SchOS/docs/architecture

The system, layer by layer

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

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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
Packages	apk	Alpine's package manager manages the root; SchOS itself is the package schos-base

Twelve virtual terminals sit under all of it -- /dev/tty1..tty12, Ctrl+Alt+F1..F12 or chvt N -- each with its own screen, line discipline and foreground process group. See terminals.md.

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The stack, bottom to top

      ┌─────────────────────────────────────────────────┐
      │ Xorg + Window Maker · Alpine's packages (apk)   │   desktop, userland
      │ toybox  mksh  musl  first-party cmd/            │
      ├─────────────────────────────────────────────────┤
      │ vgpud, ldlsd — drivers that are ordinary        │
      │ processes (and Linux's r8169 inside ldlsd)      │
      ├─────────────────────────────────────────────────┤
      │  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 AHCI NVMe e1000 8139 r8169 AC97   │
      │          virtio-gpu VBE IDE PS/2 I2C-HID        │
      ├─────────────────────────────────────────────────┤
      │  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, and ldlsd runs Linux's own drivers the same way. See devices.md and ldls.md.

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Where it came from

SchOS began as xv6 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.

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The documents

Read in this order if you are new:

boot.md	Firmware to the first userspace process. The shortest path through the whole system.
kernel.md	Memory, processes, scheduling, traps, signals, SMP.
syscalls.md	The one table everything is generated from, and how a musl binary runs unmodified.
scheduler.md	Fair share by virtual runtime, slices, and nice.
filesystem.md	The VFS, the buffer cache, mounts, procfs.
schfs.md	The copy-on-write root filesystem.
fat.md	FAT12/16/32 with long names: USB sticks, SD cards, the ESP.
devices.md	Buses, classes, binding, and the vtables consumers talk to.
usb.md	xHCI, the USB core, and the three class drivers.
udev.md	eudev on SchOS: the device table, the /sys tree, uevents, datagram sockets.
graphics.md	Framebuffer, /dev/fb0, text rendering, the console, PSOGM and GMMOS.
desktop.md	Xorg on the framebuffer, Window Maker, and what they stand on.
packages.md	The apk root: Alpine's packages, and SchOS as one of them.
scff.md	The compressed image the live root ships as.
terminals.md	The twelve virtual terminals, ptys, and handing one to a display server.
networking.md	NIC drivers, lwIP, and the socket families.
audio.md	/dev/dsp, AudioEngine, and the AC'97 driver under it.
init.md	schinit, PID 1 as a role, and the job manifests.
debugging.md	Symbols, a log that outlives the kernel, and asking the kernel a question.
ldls.md	Running somebody else's drivers: a Linux .ko, in a process, reaching hardware through /dev/pci.

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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.