How can I build a small operating system on an old desktop computer? [closed]

Answer 1

It is very easy thanks to the fact that the BIOS already have many input/output functions built into it for changing screen mode, changing pixel color, writing text to the screen and many other things. However, it does not include support for file systems, that is one of very few things you must incorporate in your OS.

The BIOS loads the first sector on the hard drive and starts executing the code from there, so your OS must be placed with the first instruction in the first sector on the hard drive.

This Wikipedia article should get you started on BIOS interrupts for writing text to the screen, receiving keys from the keyboard and other stuff. https://en.wikipedia.org/wiki/BIOS_interrupt_call

Even if you plan to use c++ I would strongly recommend reading up on assembly programming, as it is vital to understanding how the hardware works.

Answer 2

I would start small and purchase an 8086 embedded development kit, and develop a multitasking OS on that. Once you have a kernel and are acquainted with working at the hardware level, you will be ready to do something more challenging.

Building even a VGA display DOS clone is a fairly challenging thing. The details are enormous. :-)

specific topics.

How do I put the code onto the computer? Is it best to do it with a floppy disk? Can most computers do it from a USB stick?

The BIOS will do elementary bootstrapping.

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What drivers do I need, and can you suggest any references to building those?

anything that isn't direct cpu/memory operations. Anything that isn't directly in the CPU reference manual.

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After the booting sequence--then what? How do I get into protected mode etc.

Protected mode will be part of the boot sequence.

then you start multitasking and figuring out how to start processes.

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How do I manage memory without the help of an operating system? Do I just use whatever addresses I want? No initialization necessary?

Correct. You will probably want to sort out a virtual memory system eventually.

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What will I undoubtedly run into that will confuse me?

not having debugging tools, not having IO

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How can I make it either a command line O/S, and a graphical one?

With grief. Look up Windows 3.1 and Linux, in particular X windows.

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What is a graphical O/S built on? Like, how would I do something like, a command line, with a font, and a picture at the top?

Look up X windows.

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final advice: study linux/x windows. It's not perfect, but it provides an understanding of one approach. Also study embedded systems.

Answer 3

I see plenty of good references to OS development sites, so I'll describe a different approach:

If you want the experience of implementing an OS from bare metal, there are way better hardware choices than an old PC. With the PC architecture, you will spend an inordinate amount of your time coding around uninteresting artifacts of its 30 year design history. For example, just the bootloader part of the project has probably burned out many a brave programmer.

For example, you'll need one set of drivers to read your kernel off of disk and/or the network. Then you'll need code to get into protected mode. At that point, you need another set of drivers! Very little of the work you do to get the chip into protected mode will be transferable after that point. You want to run it on a different PC +- 4 years and you'll need yet another set of drivers.

Look into bootstrapping an ARM or other 32-bit "embedded" chip. Inexpensive development boards are available, or you can solder your own! Some have built-in ethernet and usb. I think you will have more fun working on a sane, non-crusty architecture, and perhaps end up with some re-usable skills.

Answer 4

First things first. Read, read, read, read, read. You need to have a firm understanding of how the OS works before you can hope to implement your own.

Grab one of Andrew Tanenbaum's books on operating systems. This is the one we used in my OS class in college:

Modern Operating Systems PDF

Modern Operating Systems on Amazon

Despite the ridiculous cover, it's a fantastic read, especially for a textbook. Tanenbaum is really an expert in this area and his explanations of how the OS works underneath the hood are clear and easy to understand. This book is mostly theory, but I believe he also has a book that discusses more of the implementation. I've never read it, though, so I can't comment on it.

That should help you bone up on process management, memory management, filesystems, and everything else your OS kernel needs to do to get it up to a bootable state. From that point on it's basically a matter of writing device drivers for the hardware you need to support, and offering implementations of the C library functions to make kernel calls for things like opening files and devices, reading and writing, passing messages between processes, etc.

Read up on x86 assembly (assuming you are designing this for an x86 machine). That should answer a lot of your questions with regards to moving between processor operating modes.

If you've got any electronics knowledge, it may be easier to start with writing an operating system for an embedded device that has ample documentation, because it will generally be simpler than an x86 PC. I've always wanted to write my own OS as well, and I'm starting with writing a microkernel embedded OS for This Development Board from Digilent. It can run the soft-core MicroBlaze processor from Xilinx, which has very thorough documentation. It's also got some RAM, flash data storage, LEDs, switches, buttons, VGA output, etc. Plenty of stuff to play around with writing simple drivers for.

One of the benefits of an embedded device is also that you may be able to avoid writing a VGA driver for a long time. In my case, the Digilent development board has an onboard UART, so I can effectively use the serial output as my console to get the whole thing up and booting to a command line with minimal fuss.

Just make sure that whatever you choose to target has a readily available and well-tested compiler for it. You do not want to be writing an OS and a compiler at the same time.

Answer 5

Many schools have OS classes that do much of what you describe. My school (CMU) taught OS in C, and we wrote a kernel, file system, and shell, and were provided with code for a boot loader.

Unfortunately, I wasn't able to find any definitive resource for this course (15-412) on the web, and it's evolved over time. But perhaps people could post links to sources and assignments for schools that do have good resources on the web.

Answer 6

I would suggest working, at least at first, on Bochs or some other virtual machine the reason being that you can take it with you wherever you want, it's easier to debug (you can see the exact state of the hardware), and if you need outside help debugging they can use the exact same 'hardware' as you.

The most useful advice I have is to get yourself into a state where you can be running C code as quickly as possible -- i.e. boot up, setup your descriptor tables, and get yourself to a point where it's safe to run compiled C. Most if not all the kernel should be in C if you want to stay sane and keep working on it. Assembly, while required in some places, is tedious and tends to be hard to debug.