55 lines
2.2 KiB
Markdown
55 lines
2.2 KiB
Markdown
---
|
|
tags:
|
|
- memory
|
|
- Linux
|
|
- kernel
|
|
---
|
|
|
|
# Virtual memory
|
|
|
|
Virtual memory is an abstracted and idealised representation of the physical
|
|
memory capacity of a machine that is presented to user space for its memory
|
|
operations.
|
|
|
|
When an OS implements virtual memory, [processes](./Processes.md) in
|
|
[user space](./User_Space.md) cannot directly read or write to the actual
|
|
memory. Instead they execute memory operations against virtual memory and the
|
|
kernel translates these into the 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](./Memory_addresses.md) 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 also possible for the kernel to present user space with an available
|
|
virtual memory capcacity that 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
|
|
|
|
Furthermore the kernel itself utilises virtual memory. The difference with
|
|
kernel virtual memory is that it has a different range of virtual addresses to
|
|
work with than user space virtual memory.
|
|
|
|
Also, unlike user address space, the kernel has access to everything running in
|
|
kernel address space. Processes in user address space are partitioned from each
|
|
other with separate address spaces that cannot interact.
|
|
|
|
// Next: the kernel also uses virtual memory however isn't also responsible for
|
|
the appportioning of virtual memory. Confused.
|
|
|
|
|