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MyOS-Simple Wiki

A field guide to building a bare-metal x86 operating system from a 512-byte boot sector up to a protected-mode C system with a shell, a clock, processes, and a calculator.

This wiki is the companion reference for MyOS-Simple — a progressive, five-stage OS tutorial. The project's README.md tells you what each stage is; this wiki explains how and why every mechanism works, down to the byte. Every article is checked against the actual source in this repository and cites it as path:line.

💡 Tidbit: Nothing in this project depends on an existing OS at runtime. From the first 512 bytes onward, the only thing beneath your code is the BIOS — and by Stage 2, not even that.

Start here

If you want to…	Go to
Boot something in two minutes	Building and running
Follow the story stage by stage	Stage 1 → 2 → 3 → 4 → 5
Understand one idea deeply	Concepts below
Look up a number, port, or byte	Reference below
Extend the OS yourself	Writing your own stage

The five stages

#	Stage	Mode	What it adds
1	Assembly boot sector	16-bit real	Boot, BIOS print, color + keyboard variant
2	C kernel in protected mode	32-bit protected	Bootloader, GDT, real→protected switch, C kernel, direct VGA
3	Interactive shell	32-bit protected	Polled keyboard, command interpreter (5 commands)
4	Clock, processes, calculator	32-bit protected	CMOS RTC, process model, fixed-point calc, history/tab/aliases (20 commands)
5	The stabilized release	32-bit protected	Consolidated command set (18), committed build artifacts

Concepts

The theory, one idea per page. Read these alongside the stage that introduces them.

Firmware & boot

• The boot process — power-on to C entry, end to end
• Real mode — 16-bit segment:offset, the 1 MiB world
• BIOS services — int 0x10, int 0x13, int 0x16
• The boot sector — the 512-byte, 0xAA55 contract
• Disk loading with int 0x13 — CHS reads, sector counts

Into protected mode

• Protected mode — the switch, the flat model, the far jump
• The Global Descriptor Table — descriptors and selectors
• Freestanding C — life without a standard library
• Linker scripts — placing a flat binary at 0x1000

Talking to hardware

• VGA text mode — the framebuffer at 0xB8000
• The 8042 PS/2 keyboard — polling ports 0x60/0x64
• Scancodes — Set 1, make/break, translation

What an OS does

• The CMOS real-time clock — wall-clock time, BCD, uptime
• Cooperative scheduling — PCBs, the ready queue (and what's only modeled)
• Fixed-point arithmetic — decimals without an FPU

Reference

Dense, authoritative lookups.

• Memory map — every address the OS touches
• I/O ports — 0x60, 0x64, 0x70, 0x71, and the BIOS interrupts
• GDT descriptor format — the 8-byte descriptor, bit by bit
• Scancode tables — both Set-1 tables, reproduced exactly
• Command reference — every shell command across stages
• Toolchain and build — tools, flags, targets, versions
• Glossary — every term, defined and linked

Guides

• Building and running
• Debugging with GDB
• Troubleshooting
• Writing your own stage

How to read this wiki

The stages are a narrative; the concepts are the encyclopedia behind it. The most rewarding path is to read a stage page, then chase its links into the concept pages whenever you want the full theory, and keep the reference pages open in another tab.

💡 Tidbit: The hardest leap in the whole tutorial is not the C — it is getting the machine into a state where C can run at all. That is the entire job of Stage 2: a GDT, the protected-mode switch, and a fixed load address.

⚠️ Caveat: This is a teaching OS. The process scheduler is an instructive model — the register-level context switch is described but not performed — and several shortcuts (no A20 handling, no RTC update-race guard, fixed sector counts tuned for QEMU) are documented honestly on the relevant pages rather than hidden. Learn the shape here; learn the production details elsewhere.