Assembly Source series of Sound

This is the past DOS era of the compilation of source code, although has passed, but for the study of the assembly is still helpful, assembly language is just a basic programmer language, most people can grasp, not necessarily in-depth research.

; Sound.asm
;
; This is a small sound example for the IBM PC
; It'll play notes of the octave starting at Middle C.
;-----------------------------------------------------------------------------
; How to generate sound on the IBM PC:
;
; The 8255 (port 61H) bit 0 controls the 8253 timer
; Bit 1 Controls the speaker
;
; The output Channel 2 from the 8253 timer is anded with the
; Speaker control bit in order to turn on/off the speaker.
;
; Timer Channel 2 Internal count register (port 42h) is loaded in
; Two successive out operations with the lower byte being loaded.
; For this to work properly, the Timer command register has to be loaded
; With b6h.
;
; Since the input clock to the timer chip is 1.19318 MHz, the counter value
; To generate the frequency X can be calculated by 1193180/x.
;-----------------------------------------------------------------------------
; CX is used as a note counter with the frequency effectively being
; Incremented by a half tone and the corresponding count being loaded
; Into the count register in each iteration.
;
; No stack segment is needed to this small program, so don ' t panic
; When you receive the ' No stack segment ' Warning.
;
; Arne Asplem 880731 (MASM 4.0)
DSEG segment; Data segment
Notes DW 262,277,294,311,330,349,370,392,415,440,466,494
Dseg ends
CSEG segment
Sound proc Far
Assume Cs:cseg, ds:dseg, ss:nothing
Start
mov ax, dseg
mov ds, ax; Set up data segment (DS) Reg.
Xor Si, si
MOV bx, 12; Note Count
; Set up Timer command Register and counter register
mov al, 0b6h; Set 8253 Command Register
Out 43h, AL; For Channel 2, mode 3
Nloop:
mov ax, 34dch; Low part of clock freq.
MOV dx, 12h; Hight part of Clock freq.
div [notes + si]; Get the From data segment
Out 42h, AL; 8253 Command register (low byte)
mov al, ah
Out 42h, AL; 8253 command Regsieter (high byte)
; Turn on low bits in 8255 output port
In Al, 61h; Read current value of 8255 port
or AL, 3; Clear Low Bits
Out 61h, AL; Send new value to port
; Loop while # is sounding
MOV cx, 6d60h
Rpta
Loop Rpta; 1/10 sec delay
; Turn off speaker, check # count, set up next
Xor Al, 3
Out 61h, AL; Turn off speaker
MOV cx, 0af0h
　　　　
RPTB:
Loop RPTB; 1/100 sec delay
Inc si; Increment Note pointer
Inc si
Dec BX; Decrement Note counter
　　　　
JNZ Nloop; Loop until BX = 0
mov ax, 4c00h; Terminate program
int 21h
Sound ENDP
Cseg ends
End Start