Forge

ForgeOS — a 16-bit OS in pure Forge

A tiny 16-bit real-mode operating system written entirely in Forge — no inline assembly anywhere in the project. It is the reference implementation of the x86_16 target and lives in examples/os/.

Three-stage boot

Stage	Source	Output	Sectors	Load address	Format
1 — boot sector	`src/boot/boot.dev`	`build/boot.bin`	0	`0x7C00` (BIOS)	`flat`
2 — loader	`src/boot/loader.dev`	`build/loader.raw`	1–2	`0x9000`	`raw` (`LOAD 0x9000`)
3 — kernel	`src/kernel/kernel.dev`	`build/kernel.raw`	3–10	`0x7C00`	`raw`

The boot sector reads sectors 1–2 into 0x9000 with _dev_bios_disk_read (INT 13h CHS) and far-jumps there with _dev_jump.
The loader (running at 0x9000, clear of the kernel’s 0x7C00..0x8C00 region) reads sectors 3–10 into 0x7C00, resets segments, and far-jumps to the kernel.
Each stage’s .fld declares its LOAD address so the compiler fixes up absolute string addresses correctly (os-loader.fld, os.fld).

Build and run

cargo build --release
cd examples/os
./build.sh            # build build/os.img
./build.sh run        # ...and boot it in QEMU (floppy drive)

build.sh enforces the stage budgets: boot.bin = 512 B (with 0x55AA), loader.raw ≤ 1024 B, kernel.raw ≤ 4096 B.

For reliable operation under QEMU use the IDE disk with multi-thread TCG (the floppy controller hangs single-thread TCG):

qemu-system-x86_64 -accel tcg,thread=multi \
  -drive file=build/os.img,format=raw,if=ide -nographic

Shell

The kernel starts an interactive shell reading keys via BIOS int 16h:

help    - this list
hello   - greet from an app module
about   - about this OS
mem     - conventional memory size
calc    - square of a number, e.g. `calc 42`
dump    - hex dump of low memory
clear   - clear the screen
reboot  - reboot the machine

Each command is a plain Forge module under src/apps/, dispatched by src/shell/commands.dev.

Driving the shell from the host

For scripting, run QEMU with a monitor socket and use socat + sendkey:

qemu-system-x86_64 -accel tcg,thread=multi \
  -drive file=build/os.img,format=raw,if=ide -nographic \
  -monitor unix:/tmp/mon.sock,server,nowait

(sleep 7; printf 'sendkey c\nsendkey a\nsendkey l\nsendkey c\nsendkey spc\nsendkey 4\nsendkey 2\nsendkey ret\n') \
  | socat - UNIX-CONNECT:/tmp/mon.sock

Testing

The integration test os_dev_boots_shell_and_calc builds all three stages with forgec, assembles os.img, boots it in QEMU, types calc 42 into the shell over the monitor, and asserts the result (sq(0x002A) = 0x06E4). It needs qemu-system-x86_64 and socat on PATH.

This site is open source. Improve this page.