data segment 
      IOPORT EQU 0C400H-0280H
      IO8253 EQU IOPORT+283H
      IO8253A EQU IOPORT+280H
      IO8253B EQU IOPORT+281H
data ends

stack1 SEGMENT STACK
  DW 100H DUP(?)
Stack1 ENDS

CODE SEGMENT
  ASSUME CS:CODE,ds:data,ss:stack1

  START:  MOV  AX,Data
          MOV  DS,AX

          MOV  AL,36H     ;計數器0,寫高低字節,方式3   
          MOV  DX,IO8253   
          OUT  DX,AL      ;寫控制字
          MOV  AX,1000    ;計數初值1000  
          MOV  DX,IO8253A 
          OUT  DX,AL      ;先寫低字節
          mov  al,ah
          out  dx,al      ;再寫高字節
          

          mov  al,76h     ;計數器1,寫高低字節，方式3
          mov  dx,IO8253
          out  dx,al      ;寫控制字
          mov  ax,1000
          mov  dx,IO8253B
          out  dx,al
          mov  al,ah
          out  dx,al

          MOV  AH,4CH     
          INT 21H        ;返回dos

CODE ENDS
      END START