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What is the kernel?

Quotation body.

Explain to me what "kernel" means, please.

The kernel is the inner part of something. What is the context of the question?

I'm "reading" Windows NT server 4.0 Administrator's Bible and UNIX System Administration Handbook (second edition) and so far I'm in the TOC. a few ch titles mention kernel. I looked it up @ Mirrium Webster, but I need to understand "homomorphism" which got me even more confused.

oops sorry, thats: homomorphorphism

Buh?

It has to do with mapping sets into or onto other sets then in loses me

back to kernel. let me try an analogy.

so, Kernel mode is what?

When you go to a bank, you can't simply withdraw money or deposit money. You have to stand in line at a window, hand your request to someone on the other side, and they decide whether you're allowed to do something and they do it for you. Only the tellers have access to what's in the bank.

i'm with you so far...

There are many things in a computer which are secured like money in a bank. These include the disks, the memory, just about anything hardware. For example, you can look at your own memory but not at other peoples'. This is both for confidentiality and just to prevent mistakes, such as a bug in one program accidentally clobbering some other program's memory.

uh huh

Ordinary programs are like ordinary people. They have no direct access to get more memory or even give memory back. They can't look at other peoples' memory. Etc., etc. An ordinary program must stand in line, hand a request for what it wants to the operating system, and see if they got away with it.

got it.

and the kernel is that bit of the os to which the requests are given?

The kernel is like the back room at a bank. There are analogs of tellers and other officials back there, who are responsible for doing the work that the ordinary person isn't allowed to do. These are called device drivers, system services, schedulers, swappers, and so on.

Like the back room of a bank, the kernel also has its own "books", that is, records of what's back there and who is entitled to it.

ok. and so the kernel is the core bit of the os?

Yes. It's the part that has to be there for the computer to be practical to operate as a multiuser system. There are other parts of the operating system, such as the utility programs, libraries, folders containing icons and so on which are also valuable but aren't a part of the back room.

ok, that helps. So why is it called the kernel?

operating system developers often have in mind an idea of layers like an onion has when they design the OS. the outer layers communicate with the next inner layers, a middle layer communicates with the layers on each side. The kernel part that I described to you doesn't have any layer inside it -- it communicates directly with the bare metal, so to speak -- that is, the hardware, which isn't the operating system at all. And it's the kernel's job to protect the hardware from broken or malevolent layers outside it.

(well, it's the job of the kernel to provide hardware services to the layers outside it but in a reliable, fair, and secure way)

Well, hey, that makes sense. Now I think I'll even be able to remember it!

that's great!

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