3.asm

里面有六个汇编程序

data    segment
	st1 db "The before letter is:$"
	st2 db "The behind letter is:$"
	st3 db "Please input one letter(A-Z,a-z):$"
data	ends
code	segment
	assume ds:data,cs:code
start:	mov ax,data
	mov ds,ax
	lea dx,st3
	mov ah,9
	int 21h
	mov ah,1
	int 21h
	mov bl,al
	mov dl,0ah
       	mov ah,2
       	int 21h
       	mov dl,0dh
       	mov ah,2
       	int 21h
	cmp bl,5ah
	je A1
	cmp bl,7ah
	je A1
	cmp bl,41h
	je A3
	cmp bl,61h
	je A3
	jmp A2
A1:	lea dx,st1
	mov ah,9
	int 21h
	mov dl,bl
	sub dl,1
	mov ah,2
	int 21h
	jmp A4
A2:	lea dx,st1
	mov ah,9
	int 21h
	mov dl,bl
	sub dl,1
	mov ah,2
	int 21h
	mov dl,0ah
	mov ah,2
       	int 21h
       	mov dl,0dh
       	mov ah,2
       	int 21h
A3:	lea dx,st2
	mov ah,9
	int 21h
	mov dl,bl
	add dl,1
	mov ah,2
	int 21h
A4:	mov ax,4c00h
	int 21h
code ends
end start