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.

During the boot_process, it runs in physical memory mode however it rapidly creates a 1:1 mapping of physical to virtual memory for itself. This direct mapping allows the kernel to easily access any physical memory location as needed, whilst retaining the benefits of virtual memory.

With its own virtual memory established, it can then build additional virtual memory structures for itself and user processes.

While the kernel uses virtual memory it can also access physical memory at any time wherease user space processes can only ever access 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.

In addition to being able to access its own virtual memory (and physical memory, as required) the kernel can also access any user processes’ virtual address.

This said, the virtual memory space of the kernel and the virtual memory space of the user processes are distinct. It is not the case that the kernel is superset of all available virtual memory. It can access user space virtual memory because it sets up the tables and locations, not because it is a subset of its own virtual memory.