Port 00F  -  8237A-5 Multiple Mask Register

        All PCs except PCjr: A write sets all mask register bits.

        xxxx 1xxx  1=set channel 3 mask bit, 0=clear
        xxxx x1xx  1=set channel 2 mask bit, 0=clear
        xxxx xx1x  1=set channel 1 mask bit, 0=clear
        xxxx xxx1  1=set channel 0 mask bit, 0=clear