INT 21 - Phar Lap 386/DOS-Extender, Intel Code Builder - INSTALLATION CHECK
        AH = 30h
        EAX = 00003000h
        EBX = 50484152h ("PHAR")
Return: AL = major DOS version
        AH = minor DOS version
        EAX bits 31-16 = 4458h ('DX') if 386/DOS-extender installed
            BL = ASCII major version number
        EAX bits 31-16 = 4243h ('BC') if Intel Code Builder installed
            EDX = address of GDA
SeeAlso: AX=FF00h,INT 2F/AX=F100h