77 lines
3.2 KiB
Markdown
77 lines
3.2 KiB
Markdown
---
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tags:
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- memory
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- Linux
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- kernel
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---
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# Virtual memory
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Virtual memory is an abstracted and idealised representation of the physical
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memory capacity of a machine that is presented to user space for its memory
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operations.
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When an OS implements virtual memory, [processes](Processes.md) in
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[user space](User_Space.md) cannot directly read or write to the physical
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memory. Instead they execute memory operations against virtual memory and the
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[kernel](The_kernel.md) translates these into the actual operations against
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the memory hardware.
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The main benefits:
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- User mode processes do not have to be concerned with the physical memory
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management
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- There is a buffer between user mode processes and physical memory, meaning
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that memory cannot be accidentally corrupted by other processes in user space.
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Because the physical memory is abstracted, it can be the case that the physical
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[memory addresses](Memory_addresses.md) are non-contiguous or even distributed
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accross different hardware components (such as the
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[cache](Register_and_cache_memory.md) and [swap](Swap_space.md)). Despite
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this, the memory addresses will appear contiguous in virtual memory. Each user
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space process is presented with the same range of available memory addresses and
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the same total capacity.
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It is also possible for the kernel to present user space with an available
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virtual memory capcacity that exceeds the current physical capacity of the
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machine:
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> _It's possible for the kernel and all running processes to request more
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> byte<!-- -->s of virtual memory than the total size of RAM. In that
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> situation, the OS can move move bytes of memory to secondary storage to make
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> room in RAM for newly requested memory._
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_How Computers Really Work_ (2021) p.206
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## Virtual memory and the kernel
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The kernel itself utilises virtual memory.
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During the [boot_process](Boot_process.md), it runs in physical memory mode
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however it rapidly creates a 1:1 mapping of physical to virtual memory for
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itself. This direct mapping allows the kernel to easily access any physical
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memory location as needed, whilst retaining the benefits of virtual memory.
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With its own virtual memory established, it can then build additional virtual
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memory structures for itself and user processes.
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While the kernel uses virtual memory it can also access physical memory at any
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time wherease user space processes can only ever access virtual memory.
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The kernel virtual memory has a different range of virtual addresses to work
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with than user space virtual memory.
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Unlike user space virtual memory, the kernel has access to everything running in
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kernel address space whereas processes in user address space are partitioned
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from each other with separate address spaces that cannot interact.
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In addition to being able to access its own virtual memory (and physical memory,
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as required) the kernel can also access any user processes' virtual address.
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This said, the virtual memory space of the kernel and the virtual memory space
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of the user processes are distinct. It is not the case that the kernel is
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superset of all available virtual memory. It can access user space virtual
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memory because it sets up the tables and locations, not because it is a subset
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of its own virtual memory.
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