Virtual Memory

Virtual memory is the separation of user logical memory from physical memory.

It allows a large virtual memory to be provided to processes even if the physical memory is small.

It offloads programmers from having to worry about writing overlay codes.

Virtual memory is implemented using demand paging or demand segmentation.

They take the responsibility off the programmer for coding overlays, fixed, or variable partitions.

A process resides in secondary storage (usually a disk).

When a process needs to be executed, it is paged into memory.

No, only the pages that are needed are brought in by a lazy swapper.

Hardware support is needed to distinguish between pages that are in memory and those on disk.

Valid means the page is in memory and legal; invalid means the page is either not in the logical address space or is on the disk.

Avoids reading pages that will not be used, decreases paging time, and decreases the need for physical memory.

A pager decides which pages to bring into memory.

A pager is concerned with individual pages of a process, while a swapper manipulates the entire process.

The pager guesses which page will be used before the process is swapped out again.

Address spaces are divided into segments.

Each segment has a linear sequence of addresses from 0 to a maximum value.

Yes, segment lengths may change during execution.

Each segment has different addresses and can shrink and grow without affecting other segments.

Segment tables are used to keep track of segments.

Demand segmentation allocates memory into segments.

Demand paging focuses on fixed-size blocks (pages), while demand segmentation uses variable-sized logical units (segments).

Both are techniques for implementing virtual memory.