SOCSAMS e-learning
VIRTUAL
MEMORY
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Introduction
To execute a program, the entire process should be
loaded into the main memory. Thus the size of the
process is limited to the size of the physical memory.
A memory management scheme called “Overlaying”
can be used to overcome this situation.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Overlaying
The programmer splits a program into smaller parts
called overlays in such a way that no two overlays are
required to be in main memory at the same time.
An overlay is loaded into memory only when it is
needed.
One major disadvantage of this technique is that it requires
major involvement of the programmer and time consuming.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Virtual Memory – overview
Virtual memory is a technique that allows execution of
a program that is bigger than the physical memory of
the computer system.
In this technique, the operating system loads only
those parts of program in memory that are currently
needed for the execution of the process. The rest part
is kept in the disk and is loaded into memory only
when needed.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Virtual Memory – overview
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Virtual Memory – overview
Virtual memory gives the illusion that the system has a
much larger memory than is actually available.
Virtual memory frees programs from the constraints of
physical memory limitation.
The virtual memory can be implemented by:
• Demand Paging.
• Demand Segmentation.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging
Demand paging is a system which a page is loaded
into the memory only when it is needed during
program execution. Pages that are never accessed are
never loaded into the memory.
A demand paging system combines the features of
paging with swapping. To facilitates swapping, the
entire virtual address space is stored contiguously on
the disk.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging
Whenever a process is to be executed, an area on the
disk is allocated to it on which pages are copied, called
swap space of the process. During the execution of a
process, whenever a page is required, it is loaded into
the main memory from the swap space. Similarly,
when a process is to be removed from main memory,
it is written back into the swap space if it has been
modified.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging
Other than swap space, some form of hardware
support is also needed to differentiate the pages that
are in memory from the ones on disk. For this, only an
additional bit valid is maintained in each page table
entry to indicate whether the page is in memory.
If a page is valid, that is it exists in the virtual address
space of the process, the associated valid bit is set to 1,
otherwise it is set to 0.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging
Whenever a process request for a page, the virtual
address is sent to MMU. The MMU checks the valid
bit in the page table entry of that page. If the valid bit
is 1, it is access as in paging. Otherwise, the MMU
raises an interrupt called page fault or a missing page
interrupt and the control is passed to the page fault
routine in the operating system.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging - Handing Page Fault
To handle the page fault, the page fault routine first of
all checks whether the virtual address for the desired
page is valid from its PCB stored in the process table.
If it is invalid, it terminates the process giving error.
Otherwise, it takes the following steps:
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging - Handing Page Fault
1. Location for a free page frame in memory and allocation it
to the process.
2. Swaps the desired page into this allocated page frame.
3. Update the process table and page table to indicate that the
page is in memory.
After performing these steps, the CPU restarts from the
instruction that it left off due to the page fault.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging – page-in
In demand paging, the process of loading a page in
memory is known as page-in operation instead of
swap-in. It is because the whole process is not loaded;
only some pages are loaded into the memory.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Demand Paging - Advantages
• It reduces the swap time since only the required pages are
swapped in instead of the whole process.
• It increases the degree of multiprogramming by reducing the
amount of physical memory required for a process.
• It minimizes the initial disk overhead as not all pages are to
be read initially.
• It does not need extra hardware support.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
COPY-ON-WRITE
A process may need to create several processes during its
execution and the parent and child processes have their own
distinct address spaces.
If the newly created process is the duplicate of the parent
process, it will contain the same pages in its address space as
that of parent. In this case, copying the parent’s address
space may be unnecessary.
To avoid copying, copy-on-write – a virtual memory technique
can be employed.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
COPY-ON-WRITE
In this technique, initially, parent and child processes are
allowed to share the same pages and these shared pages are
marked as copy-on-write pages.
Now, if either process attempts to write on a shared page, a
copy of that page is created for that process. Only the pages
that can be modified (containing data) are marked as copyon-write pages while the pages that cannot be modified
(containing executable code) are shared between parent and
child processes.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
COPY-ON-WRITE
To implement this, a bit is associated in page table entry of each
shared page to indicate that it is a copy-on-write page.
Whenever either process say, child process tries to modify a
page, the operating system creates a copy of that page, maps
it to the address space of the child process and turns off the
copy-on-write bit.
Now the child process can modify the copied page without
affecting the page of the parent process. Thus, it saves
memory.
Dept. of Computer Applications, MES College Marampally