Virtual Memory

Definition: Virtual memory is a memory management technique that makes an application think it has contiguous, large private address space, independent of physical RAM size, by automatically moving data between RAM and disk (swap/backing store) via paging/segmentation. It gives illusion of virtual size >> physical size.

OS + MMU provide address translation per reference; only active working set stays in RAM, rest on disk, loaded on demand (page fault).

Why Virtual Memory?

• Run programs larger than physical memory.
• Multiprogramming isolation & protection (each process virtual space).
• Simplified linking/loading, no external fragmentation (with paging).

Logical (Virtual) vs Physical Address - Roles

Logical / Virtual Address - Generated by CPU before translation.

• Address seen by process/compiler: e.g. $32$-bit virtual → $4GB$ per process ($2^{32}$), $48$-bit in x86-64.
• Partitioned: | Virtual Page Number (VPN) | Offset | (offset = page size bits; e.g. $4KB$ page → $12$ bits offset).
• Never used to access RAM directly; goes through MMU.
• Contiguous from process view; sparse vs file mapping allowed.
• Protect bits checked per virtual page.

Physical Address - Actual address on RAM bus after translation.

• After MMU/TLB lookup: | Physical Frame Number (PFN) | Offset |.
• Range limited by installed RAM: e.g. $1GB$ RAM → $30$-bit physical.
• Used by cache/memory controller.
• Multiple virtual pages (even from different processes) can map to same physical frame (shared memory) or one virtual page can map nowhere (paged out → page fault).

Translation flow:

CPU issues VA ──► TLB (VPN→PFN) hit? ──► PA to cache/RAM
                  │ miss
                  └─► Page Table walk (in RAM) ──► PFN + valid bit
                               │ invalid (not resident)
                               └─► Page Fault → OS loads frame from disk, updates table, retries

Example: VA 0x00403F12 ($VPN=0x403, offset=0xF12$) with $4KB$ pages, page table maps $VPN\ 0x403 → PFN\ 0x1A2$ → PA 0x1A2F12.

Key distinction: Logical addresses give program portability & protection; physical addresses give actual storage location. Compiler/linker deals in logical; MMU/OS manages physical. Without translation, process would see raw RAM limits and fragmentation.

Modern extensions: Demand paging, copy-on-write, memory-mapped files, TLB, multi-level page tables all build on this split.