#!/usr/bin/env python3
"""disasm16.py -- linear-sweep 16-bit x86 disassembler built on FCML.

Why this exists
---------------
We need an *independent* check on the bytes our Modula-2 compiler and runtime
emit.  The obvious candidates all turned out to be unusable:

  * objdump / binutils:  has no 16-bit x86 disassembler at all.  `-m i8086`
    silently falls back to the 32-bit i386 rules.
  * unicorn 2.1.4:      UC_MODE_16 mis-decodes 16-bit ModRM memory operands.

FCML (libfcml, Debian package `fcml`) does have a real 16-bit x86
disassembler, and we validated it against GNU as's `.code16` *encoder*
(assemble there, disassemble here).  See tests/FCML_PROBE.md.

FCML is driven through the `fcml-disasm` CLI because the package ships no
development headers, so ctypes struct layout would be guesswork.  The CLI
happens to be a linear sweeper that reports "Instruction code length", which
is all this driver needs.

The length agreement is the important part: if we emit a 3-byte instruction
where a 4-byte one was required, the sweep desynchronises and the very next
line points straight at the offending offset.

Usage:
    disasm16.py FILE [BASE]        # BASE defaults to 0, decimal or 0x-hex
    disasm16.py --bytes '89 6e 04' # inline bytes, for one-off probes
"""

import re
import subprocess
import sys

FCML = "fcml-disasm"

# The Debian 1.3.0 `fcml-disasm` wrapper aborts (SIGABRT, rc=134) on any
# buffer of 16 bytes or more: it linearly sweeps the *whole* input and blows
# up on the resulting instruction list.  Reproduce with 16 NOPs:
#     fcml-disasm -m16 0x90909090909090909090909090909090   # rc=134
# 15 bytes is safe, and 15 >= the longest real-mode instruction (7 bytes,
# e.g. `EA off16 seg16`), so a 15-byte window always contains the whole first
# instruction.  We only ever read the first instruction's length.
MAX_WINDOW = 15

RE_LEN = re.compile(r"^\s*Instruction code length:\s*(\d+)\s*$")
RE_TEXT = re.compile(r"^\s*Disassembled instruction:\s*(.*?)\s*$")


def decode(code, base):
    """Decode the first instruction of `code` (bytes). Returns (text, length).

    `code` is the remaining stream; at most MAX_WINDOW bytes are handed to
    FCML.  No padding is invented: an under-length buffer must surface as a
    decode error rather than being silently completed with made-up bytes.
    """
    win = code[:MAX_WINDOW]
    hexs = "".join("%02X" % b for b in win)
    out = subprocess.run(
        [FCML, "-m16", "-rh", "-rz", "-ip", hex(base), "0x" + hexs],
        capture_output=True, text=True)
    if out.returncode != 0:
        return None, 0
    text = None
    length = None
    for line in out.stdout.splitlines():
        m = RE_TEXT.match(line)
        if m:
            text = m.group(1)
        m = RE_LEN.match(line)
        if m:
            length = int(m.group(1))
    if length is None or length == 0 or length > len(code):
        return text, 0
    return text, length


def disasm(code, base=0, out=sys.stdout):
    """Linear-sweep `code` from `base`, printing one line per instruction."""
    pc = 0
    ninstr = 0
    while pc < len(code):
        text, length = decode(code[pc:], base + pc)
        if length == 0:
            out.write("%04X: %-24s <DECODE ERROR>\n"
                      % (base + pc, " ".join("%02X" % b
                                             for b in code[pc:pc + 8])))
            return ninstr, False
        shown = code[pc:pc + length]
        out.write("%04X: %-24s %s\n"
                  % (base + pc,
                     " ".join("%02X" % b for b in shown),
                     text if text else "<no text>"))
        pc += length
        ninstr += 1
    return ninstr, True


def main(argv):
    if len(argv) < 2:
        sys.stderr.write(__doc__)
        return 2
    if argv[1] == "--bytes":
        code = bytes.fromhex(argv[2].replace(" ", ""))
        base = int(argv[3], 0) if len(argv) > 3 else 0
    else:
        with open(argv[1], "rb") as f:
            code = f.read()
        base = int(argv[2], 0) if len(argv) > 2 else 0
    ninstr, ok = disasm(code, base)
    sys.stderr.write("-- %d instructions, %d bytes\n" % (ninstr, len(code)))
    return 0 if ok else 1


if __name__ == "__main__":
    sys.exit(main(sys.argv))