Kautalya Mishra
 Programmers when confronted with the problem of
having programs that were too big to fit in to the
available memory came up with a solution of splitting
the program in to pieces called ‘Overlays’.
 Overlay 0 would start running first followed by the
next overlay. The overlays were kept on the disk and
swapped in and out of the memory by the operating
system dynamically.
 Although the work of swapping Overlays in and out
was done by the system, the LABORIOUS work of
splitting the program in to Overlays had to be done by
the programmer.
 The basic idea of Virtual Memory is to keep only those
parts of the program currently in use in the memory and
the rest on the disk drive.
 For example: a 16M program can run on a 4M machine by
carefully choosing which 4M to keep in memory at each
instant, with pieces of the program being swapped
between disk and memory as needed.
 It can work in a Multiprogramming system, with bits and
pieces of many programs brought to the memory at once by
the Operating System. While a program is waiting for part
of itself to be brought in, it is waiting for I/O and cannot
run, so the CPU can be given to another process, the same
way as for any other multiprogramming system.
The position and function of the MMU
 When a page being referred to by the virtual address is
not loaded as a page frame in to the memory, a page
fault is occurred which causes the CPU to trap the
Operating System.
 The Operating System removes a page frame from the
memory and loads the new page in to the freed
location at the same time making appropriate changes
to the page table.
 There are a number of page replacement algorithms
available such as The Optimal Page Replacement Algo,
The Not Recently Used Page Replacement Algo, FIFO
etc.
Problems faced with Virtual Memory implementation:
• Possibly large page table
• Thrashing
Solution:
• Use multilevel page tables.
• Use TLB (Translation Lookaside Buffer) or
sometimes called ‘Associative Memory’.
• Page fault rates can be reduced by using a ‘Working
Set Model’ approach or a ‘Prepaging’ approach.
• Increase RAM!
 Some advanced systems give programmers some
control over the memory map so that they can used it
in a non-traditional way. It allows for the possibility of
sharing the same memory space between two or more
processes.
 Distributed Shared Memory – The idea here is to allow
multiple processes over a network to share a set of
pages, possibly, but not necessarily, as a single shared
linear address space
 Operating Systems – Design and Management
Andrew S. Tanenbaum and Albert S. Woodhull
 http://computer.howstuffworks.com/virtualmemory.htm
 http://en.wikipedia.org/wiki/Virtual_memory