real mode

mem.py

#!/usr/bin/env python3
import argparse
import subprocess
import tempfile
import pathlib
import sys

PRESETS = {
    "ivt_int0":     (0x0000, 0x0000, "IVT: int 0 (divide-by-zero) handler offset"),
    "ivt_int0_seg": (0x0000, 0x0002, "IVT: int 0 handler segment"),
    "ivt_int10":    (0x0000, 0x0040, "IVT: int 0x10 (video) handler offset"),
    "ivt_int13":    (0x0000, 0x004C, "IVT: int 0x13 (disk) handler offset"),
    "bios_equip":   (0x0000, 0x0410, "BIOS Data Area: equipment word"),
    "bios_memkb":   (0x0000, 0x0413, "BIOS Data Area: base memory size (KB)"),
    "bios_tick":    (0x0000, 0x006C, "BIOS Data Area: timer tick counter (live!)"),
    "video_mem":    (0xB800, 0x0000, "Video memory: char+attr of top-left screen cell"),
}


def build_asm(seg: int, off: int, label: str) -> str:
    live = (label == PRESETS["bios_tick"][2])
    return f"""
BITS 16
ORG 0x7C00
mov ax, {seg:#06x}
mov ds, ax
{ '.watch:' if live else '' }
mov bx, [{off:#06x}]
mov cx, 4
.loop:
    rol bx, 4
    mov al, bl
    and al, 0x0F
    add al, '0'
    cmp al, '9'
    jbe .p
    add al, 7
.p:
    mov ah, 0x0E
    int 0x10
    loop .loop
{ 'mov ah, 0x0E' if live else '' }
{ "mov al, 13" if live else '' }
{ "int 0x10" if live else '' }
{ "jmp .watch" if live else "hlt" }
times 510 - ($ - $$) db 0
dw 0xAA55
"""


def main():
    parser = argparse.ArgumentParser(
        description="Boot a tiny NASM stub in QEMU that dumps a real-mode memory location to the screen."
    )
    parser.add_argument(
        "preset",
        nargs="?",
        choices=sorted(PRESETS.keys()),
        help="Which memory location to peek at.",
    )
    parser.add_argument(
        "-l", "--list",
        action="store_true",
        help="List available presets and exit.",
    )
    args = parser.parse_args()

    if args.list or not args.preset:
        print("Available presets:\n")
        for name, (seg, off, label) in PRESETS.items():
            print(f"  {name:14s} {seg:04x}:{off:04x}  {label}")
        if not args.preset:
            print("\nUsage: python3 peek.py <preset>")
            sys.exit(0 if args.list else 1)
        return

    seg, off, label = PRESETS[args.preset]
    asm_src = build_asm(seg, off, label)

    with tempfile.TemporaryDirectory() as tmp:
        asm_path = pathlib.Path(tmp, "b.asm")
        img_path = pathlib.Path(tmp, "b.img")
        asm_path.write_text(asm_src)

        print(f"Peeking at {seg:04x}:{off:04x} — {label}")
        subprocess.run(["nasm", "-f", "bin", str(asm_path), "-o", str(img_path)], check=True)
        subprocess.run(["qemu-system-i386", "-fda", str(img_path), "-boot", "a"], check=True)


if __name__ == "__main__":
    main()