Writing a Simple Operating System — from Scratch

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Source: https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf

Writing a Simple Operating System — from Scratch

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We’ve all used an operating system (OS) before (e.g. Windows XP, Linux, etc.), and perhaps we have even written some programs to run on one; but what is an OS actually there for? how much of what I see when I use a computer is done by hardware and how much is done by software? and how does the computer actually work?

The late Prof. Doug Shepherd, a lively teacher of mine at Lancaster University, once reminded me amid my grumbling about some annoying programming problem that, back in the day, before he could even begin to attempt any research, he had to write his own operating system, from scratch. So it seems that, today, we take a lot for granted about how these wonderful machines actually work underneith all those layers of software that commonly come bundled with them and which are required for their day-to-day usefulness.

Here, concentrating on the widely used x86 architecture CPU, we will strip bare our computer of all software and follow in Doug’s early footsteps, learning along the way about:

• How a computer boots

• How to write low-level programs in the barren landscape where no operating system yet exists

• How to configure the CPU so that we can begin to use its extended functionality

• How to bootstrap code written in a higher-level language, so that we can really start to make some progress towards our own operating system

• How to create some fundamental operating system services, such as device drivers, file systems, multi-tasking processing.

Note that, in terms of practical operating system functionality, this guide does not aim to be extensive, but instead aims to pool together snippets of information from many sources into a self-contained and coherent document, that will give you a hands-on experience of low-level programming, how operating systems are written, and the kind of problems they must solve. The approach taken by this guide is unique in that the particular languages and tools (e.g. assembly, C, Make, etc.) are not the focus but instead are treated as a means to an end: we will learn what we need to about these things to help us achieve our main goal.