汇编:cmp

assume cs:code, ds:data, ss:stack

data segment
	db	"Beginer's All - purpose symbolic instraction Code.",0
data ends

stack segment
	db 128 dup(0)
stack ends

code segment
	start:	mov dx, stack
		mov ss, dx
		mov sp, 128

		call clear_screen

		call init_reg

		mov si,0
		mov di,160*10+20*2
		call show_string	
		
		call up_Letter

		mov di, 160*11 + 20*2
		call show_string

		mov ax, 4C00H
		int 21H


;==========================================
up_Letter:	
		push ax
		push bx
		push cx
		push dx
		push si
		push di
		push ds
		push es

		
upLetter:	mov dl, ds:[si]
		cmp dl,0
		je upLetterRet    ;如果dl 等于0
		cmp dl, 'a'      
		jb nextLetter     ; 如果dl小于'a'
		cmp dl, 'z'
		ja nextLetter     ; 如果dl大于'z'
		and byte ptr ds:[si], 11011111B
		
nextLetter:
		inc si
		jmp upLetter



upLetterRet:
		pop es
		pop ds
		pop di
		pop si
		pop dx
		pop cx
		pop bx
		pop ax

		ret

;==========================================
show_string:
		push ax
		push bx
		push cx
		push dx
		push si
		push di
		push ds
		push es

showString:	mov dl, ds:[si]
		cmp dl,0
		je showStringRet  ; 等于0的话，跳到showStringRet
		mov es:[di],dl
		add di,2
		inc si
		loop showString

showStringRet:	pop es
		pop ds
		pop di
		pop si
		pop dx
		pop cx
		pop bx
		pop ax


		ret

;==========================================
clear_screen:	mov dx, 0B800H
		mov es, dx

		mov cx, 2000
		mov bx,0

	clrScr:	mov dx, 0700H
		mov es:[bx],dx
		add bx,2
		loop clrScr

		ret
;==========================================
init_reg:	mov dx, data
		mov ds, dx

		mov dx, 0B800H
		mov es, dx
		ret
code ends

end start