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