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page 80,132
title CP/DOS DosBeep mapper
dosxxx segment byte public 'dos'
assume cs:dosxxx,ds:nothing,es:nothing,ss:nothing
;**********************************************************************
;*
;* MODULE: dosbeep
;*
;* FUNCTION: generate a tone with desired frequency and duration
;*
;* CALLING SEQUENCE:
;*
;* push word frequency
;* push word duration (in milliseconds)
;* call dosbeep
;*
;* MODULES CALLED: none
;*
;*********************************************************************
public dosbeep
.sall
include macros.inc
inv_parm equ 0002h ;invalid parameter return code
str struc
old_bp dw ?
return dd ?
duratn dw ? ; duration
frqncy dw ? ; frequency
str ends
dosbeep proc far
Enter DosBeep
mov al,10110110b ; Set 8253 chip channel 2
out 43h,al ; to proper mode for tone
; Channel 2 is now set up as a frequency divider. The sixteen bit
; value sent to that port (in low-high format) is divided into
; 1.19 MHz, the clock speed. In order to send the proper value
; to the register, then, the frequency requested must be divided
; into 1,190,000.
mov dx,012h ; MSB of 1.19M
mov ax,2970h ; LSB of 1.19M
mov cx,[bp].frqncy ; divisor
mov bx,025h ; check frequency range
cmp cx,bx ; frequency ok ??
jl error ; branch if error
mov bx,7fffh
cmp cx,bx
jg error
div cx ; then divide
out 42h,al ; and output
mov al,ah ; directly to
out 42h,al ; the 8253 port.
; Turn on speaker
in al,61h ; Save original value
mov ah,al ; in ah
or al,3 ; Turn on control bit
out 61h,al ; in 8255 chip
; Now loop for DURATN milliseconds
mov cx,[bp].duratn ; load value
delay: mov bx,196 ; inner loop count
del2: dec bx ; a millisecond
jne del2 ; for each
loop delay ; iteration
; Turn speaker off
mov al,ah ; replace
out 61h,al ; original value
sub ax,ax ; set no error code
jmp exit ; return
error: mov ax,inv_parm
exit: MExit ; pop registers
ret size str - 6 ; return
dosbeep endp
dosxxx ends
end
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