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/          Stage 1 MBR + stage 2 loader (splash, F8 menu, kernel load)
├─ kernel/        The kernel, by subsystem: arch/ core/ dev/ fs/ proc/ sys/
├─ include/       Headers shared by kernel, userland, and tools/mkfs
├─ 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>
├─ 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)
```

## 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

```sh
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

```sh
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

```sh
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
cannot be combined with Apache-2.0 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.