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Operating System written for me
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Zig 94.2%
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Daniel Samson 5d57e7b01c
M3 step 3: syscall/sysret fast path 
Per-core MSR setup (EFER.SCE, STAR, LSTAR, SFMASK) enables syscall; the
GDT layout was already chosen so sysret lands on CS 0x23 / SS 0x1B. A new
syscall_entry stub swaps in the kernel GS, switches to the task's kernel
stack via the per-CPU block, builds a CpuState frame identical to the
interrupt path's, and reuses interruptDispatch (vector 128) — then
sysretq back. enter_user now also publishes kernel_rsp so the borrowed
path's syscalls land on a good stack. /sbin/init uses the `syscall`
instruction. int 0x80 stays for the test blobs. Suite 27/27.

(Noted in the stub: sysretq #GPs in ring 0 on a non-canonical return
RIP — a hardening item once untrusted user code exists.)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
 		2026-07-08 23:17:31 +01:00
docs M2 step 5: drop the low half — a true higher-half kernel 2026-07-08 22:54:52 +01:00
sbin M3 step 3: syscall/sysret fast path 2026-07-08 23:17:31 +01:00
src M3 step 3: syscall/sysret fast path 2026-07-08 23:17:31 +01:00
test isolation M1: ring 3 + a real /sbin/init, end to end 2026-07-08 22:15:07 +01:00
.gitignore started project with basic efi setup 2026-07-01 18:32:56 +01:00
.zig-version Pin zig version to 0.16 LTS 2026-07-03 13:49:47 +01:00
README.md moving kernel code to kernel/ 2026-07-05 10:19:11 +01:00
build.zig M2 step 4: relink the kernel into the higher half 2026-07-08 22:45:56 +01:00
build.zig.zon started project with basic efi setup 2026-07-01 18:32:56 +01:00

README.md

DanOS

Codename: Shodan Version: 1

A small operating system, written from scratch in Zig — a bootloader (src/boot/) and a microkernel (src/kernel/), sharing a neutral handoff contract (src/root.zig). It boots x86-64 via UEFI, and so far has a framebuffer console, a physical frame allocator, its own paging with W^X permissions, interrupt/exception handling, a LAPIC timer, a kernel heap, a fixed-priority preemptive scheduler, and in-kernel IPC channels. See docs/ for how each piece works.

Prerequisites

  • Zig 0.16.x — the build is pinned to this line (.zig-version); other minor versions are rejected, because Zig makes breaking changes between releases pre-1.0. A toolchain manager such as zvm or zigup will pick up .zig-version automatically.
  • QEMU (qemu-system-x86_64) — to run and test the kernel. On macOS, brew install qemu also bundles the OVMF firmware below.
  • OVMF UEFI firmware — the edk2-ovmf package (Arch), ovmf (Debian/Ubuntu), or edk2-ovmf (Fedora); on macOS it ships inside the Homebrew qemu formula. Both the build and the test harness probe the known Arch/Debian/Fedora/macOS layouts and use the first that exists, so no configuration is normally needed. Override with -Dovmf-code= / -Dovmf-vars= (build) if yours lives elsewhere.
  • Python 3 — for the QEMU integration test harness.

Build

zig build

Produces the UEFI bootloader (zig-out/bin/BOOTX64.efi) and the kernel ELF (zig-out/bin/kernel).

Run

Boot it in QEMU with OVMF (opens a display window):

zig build run-x86-64
# distro with OVMF elsewhere:
zig build run-x86-64 -Dovmf-code=/path/OVMF_CODE.fd -Dovmf-vars=/path/OVMF_VARS.fd

Test

zig build test            # host unit tests (the platform-independent shared code)
python3 test/qemu_test.py  # QEMU integration tests: boots the kernel and asserts
                           # on its serial output (see docs/testing.md)

The integration harness builds and boots the kernel once per test case, checking memory, the frame allocator, paging (incl. NX and the null guard), the heap, interrupts, and exception handling. It exits non-zero on any failure, so it drops straight into CI.

Documentation

Design notes explaining the why behind the code live in docs/ — start with docs/README.md.

San Serif Text "Dan OS" with a black karate belt around it.