Clear keyboard buffer (INT 21h, 0Ch)

        Entry:
          AH = 0Ch AL must be 1, 6,  7,  8,  or  0Ah.

        Flushes all typeahead input, then executes function specified
        by AL (effectively moving it to AH and repeating the INT 21
        call). If AL contains a value not in the list above, the keyboard
        buffer is flushed and no other action is taken.