Answer by Mokubai for Would an executable need an OS kernel to run?

The kernel is "just" more code. It's just that that code is a layer that lives between the lowest parts of your system and the actual hardware.

All of it runs directly on the CPU, you just transition up through layers of it to do anything.

Your program "needs" the kernel in just the same way it needs the standard C libraries in order to use the printf command in the first place.

The actual code of your program runs on the CPU, but the branches that code makes to print something on screen go through the code for the C printf function, through various other systems and interpreters, each of which do their own processing to work out just howhello world! actually gets printed on your screen.

Say you have a terminal program running on a desktop window manager, running on your kernel which in turn is running on your hardware.

There's a lot more that goes on but lets keep it simple...

1. In your terminal program you run your program to print hello world!
2. The terminal sees that the program has written (via the C output routines) hello world! to the console
3. The terminal program goes up to the desktop window manager saying "I got hello world! written at me, can you put it at position x, y please?"
4. The desktop window manager goes up to the kernel with "one of my programs wants your graphics device to put some text at this position, get to it dude!"
5. The kernel passes the request out to the graphics device driver, which formats it in a way that the graphics card can understand
6. Depending on how the graphics card is connected other kernel device drivers need to be called to push the data out on physical device buses such as PCIe, handling things like making sure the correct device is selected, and that the data can pass through relevant bridge or converters
7. The hardware displays stuff.

_{This is a massive oversimplification for description only. Here be dragons.}

Effectively everything you do that needs hardware access, be it display, blocks of memory, bits of files or anything like that has to go through some device driver in the kernel to work out exactly how to talk to the relevant device. Be it a filesystem driver on top of a SATA hard disk controller driver which itself is sitting on top of a PCIe bridge device.

The kernel knows how to tie all these devices together and presents a relatively simple interface for programs to do things without having to know about how to do all of these things themselves.

Desktop window managers provide a layer that means that programs don't have to know how to draw windows and play well with other programs trying to display things at the same time.

Finally the terminal program means that your program doesn't need to know how to draw a window, nor how to talk to the kernel graphics card driver, nor all of the complexity to do with dealing with screen buffers and display timing and actually wiggling the data lines to the display.

It's all handled by layers upon layers of code.