INT 21 - DOS 2.x and 3.3+ only - "AVAILDEV" - SPECIFY \DEV\ PREFIX USE
AH = 37h
AL = subfunction
02h get availdev flag
Return: DL = 00h \DEV\ must precede character device names
= nonzero \DEV\ is optional
03h set availdev flag
DL = 00h        \DEV\ is mandatory
= nonzero    \DEV\ is optional
Return: AL = status
00h successful
FFh unsupported subfunction
Notes:  all versions of DOS from 2.00 allow \DEV\ to be prepended to device
names without generating an error even if the directory \DEV does
not actually exist (other paths generate an error if they do not
exist).
although DOS 3.3+ accepts these calls, they have no effect, and
AL=02h always returns DL=FFh


2137 - DOS 2.x and 3.3+ only - "AVAILDEV" - SPECIFY \DEV\ PREFIX USE