dosint21 — raw INT 21h dispatch

import dosint21

A port-specific module that exposes the lower-level dos_int21_open / _read / _write / _close / _lseek primitives directly. Useful when you need DOS-flavoured semantics (get-error-code, specific access modes) that open() doesn’t expose.

⚠️ The Python open() builtin already uses this path. Only reach for the raw API when you need something open() can’t give you.

Functions

dosint21.open(path, mode) → fd

mode is the DOS AH=3D access-mode field:

Returns the DOS file handle on success (0–255). Raises OSError on failure.

import dosint21
fd = dosint21.open('CONFIG.SYS', 0)
print('opened fd', fd)

dosint21.read(fd, n) → bytes

data = dosint21.read(fd, 1024)
print(data.decode('cp437'))    # DOS default codepage

dosint21.write(fd, buf) → int

n = dosint21.write(fd, b'hello\r\n')

dosint21.close(fd)

dosint21.close(fd)

dosint21.lseek(fd, offset, whence=0) → int

whence: 0 = SEEK_SET, 1 = SEEK_CUR, 2 = SEEK_END.

dosint21.lseek(fd, 0, 2)     # to end
size = dosint21.lseek(fd, 0, 1)    # current position = size

dosint21.fsize(fd) → int

Convenience: returns file size in bytes without disturbing the current seek position.

How it works under the hood

All calls go through:

python → C dos_int21_open(...)
       → dos_int21_call(rmcs)
       → pktdrv_int_invoke(0x31, dpmi)   ; DPMI fn 0x0301
       → INT 0x31 (PM)
       → DPMI host's int 31 handler
       → switches to real mode at our CD 21 CB thunk
       → real INT 0x21
       → returns through DPMI to PM caller

The thunk + bounce buffer are pre-allocated at main() entry while the C stack is shallow, because PMODE/W’s DPMI dispatch has deep-stack sensitivities.

See also

This is a port-specific module; no upstream-MicroPython counterpart.