Virtual memory is a technique that allows processes that may not be entirely in the memory to execute by means of automatic storage allocation upon request. The term virtual memory refers to the abstraction of separating LOGICAL memory--memory as seen by the process--from PHYSICAL memory--memory as seen by the processor. Because of this separation, the programmer needs to be aware of only the logical memory space while the operating system maintains two or more levels of physical memory space.
The virtual memory abstraction is implemented by using secondary storage to augment the processor's main memory. Data is transferred from secondary to main storage as and when necessary and the data replaced is written back to the secondary storage according to a predetermined replacement algorithm. If the data swapped is designated a fixed size, this swapping is called paging; if variable sizes are permitted and the data is split along logical lines such as subroutines or matrices, it is called segmentation. Some operating systems combine segmentation and paging. [D1] [S2]
The diagram illustrates that a program generated address ( 1 ) or "logical address" consisting of a logical page number plus the location within that page (x) must be interpreted or "mapped" onto an actual (physical) main memory address by the operating system using an address translation function or mapper ( 2 ). If the page is present in the main memory, the mapper substitutes the physical page frame number for the logical number ( 3 ). If the mapper detects that the page requested is not present in main memory, a fault occurs and the page must be read into a frame in main memory from secondary storage ( 4 , 5 ). [D2] pp. 161-165
What do you think are the main considerations in implementing this
virtual memory system?
