# The system, layer by layer

What SchOS is today. Everything in this directory is built and running; what is
merely intended lives in [../plan/](../plan/).

---

## The names, and what each one covers

`uname -s` reports `SchIFT`, the way every Unix reports its kernel rather than
its distribution.

| Layer | Name | What it is |
|---|---|---|
| System | **SchOS** | The distribution — kernel, userland, tooling |
| Kernel | **SchIFT** | Multi-core x86-64 kernel with a modular VFS |
| Filesystem | **SchFS** | Transactional, copy-on-write, the root filesystem |
| API | **SchAPI** | The native programming interface — *designed, not built* |
| Compositor | **Schcom** | Owns the display; v0 runs |

Six virtual terminals sit under all of it -- `/dev/tty1`..`tty6`, AltGr+F1..F6
or `chvt N` --
each with its own screen, line discipline and foreground process group. See
[terminals.md](terminals.md).

---

## The stack, bottom to top

```
      ┌─────────────────────────────────────────────────┐
      │ toybox  kilo  mksh  zsh  musl  first-party cmd/ │   userland
      ├─────────────────────────────────────────────────┤
      │  vgpud — a driver that is an ordinary process   │
      ├─────────────────────────────────────────────────┤
      │  schinit — PID 1, job manifests, supervision    │
      ├─────────────────────────────────────────────────┤
      │  syscall table — Linux numbering, one .inc file │   ← the ABI seam
      ╞═════════════════════════════════════════════════╡
      │  VFS │ sockets │ signals │ mmap │ poll │ procfs │
      ├──────┴─────────┴─────────┴──────┴──────┴────────┤
      │  SchFS  │  lwIP  │  vt/gfx  │  buffer cache     │
      ├─────────┴────────┴──────────┴───────────────────┤
      │  device model — class vtables, bus binding      │   ← the driver seam
      ├─────────────────────────────────────────────────┤
      │  /dev/pci — MMIO, DMA and IRQs to a process     │   ← the eviction seam
      ├─────────────────────────────────────────────────┤
      │ drivers: xHCI  e1000  virtio-gpu  VBE  IDE  PS/2│
      ├─────────────────────────────────────────────────┤
      │  scheduler │ paging │ traps │ IRQ │ SMP │ DMA   │   SchIFT core
      └─────────────────────────────────────────────────┘
                            ▲
                   UEFI loader (boot/uefi)
```

Three seams matter. Above the **syscall table**, software is ordinary and
portable — musl and toybox are unmodified. Below the **device model**, a driver
never names its consumer and a consumer never names a driver. And at
**`/dev/pci`**, a driver stops having to be in the kernel at all: a process is
handed a device's registers, DMA memory and interrupts, and the in-kernel driver
that held it releases it cleanly. `vgpud` is the virtio-gpu driver living out
there today. See [../plan/hybrid.md](../plan/hybrid.md).

---

## Where it came from

SchOS began as [xv6](https://github.com/mit-pdos/xv6-public) and is now mostly
something else. What survives from xv6 is the *shape* of the process table, the
sleep/wakeup discipline, and the buffer cache. What has been replaced: the
scheduler and trap path (rewritten for x86-64 and for more than one processor),
the memory manager, the entire filesystem, the syscall layer, and every driver.

That inheritance is worth knowing when reading the code, because the parts that
still look like xv6 follow xv6's conventions and the parts that do not, do not.

---

## The documents

Read in this order if you are new:

| | |
|---|---|
| [boot.md](boot.md) | Firmware to the first userspace process. The shortest path through the whole system. |
| [kernel.md](kernel.md) | Memory, processes, scheduling, traps, signals, SMP. |
| [syscalls.md](syscalls.md) | The one table everything is generated from, and how a musl binary runs unmodified. |
| [filesystem.md](filesystem.md) | The VFS, the buffer cache, mounts, procfs. |
| [schfs.md](schfs.md) | The copy-on-write root filesystem. |
| [devices.md](devices.md) | Buses, classes, binding, and the vtables consumers talk to. |
| [usb.md](usb.md) | xHCI, the USB core, and the three class drivers. |
| [graphics.md](graphics.md) | Framebuffer, text rendering, the console, and PSOGM. |
| [terminals.md](terminals.md) | The six virtual terminals, and the chord that switches them. |
| [networking.md](networking.md) | NIC drivers, lwIP, and both socket families. |
| [audio.md](audio.md) | `/dev/dsp`, AudioEngine, and the AC'97 driver under it. |
| [init.md](init.md) | `schinit`, PID 1 as a role, and the job manifests. |
| [debugging.md](debugging.md) | Symbols, a log that outlives the kernel, and asking the kernel a question. |
| [ldls.md](ldls.md) | Running somebody else's drivers: a Linux `.ko`, in a process, reaching hardware through `/dev/pci`. |

---

## Three rules that hold across all of it

**The kernel and the host tools compile the same SchFS source.** `mkschfs`
cannot disagree with the kernel about the on-disk format, because there is only
one implementation. Format bugs are found on the host, where a test takes a
second instead of a boot.

**Syscalls are declared in exactly one place.** `kernel/sys/table.inc`.
The constants, the dispatch table and the Linux translation map are all
generated from it, so a syscall cannot be half-registered.

**A list you can read beats a list assembled by side effects.** Drivers are
written out by hand in `kernel/drivers/registry.c` rather than collected by a
linker section, and platform devices are registered in one function rather than
inside each driver's init. The tradeoff is deliberate: self-registration is
nicer to write, and it means the list of what is in your kernel is not written
down anywhere you can read.