2719b93530
These were the awkward ones. Each began with an operation packed into a single byte — two of them with a version wedged in beside it — so there was no wrapping them: the layouts had to be rebuilt. The device manager's own enumerate and subscribe become the reserved verbs that mean the same thing everywhere, its replies lose three status structs the envelope already carries, and a device id becomes the packet's target. Power drops the version it repeated on every request, because describe is the handshake, and stops claiming a 64-byte ceiling it never needed for calls. USB moves a control transfer's data to the packet tail in both directions, which makes the status length the transferred length and retires a field that had been saying the same thing twice. The danger in this one was not the protocols but their readers. Init recognised a power button by two bytes at the head of a message, the ACPI service dispatched on the first byte, the xHCI driver read its operation with a raw integer load, and the HID drivers reinterpreted a report wholesale — none of which would have failed to compile once the layouts moved. They would simply have stopped: no shutdown on the power button, no reports from the keyboard. Every one of them now reads through the generated types, and the shutdown gate that answers only a subscriber is the same code it was. Two sizes were decided by measuring rather than assuming. The child-added message is both a request and the event broadcast to subscribers, and alignment rounds it to 48 bytes, which puts its packet exactly on the 64-byte push floor — a test pins that, because a field added carelessly would now overflow it. The interrupt report gives up eight bytes of inline room to make space for the header; the two drivers that produce reports send eight and four. Suite 110/110. |
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|---|---|---|
| .. | ||
| README.MD | ||
| acpi.md | ||
| architecture.md | ||
| arm.md | ||
| communication.md | ||
| discovery.md | ||
| efi.md | ||
| frame-allocator.md | ||
| framebuffer.md | ||
| gop.md | ||
| halting.md | ||
| heap.md | ||
| interrupts.md | ||
| logging.md | ||
| memory-map.md | ||
| paging.md | ||
| power.md | ||
| process-lifecycle.md | ||
| process-management.md | ||
| protocol-namespace.md | ||
| release-iso.md | ||
| resilience.md | ||
| scheduling.md | ||
| shared-fate-plan.md | ||
| smep-smap.md | ||
| smp.md | ||
| syscall.md | ||
| system-image.md | ||
| sysv.md | ||
| threading-plan.md | ||
| threading.md | ||
| timers.md | ||
| vdso.md | ||
README.MD
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 that programs call into, and the documented IPC protocols such as the VFS protocol. 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, notfork. 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?