INT 2F U - HIMEM.SYS v2.77+ - GET A20 HANDLER NUMBER
        AX = 4308h
Return: AL = 43h if supported
            BL = A20 handler number (value of /MACHINE:nn switch)
            BH = AT A20 switch time (00h medium, 01h fast, 02h slow)
SeeAlso: AX=4309h,AX=4330h

.