BBoeOS

A minimal x86 operating system with a two-stage bootloader, shell, filesystem, networking stack, self-hosted assembler, and C compiler — all running in 16-bit real mode on a floppy disk.

Dependencies

• nasm: brew install nasm
• python3 (for add_file.py)

Building and running BBoeOS

• Build the binary

  ./make_os.sh

• Run with QEMU:

  qemu-system-i386 -drive file=drive.img,format=raw

• Run with serial console:

  qemu-system-i386 -drive file=drive.img,format=raw -serial stdio

• Add a file to the filesystem:

  ./add_file.py

• Run the self-hosting assembler test suite (diffs each program in static/ against NASM output after reassembling it inside the OS):

  ./test_asm.py # full suite ./test_asm.py edit # one program; artifacts kept in a temp dir

File Structure

src/include/          Shared includes
  constants.asm       Shared constants (memory addresses, filesystem params)
src/kernel/           Kernel assembly source
  ansi.asm            ANSI escape sequence parser, serial output
  bboeos.asm          Stage 1 boot code, shell loader, shared functions
  fd.asm              File descriptor table management (open, read, write, close)
  io.asm              Filesystem I/O (find_file, read_sector), visual_bell
  net.asm             NE2000 NIC driver: ARP, IP, ICMP, UDP
  syscall.asm         INT 30h syscall handler
  system.asm          Graphics mode, reboot, shutdown
src/asm/              User-space programs (assembly sources)
src/c/                User-space programs (C sources, compiled by cc.py)
add_file.py           Host-side script to add files to drive image
make_os.sh            Build script

Known limitations / TODO

• edit cannot open asm.asm. The gap buffer is 20 KB at 0x2000 with the 2.5 KB kill buffer at 0x7000, sandwiched between the edit binary (loaded at PROGRAM_BASE = 0x0600) and the resident kernel (stage 1 MBR at 0x7C00, stage 2 above it through ~0xE000). The hard ceiling for a contiguous gap buffer in segment 0 is ~27 KB (the gap from just past the edit binary up to 0x7C00); a separate ~4.5 KB of slack exists above the NIC buffers at 0xEE00–0xFFFF and could host the kill buffer, but static/asm.asm is ~118 KB so neither rearrangement helps. The real fix is to relocate the gap buffer into its own segment(s): one segment at e.g. 1000h:0000 gets 64 KB; splitting across two segments gets 128 KB and clears asm.asm with headroom. Requires widening gap_start/gap_end to 17-bit (or dword) and routing every BUF_BASE access through a segment-aware helper.

Resources

• https://neosmart.net/wiki/mbr-boot-process/
• https://en.wikibooks.org/wiki/X86_Assembly/Bootloaders
• http://www.ousob.com/ng/asm/ng1f806.php
• https://en.wikipedia.org/wiki/BIOS_interrupt_call
• ftp://ftp.embeddedarm.com/old/saved-downloads-manuals/EBIOS-UM.PDF