求解释电子琴汇编代码，跪求各位大侠给出每步的注释

;*******************************;
;* 电子琴实验 *;
;*******************************;

data segment
io8255a equ 288h
io8255b equ 28bh
io8253a equ 280h
io8253b equ 283h
table dw 524,588,660,698,784,880,988,1048;高音的
;table dw 262,294,330,347,392,440,494,524;低音的
msg db 'Press 1,2,3,4,5,6,7,8,ESC:',0dh,0ah,'$'
data ends

code segment
assume cs:code,ds:data
start:
  mov ax,data
  mov ds,ax

  mov dx,offset msg
  mov ah,9
  int 21h ;显示提示信息
sing:  
  mov ah,7
  int 21h ;从键盘接收字符,不回显
  cmp al,1bh
  je finish ;若为ESC键,则转finish
  cmp al,'1'
  jl sing
  cmp al,'8'
  jg sing ;若不在'1'-'8'之间转sing
   
  sub al,31h
  shl al,1 ;转为查表偏移量
  mov bl,al ;保存偏移到bx
  mov bh,0
   
  mov ax,4240H ;计数初值 = 1000000 / 频率, 保存到AX
  mov dx,0FH
  div word ptr[table+bx]
  mov bx,ax
   
  mov dx,io8253b ;设置8253计时器0方式3, 先读写低字节, 再读写高字节
  mov al,00110110B
  out dx,al

  mov dx,io8253a  
  mov ax,bx
  out dx,al ;写计数初值低字节
   
  mov al,ah
  out dx,al ;写计数初值高字节
   
  mov dx,io8255b ;设置8255 A口输出
  mov al,10000000B
  out dx,al
   
  mov dx,io8255a  
  mov al,03h
  out dx,al ;置PA1PA0 = 11(开扬声器)
  call delay ;延时
  mov al,0h
  out dx,al ;置PA1PA0 = 00(关扬声器)
   
  jmp sing
finish:
  mov ax,4c00h
  int 21h
   
delay proc near ;延时子程序
  push cx
  push ax
  mov ax,15
x1: mov cx,0ffffh
x2: dec cx
  jnz x2
  dec ax
  jnz x1
  pop ax
  pop cx
  ret
delay endp
code ends
end start

超简单的代码，还要.....