串行接口

;************************;

;*8251串行通讯(自发自收)*;

;************************;

data segment

io8253a equ 280h

io8253b equ 283h

io8251a equ 2b8h

io8251b equ 2b9h

mes1 db 'you can play a key on the keybord!',0dh,0ah,24h

mes2 dd mes1

data ends

code segment

assume cs:code,ds:data

start: mov ax,data

mov ds,ax

mov dx,io8253b ;设置8253计数器0工作方式

mov al,16h

out dx,al

mov dx,io8253a

mov al,52 ;给8253计数器0送初值

out dx,al

mov dx,io8251b ;初始化8251

xor al,al

mov cx,03 ;向8251控制端口送3个0

delay: call out1

loop delay

mov al,40h ;向8251控制端口送40H,使其复位

call out1

mov al,4eh ;设置为1个停止位,8个数据位,波特率因子为16

call out1

mov al,27h ;向8251送控制字允许其发送和接收

call out1

lds dx,mes2 ;显示提示信息

mov ah,09

int 21h

waiti: mov dx,io8251b

in al,dx

test al,01 ;发送是否准备好

jz waiti

mov ah,01 ;是,从键盘上读一字符

int 21h

cmp al,27 ;若为ESC,结束

jz exit

mov dx,io8251a

inc al

out dx,al ;发送

mov cx,40h

s51: loop s51 ;延时

next: mov dx,io8251b

in al,dx

test al,02 ;检查接收是否准备好

jz next ;没有,等待

mov dx,io8251a

in al,dx ;准备好,接收

mov dl,al

mov ah,02 ;将接收到的字符显示在屏幕上

int 21h

jmp waiti

exit: mov ah,4ch ;退出

int 21h

out1 proc near ;向外发送一字节的子程序

out dx,al

push cx

mov cx,40h

gg: loop gg ;延时

pop cx

ret

out1 endp

code ends

end start