INT 21 - Quarterdeck DOS-UP.SYS v2.00 - INSTALLATION CHECK                      
	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