could cause the processor to do
almost anything. If the operating system were one of the programs over-
written, then the entire computer might lock up.
Virtual memory fixes these problems by translating each address
before memory is accessed. The address generated by the program using
the available addressing modes is called the virtual address. Before
each memory access the virtual address is translated to a physical
address. The translation is controlled by the operating system using a
lookup table stored in memory.
The lookup table needed for translations would become unmanageable
if any virtual address could be assigned any physical address. Instead,
some of the least significant virtual address bits are left untranslated.
These bits are the page offset and determine the size of a memory page.
The remaining virtual address bits form the virtual page number and
are used as an index into the lookup table to find the physical page
number. The physical page number is combined with the page offset to
make up the physical address.
The translation scheme shown in Fig. 4-3 allows every program to
assume that it will always use the exact same memory addresses, it is
the only program in memory, and the total memory size is the maximum
amount allowed by the virtual address size. The operating system deter-
mines where each virtual page will be located in physical memory. Two
programs using the same virtual address will have their addresses

Virtual page number Page offset Virtual address

Translation

Physical page number Page offset Physical address
Figure 4-3 Virtual to physical address translation.