| UMBC CMSC 211 Fall 1999 |
| Bit | Meaning | |
|---|---|---|
| 0 | 0 - binary data, 1 - BCD | |
| 1-3 | Number of mode used (000-101) | |
| 4-5 | Operation Type 00 - send the value of counter 01 - read/write high byte 10 - read/write low/byte 11 - read/write both high and low bytes | |
| 6-7 | Number of channel to be programmed |
Channel 0 is used by teh system clock to calculate the time of day. It is programmed for 18.2 pulses per second (called ticks) and stored in the BIOS data area at 0040h:006Ch. Every pulse initiates a timer interrupt 8h (IRQ0). Don't mess with this one!
Channel 1 is responsible for refreshing RAM and counts the pulses during disk operations, so that it can reset the timer counter upon completion of an operation. This is also not a good one to mess with.
Channel 2 (port 42h) is connected to the computer's speaker and issues square wave pulses used to make sounds. You can change the sound frequency with this channel.The 8255 chip, (PPI) is also involved in generating sound and that bits 0 and 1 of port 61h also control the speaker.
;***************************************************************
; Program Sound ( Chapter 10 )
;
; The program for outputting a sound of prescribed tone
;
; Author: A.I.Sopin Voronezh, Russia 1990 --- 1992
; ------
;
; Call from Assembler programs:
;
; Call SOUND
;
; Parameters passed through the registers:
;
; Frequency - DI register (from 21 to 65535 hertz)
;
; Duration -BX register (in hundredth of second)
;
; Registers AX, CX, DX, DS, ES, SI are retained by the program
;
;
;
;***************************************************************
PUBLIC SOUND
CODE SEGMENT
ASSUME CS:CODE
SOUND PROC FAR
push ax
push cx
push dx
push ds
push es
push si
;-----------------------------------------------------------
in al,61h ; Read current port mode B (8255)
mov cl,al ; Save current mode
or al,3 ; Switch on speaker and timer
out 61h,al ;
mov al,0B6h ; set for channel 2 (8253)
out 43h,al ; command register 8253
mov dx,14h ;
mov ax,4F38h ; divisor of frequency
div di ;
out 42h,al ; lower byte of frequency
mov al,ah ;
out 42h,al ; higher byte of frequency
; Generation of sound delay
mov ax,91 ; multiplier - AX register !
mul bx ; AX =BX*91 (result in DX:AX)
mov bx,500 ; divisor, dividend in DX:AX
div bx ; result in AX, remainder in DX
mov bx,ax ; save result
mov ah,0 ; read time
int 1Ah ;
add dx,bx ;
mov bx,dx ;
Cycle: int 1Ah ;
cmp dx,bx ; Has time gone ?
jne Cycle ;
in al,61h ; Read mode of port B (8255)
mov al,cl ; Previous mode
and al,0FCh ;
out 61h,al ; Restore mode
;-----------------------------------------------------------
; Restoring registers and exit
Exit: pop si ;
pop es ;
pop ds ;
pop dx ;
pop cx ;
pop ax ;
RETF ; exit from subroutine
SOUND ENDP
CODE ENDS
END