assume cs:code

stack segment
  db 128 dup (0)
stack ends  

code segment

start:       mov ax,stack
             mov ss,ax
             mov sp ,128
             
             push cs
             pop ds
             mov ax,0
             mov es,ax
             mov si,offset int7ch
             mov di,200h
             mov cx,offset int7chend-offset int7ch
             cld
             rep movsb
             
             cli
             mov word ptr es:[7ch*4],200h
             mov word ptr es:[7ch*4+2],0
             sti
             
             mov ax,4c00h
             int 21h                      ;将7ch中断安装到0:200处；并将7ch的中断入口地址设置为0:200
             
int7ch:      push es
             push bx
             push cx
             push dx
             push bp
             push ax                      ;将用到的寄存器入栈
             mov ax,dx
             mov dx,0
             mov bp,1440
             div bp                       ;逻辑扇区号/1440
             push ax                      ;（逻辑扇区号/1440）的商入栈
             mov ax,dx
             mov dx,0
             mov bp,18
             div bp                       ;rem（逻辑扇区号/1440）/18
             mov ch,al                    ;rem（逻辑扇区号/1440）/18的商存入ch，为磁道号
             mov cl,dl                    
             inc cl                       ;rem（逻辑扇区号/1440）/18的余数存入cl，再加1，为扇区号
             mov dl,0                     ;设置驱动器号为0，即软驱A
             pop ax                       
             mov dh,al                    ;（逻辑扇区号/1440）的商出栈，结果存入dh，为面号
             pop ax                       ;参数输入值出栈，ah传递功能号，al传递扇区数
             add ah,2                     ;ah功能号加2，对应13h号中断中规定的读写号码
             int 13h                      ;调用13h中断
             pop bp
             pop dx
             pop cx
             pop bx
             pop es
             iret
             
int7chend:   nop

code ends
end start