path: root/README.md

# gbos

A tiny **Unix-flavored microkernel for the Game Boy Color** (MBC5 cartridge).
Not Linux, not FUZIX — a from-scratch cooperative kernel that borrows the V7
*process model* (separate read-only text + per-process data, a syscall trap,
round-robin scheduling) and squeezes it into the GBC's banked, MMU-less memory.

This is a **scaffold**, but a *working* one: it assembles into a valid 32 KiB
ROM, brings up a process table, and round-robin context-switches between two
demo tasks that each `write()` a byte and `yield()`. Verified end-to-end in the
`~/dev/gbc` emulator (headless serial console) — it streams `ABABAB...`.

## Why this shape (the hardware reality)

- No MMU, no memory protection. "Unix" here = the API/process model, not isolation.
- CPU only ever sees **64 KiB**; RAM is banked.
- **Kernel code lives in ROM** (`$0000-$3FFF`, up to 8 MiB via MBC5) — free, read-only.
- **Program text lives in ROM banks** (`$4000-$7FFF`) — the V7 "pure text" idea.
- **Per-task RW state** (data/bss/heap/stack) lives in one **8 KiB cart-RAM bank**
  (`$A000-$BFFF`, MBC5, up to 128 KiB / 16 banks).
- The context switch swaps banks, so it **must** run from **HRAM** (never banked)
  and never touch a swappable stack mid-swap.

## Memory map

```
$0000-$3FFF ROM0   kernel core (rst/IRQ vectors, init, sched, syscalls) - fixed
$4000-$7FFF ROMX   current task TEXT (read-only program code)
$8000-$9FFF VRAM   graphics (unused so far)
$A000-$BFFF SRAM   current task DATA/BSS/HEAP/STACK  (MBC5 RAM bank)
$C000-$CFFF WRAM0  kernel globals + kernel stack (never swapped)
$D000-$DFFF WRAMX  per-process u-area (SVBK bank)  (reserved, not used yet)
$FF80-$FFEE HRAM   context-switch trampoline (111 bytes)
$FFEF-$FFF2 HRAM   bank shadows + switch target
```

## Process control block (`include/gbos.inc`)

`STATE, PID, SP, ROMB(16b text bank), RAMB(data bank), WRAMB(u-area), PARENT, EXIT`
— 10 bytes. `MAX_PROCS = 8`.

## Syscall ABI

User loads `C` = syscall number, args in `DE`/`B`/`HL`, then `rst $30`.
Return in `A` (and `B` for `wait`). **The trap clobbers `BC`/`DE`/`HL`** (it uses
`HL` to index the dispatch table), so userland must save any pointer it needs
across a syscall (`push hl` / `pop hl`). Libc syscall stubs handle this for C.

| # | name    | status |
|---|---------|--------|
| 0 | exit    | ✅ zombie + reparent orphans + wake waiter |
| 1 | fork    | ✅ copies the 8 KiB bank, child returns 0 |
| 3 | write   | ✅ `DE`=buf `B`=len → serial console |
| 6 | exec    | ✅ `B`=program id → maps ROM text bank, jumps in |
| 7 | wait    | ✅ blocks; reaps a zombie child → `A`=pid `B`=code |
| 8 | getpid  | ✅ |
| 11| yield   | ✅ cooperative switch |
| 2 | read    | ✅ blocking console input (1 byte) via external-clock serial |
| 4,5,9,10 | open/close/kill/brk | ⛔ `ENOSYS` |

### The shell (PROG_SH)

init `exec`s a tiny shell: **prompt → read a line (with echo) → match a command
→ `fork`+`exec`+`wait`**. Commands live in a table (`worker`, `hello`). Runs
entirely over the headless serial console:

```
$ hello
hello world!
$ worker
worker!
$ foo
?
$
```

### Process lifecycle (exit / wait / reaping)

The classic Unix zombie/reap dance, adapted to banked memory:

- **`exit(code)`** sets `PS_ZOMBIE` + status, **reparents** any children to init
  (pid 1), **wakes** the parent if it's blocked in `wait()`, then schedules
  away forever. Its resources are freed by the reaper, not here.
- **`wait()`** scans for a `PS_ZOMBIE` child. Found → **reap**: free its
  cart-RAM bank (back to the allocator) and its PCB slot, return pid + code.
  Children exist but none dead → set self `PS_BLOCKED` and yield, retry on wake.
  No children → return `$FF` (`ECHILD`).
- The scheduler skips non-`READY` procs; `FindNextReady` returns carry when
  nothing is runnable so `yield` just keeps the caller running (idle).
- **Cart-RAM banks are a free list** (`wBankUsed` bitmap): `AllocRamBank` /
  `FreeRamBank`. Verified by running 6 workers through only 3 child banks
  (`w2w3w4w5w6w7`) — each reaped bank is recycled by the next `fork`.

Lifecycle demo (`tasks.asm`): init forks 3 workers, each `exec`s PROG_WORKER
(prints `w`, exits with its pid), init `wait`s and reaps all three →
`wwwR2R3R4!`.

### exec (`sys_exec`, src/proc.asm + programs.asm)

Where the text-in-ROM model pays off. Programs are linked to run from the ROMX
window (`$4000-$7FFF`) and stored in their own ROM banks (`programs.asm`).
`exec(B=program id)`:

1. looks up `ProgramTable[B]` = (ROM bank, entry),
2. sets `PROC_ROMB` and maps the bank live into `$4000-$7FFF`,
3. resets the stack to the top of the task's (already mapped) cart-RAM bank,
4. `jp`s to the entry — it never returns.

Proof it's really bank-driven: both demo programs are ORG'd at the **same**
address `$4000` in **different** banks (02 and 03). The parent `exec`s
`PROG_PING`, the child `exec`s `PROG_PONG`, and the output `1212...` (2000/2000,
zero garbage) can only happen if each process executes its own bank via
`PROC_ROMB`. A fuller exec would also copy `.data` from ROM and zero `.bss`.

### fork (`sys_fork`, src/proc.asm)

The interesting syscall. It enters on the parent's user stack (which lives in
the `$A000-$BFFF` cart-RAM window), so before it can swap that window it
**switches to a kernel stack in WRAM0**. Then it:

1. allocates a free PCB slot + a fresh cart-RAM bank (bump allocator),
2. copies the parent's whole 8 KiB bank → the child bank, 256 bytes at a time
   through a WRAM bounce buffer (only one cart-RAM bank is visible at once),
3. plants a **switch-in frame** in the child at `Uafter-8` (`Uafter` = parent
   SP at entry): the copied return address is already there, so it just zeroes
   the saved `hl/de/bc/af` slots — the child resumes at the post-`fork` PC with
   `A=0`,
4. fills the child PCB (shared ROM text bank, new RAM bank, parent as PPID),
5. restores the parent stack and returns the child pid.

Verified: `init` forks a child (P/C alternate on serial); nested forks yield
three independent processes with distinct pids (`123123...`).

## The context switch (`src/hram.asm`)

The crux. `hSwitchTo(DE=&incomingPCB)`:
1. push full register context onto the **outgoing** task's stack
2. save `SP` into the outgoing PCB (WRAM0, always mapped)
3. program incoming banks: **RAM bank → SVBK → ROM text bank**
4. load `SP` from the incoming PCB (its banks are now mapped)
5. pop context, `ret` into the incoming task

Runs from HRAM so changing `SVBK`/RAM-bank never pulls the rug out from `PC` or
the stack. New tasks are bootstrapped with a fake frame (`ProcSetupStack`) so the
first switch-in `ret`s straight to their entry point.

## Build

```sh
make            # -> gbos.gb (RGBDS: rgbasm/rgblink/rgbfix)
make clean
```

Console output goes to the **serial port** (`tty.asm`). Easiest way to watch it
is the project emulator's headless serial console:

```sh
~/dev/gbc/build/gbc gbos.gb --headless --uncapped    # prints ABABAB...
```

A real gbos would render to VRAM; serial is the zero-VRAM debug channel.

### Bring-up bugs already found & fixed (kept as cautionary tales)

- **Stack vs. BSS clear:** the kernel stack (`SP=$D000`) grows down into
  `$CxFF`, so zeroing all of WRAM0 wiped the live return address. `ClearKernelRAM`
  now clears only `$C000-$CBFF`, leaving the stack region alone.
- **Syscall ABI vs. dispatch:** `SyscallTrap` originally used `DE` to index the
  jump table, clobbering the `write()` buffer pointer passed in `DE`. Dispatch
  now indexes via `A`/`HL` only, preserving `DE`/`B` for the handler.

## Writing programs in C

gbos programs can be written in **C**, compiled with **SDCC** (the `sm83` port).
This makes porting tiny Unix CLI tools realistic. The toolchain (`c/`):

- `c/gbos.h` — the userland API (`writes`, `readc`, `nl`, `getpid`, `sexit`).
- `c/libc.s` — syscall wrappers (asxxxx asm). Each saves
  `BC/DE/HL` around `rst $30`, since the trap clobbers them but SDCC expects
  them preserved.
- `c/crt0.s` — entry: `call main`; then `exit(0)`. Linked first so `_start` is
  the `$4000` entry.
- `c/build.sh` — compiles a `.c` with SDCC's **native** toolchain
  (`sdasgb` + `sdldgb`), links code at `$4000`, and extracts the ROM-bank blob.
  The Makefile INCBINs it and registers it as a program.

```c
#include "gbos.h"
void main(void) {
    writes("hello from C on gbos!", 21); nl();
    writes("my pid is ", 10);
    { char d = getpid() + '0'; writes(&d, 1); } nl();
}
```

### libc

`writes`, `putc`, `puts`, `nl`, `strlen`, `readc` (returns `EOF`=4 at end of
input), `getargs` (this program's argument string), `getpid`, `sexit`.
Arguments: the shell splits the command line at the first space and leaves
`"cmd\0args\0"` at `$A000`; the child inherits it through `fork`, and
`getargs()` returns the args.

### CLI tools (in `c/`)

| tool | what it does |
|------|--------------|
| `echo` | print its arguments (`echo hello world`) |
| `cat`  | copy stdin to stdout until EOF (Ctrl-D / pipe end) |
| `uname`| print the system name |
| `pid`  | print the process's pid (decimal) |
| `true` / `false` | exit 0 / 1 |
| `chello` | the C "hello" demo |

```
$ uname
gbos sm83 (Game Boy Color)
$ echo C tools on a Game Boy
C tools on a Game Boy
$ pid
5
$ cat
hi from cat
hi from cat
```

Each tool is a separate `c/<name>.c`, built to a ROM-bank blob, INCBIN'd, and
registered in the program table + shell command table (`src/programs.asm`).

**Toolchain gotchas found the hard way:** SDCC's `--asm=rgbds` mode mis-orders
instructions (emits `ld [hl],a` before the `ld hl,sp+0` that sets the pointer) —
so we use SDCC's native asxxxx path and INCBIN the blob instead. String literals
with embedded control chars are also mangled in rgbds mode; C programs pass
explicit lengths and emit newlines via `nl()`.

## Roadmap

- [x] `fork`: copy parent's 8 KiB RAM bank → free bank, child returns 0
- [x] `exec`: point `PROC_ROMB` at a program in a ROM bank, reset stack, enter
- [x] `exit` / `wait` / zombie reaping + cart-RAM bank recycling (free list)
- [ ] `exec` refinement: copy `.data` from ROM + zero `.bss` for RW globals
- [x] a real shell program: `fork`+`exec`+`wait` driven from the serial console
- [x] C toolchain: SDCC (sm83) + libc shim, C programs run as gbos processes
- [x] grow libc (`puts`/`putc`/`strlen`/`getargs`/EOF) + args via the shell
- [x] CLI tools in C: `echo`, `cat`, `uname`, `pid`, `true`, `false`
- [x] Makefile builds all C programs (a `CBLOBS` list)
- [ ] more tools (`wc`, `head`, `rev`), a real `argc/argv`, a RAM filesystem
- [ ] Preemptive scheduling: real context save in `TimerISR` → `hSwitchTo`
- [ ] Use the `$D000-$DFFF` u-area (SVBK) for per-process kernel state / kstack
- [ ] `brk`/heap allocator inside the task bank (heap up, stack down, collision = ENOMEM)
- [ ] A filesystem in remaining cart SRAM/flash (minix/v7-ish), paged 8 KiB at a time
- [ ] Swap whole tasks to cart flash when RAM banks are exhausted (UZI-style)
- [ ] VRAM console + keyboard/joypad `read()`
```