# OS Development

This document explains the architectural decisions behind the operating system.

## Written in Zig?

The OS is written in Zig because it has excellent EFI support, so the OS boots quickly without a third-party bootloader.

Zig comes batteries included for systems work — cross-compilation, a build system, and a test runner are all part of the toolchain. Building with `-Doptimize=ReleaseSafe` keeps runtime safety checks on in the shipped kernel, which removes entire classes of bugs. The built-in test suite, combined with a QEMU integration harness, means every feature is proven to work, before it is shipped.

The codebase of the OS prioritizes readability. The aim is a codebase where someone new to OS development can find their way around without a guide.

## A microkernel?

The kernel is a thin layer: it schedules processes and manages memory. Everything else — drivers, file systems, the display — runs in user space as separate, isolated processes.

The payoff is resilience. When a driver crashes, it doesn't take the OS down with it; it gets restarted. That makes this an ideal environment for *developing* an operating system, because a buggy driver is an ordinary bug: patch it, restart the service, and keep going.

There is a security benefit too. Processes are isolated and talk over Inter-Process Communication (IPC) channels, so compromising one service doesn't hand an attacker the whole machine. Vulnerabilities tend to stay contained in the process they started in.

Other operating systems choose to pack all of these duties into one binary as a Monolithic kernel, mostly for performance: a function call inside the kernel is faster than passing a message between isolated processes. That cost is real — an IPC round-trip is a few microseconds where a function call is nanoseconds — but it is also workload-shaped. Compute-bound programs don't notice it at all. For bulk data like file contents and pixels, the design moves data through shared memory and DMA so it is copied once, the same as a monolithic kernel; only small control messages cross the IPC boundary. What remains is the per-message cost on chatty paths, and the scheduler and memory management are designed to keep that small.

## Private ABI

The syscall layer is private. The numbers and structures in `abi.zig` are an internal detail shared between the kernel and the system's own libraries, and they are free to change between builds.

The public boundary sits one level up: the [vDSO](vdso.md) that programs call into, and the documented IPC protocols such as the [VFS protocol](../file-system-development/vfs-protocol.md). Programs that stick to those interfaces keep working while the kernel rearranges itself underneath. This is the opposite of the Linux approach, where raw syscall numbers are frozen forever; here, stability is promised at the library and protocol level, and nowhere below it.

## Steal the best bits and dump the legacy

The OS is Unix-like, but selectively. It borrows the ideas that have aged well — everything is a file, small services composed over clean interfaces — and skips the parts of POSIX that have caused decades of headaches.

Some concrete choices:

- **`spawn`, not `fork`.** Creating a process starts a fresh program and returns the child's id. There is no clone-the-whole-address-space-then-immediately-throw-it-away dance, and none of the subtle state-inheritance bugs that come with it.
- **Time is a syscall.** The kernel owns the clock and timers directly. There is no time daemon to keep alive and no ambiguity about where the truth lives.
- **Lifecycle events arrive as messages.** A supervisor learns that a child exited through an IPC message on an endpoint it already owns — delivered like any other message, not as an interrupt that can fire between any two instructions.

The test for keeping an idea is simple: does it still pull its weight, or is it only there because it was there in 1979?
