SchOS/docs/guides/layout.md

Repository layout

Where everything is, and how to add to it.

One rule holds the whole thing together:

A directory contains either hand-written source or generated output. Never both.

Everything generated lives under build/. Delete build/ and you lose nothing that can't be rebuilt (except upstream tarballs, which live in dist/ and are deliberately kept out of clean).

schos/
├─ boot/          uefi/ the PE/COFF loader (splash, F8 menu, kernel load),
│                 common/ the UI it shares. UEFI only -- there is no BIOS path.
├─ kernel/        The kernel, by subsystem: arch/ core/ dev/ fs/ proc/ sys/
├─ include/       Headers shared by kernel, userland, and tools/mkfs, by
│                 subsystem -- see "include/" below
├─ lib/           First-party userland runtime: _start glue, syscall stubs,
│                 the initcode blob embedded in the kernel, user.ld
├─ cmd/           First-party programs — one directory per program:
│                 cmd/<name>/main.c  →  sysroot/bin/<name>
│                 schinit and schinitd are PID 1 and its supervisor; vgpud is
│                 the virtio-gpu driver, which runs out here rather than in
│                 the kernel; socktest/shmtest/nettest/pcitest are suites.
├─ ports/         Third-party software — one directory per port.
│                 Contains ONLY a Makefile and optional patches/.
├─ projects/      Things that run ON SchOS rather than things that ARE it:
│                 the compositor, window manager, desktop, and installer.
│                 Not part of the kernel build. See projects/README.md.
├─ tools/         Host-side helpers, SOURCE ONLY: mkfs.c, the SchFS test
│                 suites, sign.pl, vectors.pl, and the PowerShell harnesses.
│                 Their binaries build into build/tools/ -- this directory
│                 used to hold both, which is the one rule this layout has.
│                 tools/windows/loc.ps1 counts how much of the tree is first-party.
├─ sysroot/       Staging for the disk image. etc/ is hand-maintained;
│                 bin/ lib/ sbin/ are populated by `make sysroot-install`.
├─ dist/          Upstream tarball cache. Survives `make clean`.
├─ Refrence/      A FreeBSD checkout (Refrence/freebsd-src), kept as reference
│                 material and a parts bin. Never built, never in an image.
│                 Read it when a specification is ambiguous; copy from it only
│                 after the per-file licence check in docs/licensing.md.
│                 A full clone is 2.1 GB and a previous NetBSD one was deleted
│                 for that reason. Only sys/dev, sys/net and sys/netinet have
│                 ever been read, so a sparse checkout is ~100 MB:
│                   git clone --filter=blob:none --sparse <url> Refrence/freebsd-src
│                   git -C Refrence/freebsd-src sparse-checkout set │                       sys/dev sys/net sys/netinet
├─ src-trunk/     A NetBSD checkout, when present -- reference material and a
│                 parts bin. NOT built, NOT installed, must never reach an
│                 image. Currently absent; it was deleted for disk space and
│                 can be re-fetched sparsely. Copying from it is governed by
│                 the per-file licence check in docs/licensing.md §3 --
│                 NetBSD is not uniformly licensed.
├─ shell.nix      NixOS development shell: the whole toolchain in one file
├─ docs/          architecture/ what it is   plan/    what's next
│                 guides/      how to work   history/ why it's like this
└─ build/         ALL generated output (see below)

include/

One folder per subsystem, and every include names it: #include "core/proc.h", not "proc.h". The compiler is given -I./include and nothing deeper, so the folder is the only way to reach a header and the include line says where to find it.

folder	what is in it
arch/	the x86-64 machine: mmu.h, x86.h, traps.h, memlayout.h, mp.h, ioperm.h, vmap.h, asm.h, and schboot.h, the loader's handoff block
core/	the kernel proper: types.h, param.h, defs.h, proc.h, the locks, kmem.h, dma.h, irq.h, klog.h, dbg.h, sbuf.h, syscall.h, uaccess.h, hwinfo.h, elf.h, date.h
fs/	the VFS and what is under it: fs.h, file.h, buf.h, blkdev.h, schfs.h, fat.h, fsstat.h
dev/	devices and buses: device.h, pci.h, pciuser.h, usb.h, virtio.h, acpi.h, input.h, kbd.h, keymap.h, vt.h, audio.h, netdev.h
gfx/	pixels: display.h, fb.h, gfx.h, font.h, text.h, ttf.h, theme.h, logo.h (generated), qr.h
net/	socket.h, netuser.h
uapi/	what userland is built against: user.h, posix.h, errno.h, stat.h, fcntl.h, schinit.h
ldls/	the Linux driver layer's headers

A side effect worth having: errno.h, stat.h and fcntl.h are no longer at the top of include/, so a host tool built with -I./include gets the system's versions of them rather than ours.

tools/fix-includes.py rewrites a bare #include "proc.h" to the folder the header is in, reading the folders from the tree. Run it with --modified on code written before the split, or after moving a header.

build/

build/
├─ boot/          uefi/ loader, common/ UI
├─ kernel/        object files, the linked kernel, kernel.asm
├─ lib/           libc glue objects, initcode blob
├─ cmd/           linked first-party programs
├─ fs.img         the filesystem image
└─ ports/<name>/
    ├─ src/       extracted upstream tarball
    ├─ obj/       out-of-tree build directory
    └─ prefix/    install prefix (musl's headers and libs land here)

Adding your own program

mkdir cmd/hello
$EDITOR cmd/hello/main.c          # include "types.h", "user.h", ...

Then add hello to CMDS in the root Makefile. It gets linked against lib/ and installed to sysroot/bin/hello by make sysroot-install.

Adding a third-party port

mkdir ports/foo
$EDITOR ports/foo/Makefile        # start with `include ../mk/port.mk`

port.mk derives everything from the directory name — it gives you $(SRCDIR), $(BUILDDIR), $(PREFIX), $(DISTDIR), $(SYSROOT), and $(MUSL) (the musl install prefix that every port links against). Follow the pattern in ports/mksh/Makefile: fetch tarball to $(DISTDIR), extract to $(SRCDIR), build into $(BUILDDIR), install into $(SYSROOT). clean removes $(PORTDIR) and nothing else.

ports/Makefile picks up the new directory automatically. If it links against musl, add foo: musl there so ordering is right.

Local modifications to upstream go in ports/foo/patches/*.patch and are applied after extraction — never by editing files under build/, which are wiped on every clean.

Ramdisk root

make            # normal: root on the IDE disk (build/fs.img as hdb)
make ramroot    # root from a SCHFS image linked into the kernel

make ramroot builds build/fs-ram.img (default 6000 blocks, override with RAMDISK_BLOCKS=) and links it into the kernel, so the system boots to a shell with no data disk attached at all. That is what makes destructive work on the real disk survivable — reformatting it, or bringing up a new on-disk format — and it is the first piece of the installer.

Use make ramroot, not make RAMROOT=1: the flag changes CFLAGS, which make cannot see, so objects compiled for one setting would be silently reused for the other and ROOTDEV would point at the wrong device. The target drops the compiled objects first. Switching back is the same — plain make after a ramroot build reuses stale objects, so run rm -rf build/kernel build/lib build/cmd first (or just make clean).

The image costs boot time: the loader reads the kernel through the firmware's file protocol, so a 6 MB ramroot kernel is visibly slower to start than the 350 KB default.

What may go in it is a licensing question, not only a size one. The image is linked into the kernel binary, so anything in sysroot/ when it is built becomes part of a distributable artifact. A file staged there under a licence that is not ours to redistribute does not merely ride along — it makes the kernel itself undistributable. RAMDISK_EXCLUDE in the Makefile is the list of paths kept out, and enforcing it in the build rather than relying on someone remembering to unstage something first is deliberate. It has been needed once already: a 33 MB test payload landed in sysroot/ and both overflowed the image and would have gone into the kernel binary.

Three knobs, all in the Makefile, none of them FSSIZE:

RAMDISK_BLOCKS	image size, 6000 (× 1 KiB BSIZE)
RAMDISK_NINODES	inode table size, 2000
RAMDISK_EXCLUDE	sysroot-relative paths to omit, with their subtrees

RAMDISK_NINODES is worth knowing about because the inode table is preallocated: mkfs's default of 12000 inodes cost 1501 of the 6000 blocks — a quarter of the image — to describe about 260 files. At 2000 it costs 251, and metadata overall fell from 1596 blocks to 345.

If mkfs reports the image is full, read its message: it now prints the metadata breakdown and names the right knob for whichever image overflowed. It used to say only "increase FSSIZE in param.h", which is a constant that has nothing to do with the ramdisk root and sends you to the wrong file.

Two things that will bite you

The second is about a bootloader that no longer exists. It is kept because the shape of the bug outlived the code.

ld -r -b binary derives symbol names from the input file's path. The kernel embeds two blobs this way (initcode, entryother), and the root Makefile has --redefine-sym lines mapping e.g. _binary_build_lib_initcode_start → _binary_initcode_start. Move either blob's build location and those symbol names change; update the Makefile to match or the kernel fails to link.

STAGE2_SECTORS is gone, with the BIOS loader it sized. It is recorded here because the shape of the bug outlived it: one number that four files each believed independently — boot/bios/bootasm.S, stage2main.c's readseg(), the dd ... seek= offset, and twice inside mkiso.sh. Raising it from 24 to 48 for the boot logo left the dd seek=25 offset behind, so the kernel was written directly over stage 2's own tail, and nothing said so.

The fix was to derive all five from one Makefile variable. The lesson worth keeping is the other half: tools/padstage2.pl failed the build when stage 2 overflowed its budget rather than truncating it, which is why the overflow to 25440 bytes against a 24576 limit was found at all. A layout constant that several artifacts must agree on wants a check that stops the build, not a comment asking people to remember.