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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 in user space cannot directly read or write to the physical 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 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
Virtual memory and the kernel
The kernel itself utilises virtual memory. The kernel virtual memory has a different range of virtual addresses to work with than user space virtual memory.
Unlike user space virtual memory, the kernel has access to everything running in kernel address space whereas 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.
// See Claude convo
