47 lines
2.3 KiB
Markdown
47 lines
2.3 KiB
Markdown
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---
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tags:
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- CPU
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- computer-architecture
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- operating-systems
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---
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# Relation between the kernel and CPU
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It can be confusing to understand how the kernel and CPU interact with one
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another. Whilst it is true to say the kernel mediates between the hardware and
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user processes, this led me to think that the kernel was somehow 'over and
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above' the actions of the CPU. This is wrong and if you think about it, how
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could this be since any process, kernel included, requires the processor? The
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kernel is executed by and loaded into the CPU just like any other instruction in
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the
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[fetch, decode, execute cycle](Fetch_decode_execute.md)
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of the CPU.
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However as a process, the kernel is the 'first amongst equals' given that it is
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the core embodiment of the operating system. At boot time, the kernel is
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injected into memory and at this point the CPU _looks to this address_ in memory
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in order to source its first instruction. That's how the kernel gets going. It
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is the first instruction that the CPU fetches and this is what allows the kernel
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to play its mediatory role. However most of the fetch, decode, execute cycles of
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the CPU take place independently of the kernel.
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> Fetch decode and execute refer to processor pipeline stages. They occur
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> automatically as part of normal processor operation, the kernel doesn’t
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> generally have any direct control over how that happens...When the boot loader
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> loads the kernel, it points the CPU’s program counter to an entry point in the
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> kernel’s code. From there, the CPU will continue executing (fetching decoding
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> and executing) kernel code until you point the program counter to a userspace
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> program, where it will continue executing userspace code until an interrupt
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> points the program counter back into kernel code. That’s basically what the
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> CPU will be doing from boot up to shut down; switching between executing
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> kernel code and executing userspace code
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> ([Reddit]())
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This is because of [context switching](), when the CPU is running its cycle, the
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kernel is idle waiting for it to complete. Once control switches back to the
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kernel it can then run its next duty which will be either process, memory or
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device management or system calls. Then it will hand control over to either the
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processor or user space.
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