eolas/zk/VirtualMemory.md

---
tags:
  - memory
  - Linux
---

# Virtual memory

Virtual memory is an abstracted and idealised representation of the physical
memory capacity of the machine that is presented to user space for its memory
operations.

When an OS implements virtual memory, processes in user space cannot directly
read or write to the actual memory. Instead they execute memory operations
against virtual memory and the kernel translates these into actual operations
against the memory hardware.

The main benefits:

- User mode processes do not have to be concerned with the physical memory
  management
- There is a buffer between user mode processes and physical memory, meaning
  that memory cannot be accidentally corrupted by other processes in user space.

Because the physical memory is abstracted, it can be the case that the physical
memory addresses are non-contiguous or even distributed accross different
hardware components (such as the cache and swap). Despite this, the memory
addresses will appear contiguous in virtual memory. Each user space process is
presented with the same range of available memory addresses and the same total
capacity.

It is possible for the kernel to present user space with an available virtual
memory capcacity that actually exceeds the current physical capacity of the
machine:

> _It's possible for the kernel and all running processes to request more bytes
> of virtual memory than the total size of RAM. In that situation, the OS can
> move move bytes of memory to secondary storage to make room in RAM for newly
> requested memory._

_How Computers Really Work_ (2021) p.206

// Next: the kernel also uses virtual memory however isn't also responsible for
the appportioning of virtual memory. Confused.

![](/img/virtual-memory-diagram.jpg)