Int 2F/AX=12FFh/BL=00h

AX = 12FFh
BL = 00h

Return:
AL = DDh if installed
BH = state (00h disabled, nonzero enabled)
BL = readonly flag (00h writable, nonzero read-only)

Program: FDAK-DDT is the FreeDOS Alternative Kernel Device Drivers Testing release by Yury A. Semenov

See Also: AX=12FFh/BL=07h