File structure

Host-side build tooling

  • add_file.py — Host-side script to add files to the drive image filesystem
  • cc.py — Host-side C subset compiler (translates src/c/*.c to NASM-compatible assembly)
  • make_os.sh — Build script (assembles kernel, compiles C programs via cc.py, creates floppy image)

Shared includes

  • src/include/constants.asm — Shared constants (BUFFER, DIRECTORY_SECTOR, SECTOR_BUFFER, EXEC_ARG, NE2K_BASE, PROGRAM_BASE, SYS_* syscall numbers, etc.)
  • src/include/dns_query.asm, encode_domain.asm, parse_ip.asm — Shared DNS/IP helpers; see source headers for calling conventions.

Boot and kernel core

  • src/arch/x86/boot/boot.asm — Pre-paging boot binary (org 0x7C00): MBR + post-MBR real-mode bootstrap (second INT 13h read of kernel.bin directly into phys 0x20000, E820 probe, PIC remap, A20, GDT load, CR0.PE flip) + 32-bit early_pe_entry (build boot PD + first kernel PT, enable paging, far-jump to high_entry at virt 0xC0020000). No real-mode-to-PE relocation copy — KERNEL_LOAD_PHYS = 0x20000 is the kernel’s final home. Post-MBR region pads to the next 512-byte boundary; BOOT_SECTORS is derived ((boot_end - post_mbr_continue) / 512) so the count auto-grows when the boot code crosses a sector. make_os.sh only has to measure kernel.bin’s sector count for the -DKERNEL_SECTORS=N second-pass nasm invocation.
  • src/arch/x86/kernel.asm — Post-paging high-half kernel (org 0xC0020000 = DIRECT_MAP_BASE + KERNEL_LOAD_PHYS): high_entry (segment / GDT / IDT / stack setup, identity-drop, bitmap init, kernel-PT allocation, kernel_idle_pd build, boot-PD freeback) followed by %includes of every kernel subsystem (drivers, fs, helpers, net stack, syscall dispatcher, IDT, post-flip entry, frame allocator, address-space helpers) and the kernel GDT + vDSO blob (incbin "vdso.bin").
  • src/arch/x86/idt.asm — 32-bit IDT with CPU exception stubs and INT 30h gate; idt_init (called from high_entry) patches the high-half handler offsets at boot since the IDT_ENTRY macro can only emit the low 16 bits in nasm -f bin mode (section-relative labels reject & 0FFFFh / >> 16 arithmetic). Any post-flip exception lands in exc_common and prints EXCnn EIP=h CR2=h ERR=h on COM1.
  • src/arch/x86/entry.asm — protected_mode_entry (TSS patch, PIT + IRQ handler install, driver / VFS / NIC inits, banner) flowing into shell_reload (loads bin/shell and jumps), program_enter (fd reset, BSS zero via the trailer-magic protocol, ESP snapshot, iretd to ring 3 at PROGRAM_BASE), and the IRQ 0 / IRQ 6 handlers. Segment / GDT / IDT / ESP setup happens in kernel.asm’s high_entry before falling into protected_mode_entry; the ring-0 stack lives at virt 0xC0000000 + KERNEL_RESERVED_BASE (~0xC0028000 for the current kernel size — see Kernel stack note in CLAUDE.md), not in entry.asm.

Memory management

  • src/memory_management/frame.asm — Bitmap physical-frame allocator: frame_alloc / frame_free / frame_init (two-pass E820 walker — first pass finds the highest type=1 frame base, between passes sizes the bitmap, second pass marks free regions) / frame_reserve_range. Bitmap is sized at boot from E820, clamped to FRAME_PHYSICAL_LIMIT (~4 GB, the 32-bit phys ceiling), and lives in the post-kernel cluster at FRAME_BITMAP_PHYS. Frames above FRAME_DIRECT_MAP_LIMIT (the kernel direct-map ceiling) reach the kernel via the kmap window — see kmap.asm.
  • src/memory_management/access.asm — access_ok user-buffer pointer-validation helper (rejects ranges that span the user/kernel boundary or wrap), invoked by syscall handlers before touching userspace memory.
  • src/memory_management/address_space.asm — Per-program PD lifecycle: address_space_create (allocate PD, copy-image kernel half from kernel_idle_pd, build user PTs for handoff frame + vDSO + program text/BSS + stack), address_space_destroy (free user frames, skipping shared-AVL PTEs, then free PTs and PD), and the page-mapping primitives (address_space_map_page, etc.) those two drive. All PD/PT reads and writes go through kmap_map/kmap_unmap so a high-physical PD or PT frame stays addressable.
  • src/memory_management/kmap.asm — kernel temporary-mapping window at PDE 1023 (virt 0xFFC00000..0xFFFFFFFF). kmap_init (called from high_entry after the idle PD takes over) allocates one frame as the window PT and installs it at kernel_idle_pd[1023]; per-program PDs inherit it via the kernel-half copy-image in address_space_create. kmap_map(eax = phys) → eax = kernel_virt fast-paths to the direct-map alias when phys is below FRAME_DIRECT_MAP_LIMIT and falls back to a slot in the window for higher frames. kmap_unmap releases the slot. KMAP_SLOT_COUNT = 4 covers the deepest concurrent nesting in the tree (PD + PT walks in address_space_destroy); slot exhaustion panics.

Syscalls and system

  • src/arch/x86/syscall.asm — INT 30h dispatcher: per-handler bodies inlined directly (fs_, io_, rtc_, sys_) and tail-jump shims into src/syscall/syscalls.c for the four net_* handlers. The .iret_cf path sign-extends AX into EAX before iret; .iret_cf_eax is the explicit-32-bit variant used by io_read / io_write, whose byte counts can exceed 32 767.
  • src/syscall/syscalls.c — C bodies for the four non-trivial network handlers: sys_net_mac, sys_net_open, sys_net_recvfrom, sys_net_sendto. Reached via call sys_net_X; jmp .iret_cf shims in syscall.asm.
  • src/arch/x86/system.c — reboot (8042 reset), shutdown (APM / QEMU / Bochs shutdown ports).

Drivers

  • src/drivers/console.c — Unified output: put_character (ANSI parser + screen + serial mirror with auto \n → \r\n) and put_string. Delegates raw bytes to serial_character (drivers/serial.c) and ANSI cursor / palette commands to the VGA helpers in drivers/vga.c.
  • src/drivers/serial.c — COM1 driver: serial_character (output) and serial_check / serial_read (input, polled by fd_read_console).
  • src/drivers/ata.c, src/drivers/fdc.c — Hardware disk drivers (ATA PIO and floppy DMA); called via fs/block.asm’s read_sector / write_sector dispatch (AX = 0-based sector number).
  • src/drivers/ne2k.c — NE2000 ISA NIC driver (polled-mode Ethernet); I/O base 0x300, IRQ 3.
  • src/drivers/ps2.c — PS/2 keyboard driver: ps2_init, ps2_check, ps2_read.
  • src/drivers/rtc.c — RTC / PIT timer: tick counter, rtc_sleep_ms busy-wait, CMOS date read.
  • src/drivers/vga.c — VGA driver: text and mode-13h helpers (vga_set_mode, vga_clear_screen, vga_fill_block, vga_set_palette_color, …) plus fd_ioctl_vga (the /dev/vga ioctl dispatcher for VGA_IOCTL_MODE / VGA_IOCTL_FILL_BLOCK / VGA_IOCTL_SET_PALETTE).

Filesystem and VFS

  • src/fs/fd.c — File descriptor table and dispatch: fd_open (synthesizes /dev/vga into FD_TYPE_VGA without touching the filesystem), fd_read, fd_write, fd_close, fd_fstat, fd_ioctl. Per-fd-type handlers live under src/fs/fd/ (console.c, fs.c, net.c).
  • src/fs/block.asm — Block I/O dispatcher: read_sector, write_sector (dispatches to fdc/ata based on boot_disk).
  • src/fs/bbfs.asm — BBoeOS filesystem (VFS backend): bbfs_chmod, bbfs_create, bbfs_find, bbfs_init, bbfs_load, bbfs_mkdir, bbfs_rename, bbfs_update_size, plus internal helpers (find_file, scan_directory_entries, etc.).
  • src/fs/ext2.asm — ext2 filesystem (second VFS backend, auto-detected by vfs_init).
  • src/fs/vfs.c — VFS layer: runtime function-pointer table (vfs_find_fn, etc.), vfs_found_* state struct, thin wrapper functions (vfs_find, vfs_create, vfs_rmdir, …). Detects bbfs vs ext2 at boot and points the function pointers at the corresponding backend.

Networking

  • src/net/net.asm — Four-line orchestrator that %includes the protocol modules: net/arp.asm, net/udp.asm, plus build/kernel-c/net/icmp.kasm and build/kernel-c/net/ip.kasm (cc.py-compiled from src/net/icmp.c and src/net/ip.c). The NE2000 hardware driver itself lives in drivers/ne2k.c.

Userland programs

  • src/c/ — user-facing programs written in the C subset: arp, asm, cat, chmod, cp, date, dns, draw, echo, edit, ls, mkdir, mv, ping, rm, rmdir, shell, uptime.
  • src/c/edit.c — Full-screen text editor with gap buffer, Ctrl+S save, Ctrl+Q quit. All editor state is file-scope so cc.py parks it in BSS rather than auto-pinning to registers that buffer_character_at clobbers (it uses EDX/ECX as scratch). The 448 KB gap buffer (edit_buffer[EDIT_BUFFER_SIZE], sized at 0x70000 to fit under the -m 1 user-pool ceiling) and 2.5 KB kill buffer (edit_kill_buffer[EDIT_KILL_BUFFER_SIZE]) are BSS arrays — the per-program PD that address_space_create builds gets enough zero-filled user pages to back them via the trailer-magic protocol. Any source file in the tree fits with room to spare (the largest, src/c/asm.c, is ~131 KB). A single read(fd, edit_buffer, EDIT_BUFFER_SIZE) fills the buffer in one call: SYS_IO_READ returns the full 32-bit byte count via EAX (the dispatcher routes io_read through .iret_cf_eax, skipping the AX sign-extend that the rest of the syscall surface uses), so no chunking is needed up to the 448 KB buffer cap.
  • src/c/asm.c — Self-hosted x86 assembler (two-pass; byte-identical to NASM for everything in static/). Phase 1 port: the driver and handlers still live inside a single file-scope asm("...") block that wraps archive/asm.asm’s original NASM source; follow-up PRs extract pieces into pure C one family at a time. Supported directives and mnemonics are documented in the inline-asm body.

Smoke tests

  • tests/programs/ — smoke tests written in the same C subset, kept separate from user programs because they exist solely to exercise specific kernel paths from tests/test_programs.py that need a real boot to verify: bigbss (256 KB BSS allocation across per-program PD pages — also kmap window end-to-end smoke), bits (bitwise operators | ^ ~ << >> & and compound-assignment forms), booltest (booleanized comparison BinOps used as values), cftest / fctest (call-flow / function-call edge cases), gptest (user-fault #GP kill path in idt.asm’s exc_common — executes cli at CPL=3, expects shell respawn), loop (basic while-loop control flow with per-character printf), loop_array (sizeof(arr)/sizeof(elt) folding + indexed string-array reads via write), nullderef (#PF on virt 0 with the unmapped PTE[0] guard), okptest (user-buffer pointer validation via access_ok), stackbomb (16-page user-stack overflow into the unmapped guard region), stacktop (asserts USER_STACK_TOP = KERNEL_VIRT_BASE to catch constant drift). Pure cc.py codegen tests (file-scope asm() escape, #include directive, asm_register global aliasing, file-scope BSS globals, brace-initialized global arrays) live in tests/test_kernel_cc.py as compile-and-inspect-asm pytest cases — no QEMU boot needed. Built and packaged into the drive image’s bin/ only when make_os.sh is run with --with-test-programs; the default build keeps them out of the image so a normal boot ships only the user programs. tests/test_programs.py passes the flag automatically.
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