INT 2F U - DOS 3.3+ internal - IOCTL
AX = 122Bh
BP = 44xxh
SS = DOS DS (must be using a DOS internal stack)
additional registers as appropriate for INT 21/AX=44xxh
Return: as for INT 21/AH=44h
Notes:  equivalent to INT 21/AH=44h, but may only be called when already inside
a DOS function call
sets user stack frame pointer to dummy buffer, moves BP to AX, performs
IOCTL, and restores frame pointer

See also: 2144

2F122B - DOS 3.3+ internal - IOCTL