Virtual Memory: Logical vs Physical Addresses and Address Translation

Virtual memory keyword (VM) allows each process to see its own private, large address space, even though the machine has limited physical RAM keyword (PM). The operating system and hardware cooperate to translate each program’s logical (virtual) address into a physical address used to access RAM, typically using paging and the Memory Management Unit (MMU) keyword.2

A logical address is what the program generates (e.g., instruction/data references). A physical address is where the MMU ultimately sends the request to the memory system. In a paged system, a logical address is split into a page number and page offset; the page number is translated through page tables into a page frame number, then recombined with the same offset to form the physical address.

Mermaid view of the translation pipeline:

This translation is the core role of virtual memory: memory references remain simple for software (use virtual addresses), while the OS can map, protect, and swap/evict pages transparently.3

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Logical vs physical address: what changes, what stays

In a typical paged architecture, the logical address structure is:

• Logical address = $(\text{page number}, \text{page offset})$
• Physical address = $(\text{page frame number}, \text{page offset})$

Only the page number changes during translation; the page offset remains the same because the page and frame sizes match.

A concrete decomposition helps: if pages are size $2^k$, then the lowest $k$ bits of the logical address are the page offset, and the remaining bits identify the virtual page. After translation, the virtual page number becomes a physical frame number, and the offset is appended unchanged.2

Mermaid diagram of bit decomposition + remapping:

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How virtual memory works (paging, page tables, and page faults)

At a high level, virtual memory is implemented by:

1. Dividing virtual memory into pages and physical memory into frames keyword.
2. Maintaining a page table that maps each virtual page to a physical frame (or marks it as not present) keyword.2
3. On each memory access, the MMU consults the page table (often via a TLB first) to find the physical frame for the virtual page.2
4. If the referenced page is not present, the CPU triggers a page fault, and the OS loads the needed page from disk (or zero-fills for demand-zero pages).2

Important: page faults are a mechanism for demand paging, meaning pages are brought into RAM only when first used.

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Role of logical address in OS features: isolation and protection

Virtual memory makes logical addresses a natural boundary for isolation:

• Per-process address spaces: each process’s logical addresses map to different physical frames, preventing unintended access.2
• Memory protection: page tables store permission metadata (e.g., read/write/execute, user/supervisor). The MMU uses it during translation to enforce access control.2
• Contiguity illusion: a process can use a contiguous virtual address range even when physical frames are scattered.

These properties emerge because software typically reasons in terms of logical addresses while hardware enforces mappings at translation time.2

Role of physical address: resource location and system-level control

Physical addresses represent actual RAM locations. Their role includes:

• Mapping outcomes: the page table (indirectly) determines which physical frames back each logical page.
• Sharing and copy-on-write (conceptual): multiple logical pages may map to the same physical frame depending on OS policies (e.g., during program start, memory sharing, or copy-on-write).
• Accounting and performance: the OS can measure physical memory usage, decide evictions, and manage free frames because physical addresses correspond to real resources.2

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