;starts playing frequency (in hertz) passed via BX.
;destroys AX, BX, DX.

sound:    mov dx, 12h         ;high word of dividend 
          cmp dx, bx
          jnc s2              ;if freq > 18 then..
           in al, 61h
           test al, 00000011b  ;if not speaker data status
           jnz s1              ; then..
            or  al, 00000011b
            out 61h, al         ;set speaker data status
            mov al, 10110110b
            out 43h, al         ;counter 2 select, low byte then high byte, square wave, 16-bit
s1:        mov ax, 34DDh       ;end if
           div bx
           out 42h, al         ;counter 2 <- 1234DDh / freq: low byte
           mov al, ah
           out 42h, al         ;counter 2 <- 1234DDh / freq: high byte
s2:       ret                 ;end if 

nosound:                    in al, 61h
                            and al, 11111100b
                            out 61h, al        ;clear speaker data status
                            ret