Int 21 Fn 2B01  - Quarterdeck Dos-up.sys V2.00 - Installation Check        [m]

   AX = 2B01h
   CX = 444Dh ('DM')
   DX = 4158h ('AX')

Return: AX = 0000h if installed
       BX = DOS-UP version (BH = minor, BL = major)
       CX = 4845h ('HE')
       DX = 5245h ('RE')
       ES = DOS-UP driver segment