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 = version??? (0002h) CX = 4845h ('HE') DX = 5245h ('RE') ES = DOS-UP driver segment