learn-osdev

This is a repository full of notes to myself regarding operating system development. In this case, the code is the documentation, at least for the code I'm learning.

The build system is zedo-based. If you don't already have a working zedo on your system, you can setup a minimal implementation with:

git submodule update --init zedo-shim
export PATH+=":$PWD/zedo-shim/bin"

I'm assuming a GNU toolchain (gcc, as, ld, objdump, &c). The examples are tested on QEMU.

Good references (or at least they look good) are in references.md. It has notes about hardware/firmware/software interfaces as well as toolchain information.

I'm hosting this in github, and using their "Pages" feature to publish out-of-band documentation from the docs directory. That's where the overview/reference documentation lives, but the most walkthrough-style documentation is in-line with the code.

Examples

So far, I only have some primitive hello-world-style bootloader examples.

Whenever these can be build with zedo <example>, you should also be able to run them under qemu with zedo <example>.run, or as zedo <example>.gdb to allow GDB to connect on :1234.

Simple BIOS Bootloaders

These are example programs that fit into (and are run from) the master boot record. I suppose a real bootloader would actually load a kernel, but these don't, and I'm still going to call them bootloaders. They are in hello-os/src/bootloader-<arch>-bios/<example>.s, though for now I only cover x86_64 (but to be fair, real and long mode examples probably also work for the appropriately earlier x86-family processors).

Build these examples with zedo hello-os/build/bootloader-x86_64-bios/<example>.bootsector. Building involves a GNU linker script hello-os/src/bootloader-x86_64-bios/bootsector.ld, so check that out.

• donothing: Just initializes and halts immediately on booting. It stays halted even if the processor gets interrupts, which some examples can't say of themselves.
• hello: Uses (deprecated) BIOS calls to print Hello BIOS! on-screen.
• selfdump: Uses BIOS calls to print its own machine code in hexadecimal.

Stage One BIOS Bootloaders

Now I'm onto actual bootloaders; you know: things that load more code to continue booting. They are also in hello-os/src/bootloader-<arch>-bios/<example>.<mode>.s.

These examples are designed so that several stage-1 programs can use the same stage-0 bootloader. I had planned for there to be three stage 0 bootloaders each hands off to stage 1 in one of real mode, protected mode, or long mode. However, it turns out that even the protected mode is a little cramped, so I've canned the idea of also switching to long mode here. Instead, I believe it is probably better engineering to only hand off to a real mode stage 1.

These bootloaders are at stage0.rm.s, and stage0.pm.s, for a real- or protected-mode stage 1 respectively. Each example also has these .{rm,pm}.s extensions as well, which helps the build system to pair up a stage-1 with an appropriate stage-0.

Build these examples with zedo hello-os/build/bootloader-x86_64-bios/<example>.<mode>.img. In addition to the bootsector.ld linker script used earlier, I also use hello-os/src/bootloader-x86_64-bios/stage1.ld.

Real-mode examples:

• donothing: Immediately halts.
• hello: Uses (deprecated) BIOS calls to print Hello BIOS! on-screen.

Protected-mode examples:

• donothing: Immediately halts.