40 lines
1.5 KiB
Markdown
40 lines
1.5 KiB
Markdown
|
---
|
||
|
|
tags:
|
||
|
|
- memory
|
||
|
|
- Linux
|
||
|
|
---
|
||
|
|
|
||
|
|
# Virtual memory and the Memory Management Unit
|
||
|
|
|
||
|
|
## What is virtual memory?
|
||
|
|
|
||
|
|
Virtual memory is an abstraction of physical memory capacity and allocation that
|
||
|
|
is accessible to user space. The kernel handles physical memory allocation and
|
||
|
|
presents this to user space as a simplified and idealised representation of the
|
||
|
|
available memory of the system.
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
When a process writes or reads from a virtual memory address this does not
|
||
|
|
directly refer to a hardware memory location. The kernel translates this into a
|
||
|
|
physical memory address but this is opaque to the user space process. In fact,
|
||
|
|
the physical memory addresses could be distributed accross multiple
|
||
|
|
non-contiguous locations such as cache and swap memory, not just DRAM.
|
||
|
|
|
||
|
|
Although the physical memory may be distributed and non-contiguous, from the
|
||
|
|
viewpoint of user space, the available virtual memory is contiguous. Each user
|
||
|
|
space process is presented with the same range of available memory addresses and
|
||
|
|
the same total capacity.
|
||
|
|
|
||
|
|
Because this is virtual, there is no risk of one process reading or overwriting
|
||
|
|
the address of another. The same virtual address for multiple programs maps to
|
||
|
|
different physical addresses.
|
||
|
|
|
||
|
|
// Next: more memory offered than is physically available.
|
||
|
|
|