INTRODUCTION:
Virtual memory managers provide an abstraction of the physical memory by creating a virtual address space in the secondary memory and then automatically determining the part of the address space to be loaded into primary memory at any given time. The system handles the transfer of address space blocks back and forth between primary and secondary memories without manual intervention.
CONCEPTS:
The concept of virtual memory is implemented by demand paging. In Virtual memory systems, when a new process is initiated, the system loader must load at least one page into real memory.
i.e. the page containing the execution start point of the process. When execution of the process commences, execution will proceed through subsequent instructions beyond the start point. This will continue as long as memory references generated by that page are also within the same page. As soon as the next logical address generated by the process falls out in the range outside the current page boundary, an interrupt occurs.
PAGE FAULT:
That is when the page whose address is generated by the CPU is not available, an interrupt to Os occurs. This interrupt is called “Page Fault”, which effectively asks for the page referenced to be loaded- hence the term demand paging is used in context of Virtual memory. The system loader tries to oblige by loading the requested page into a free real memory page frame .
When this is accomplished, execution can continue. By a series of page faults occurring in this way, a subset of the pages of a process will gradually accumulate in real memory; this subset is termed as resident set of the process.
RESULT:
1.When the process terminates, the operating system releases all the pages belonging to the process, making them available to other processes. Thus this whole process requires a sufficient amount of disk memory to roll out the partially executed processes and making them available later on.
2.This processes of rolling out/in the processes or the pages, is called Swapping. The record of the pages which are in the memory at a particular point of time is maintained in a table called PMT (Page Map Table). As the page table is referred for each instruction, it is usually kept in a fast speed associative memory to increase the system performance.
Manjeet kaur,Dr.Vikram Singh ,PH.D,NET/JRF,M.SC.
Tags: Increase, loaded, Memory, Performance, Research, Virtual
