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计算器实现简单的加减乘除运算（小于65536的正数）
①从键盘输入算数的等式
②不接受复合算数如3+5*7
③按‘=’号输出结果
④按‘C’号作为CLEAR清零
程序分为输入模块，判断模块，计算模块，输出模块。
其他：增加了一些特性
①能够得出两个正数相减得到的负数
②能够计算除法中的余数
③比较美观
代码过长由于增加了一些特性

注意 C清除，不要按回车键，按等于号自动出结果

DATA SEGMENT
divisors DW 10000, 1000, 100, 10, 1
results DB 0,0,0,0,0,"$" ;存放五位数ASCII码
data ends

STACK SEGMENT
STACK ENDS

CODE SEGMENT
ASSUME CS:CODE
MAIN PROC FAR
MOV AX,DATA
MOV DS,AX

NOW:
MOV AX,0
CALL INPUT

PUSH BX
;------------------------
CMP AL,'+'
JZ PLUS
CMP AL,'-'
JZ MINUS
CMP AL,'*'
JZ BY
CMP AL,'/'
JZ DIVD

PLUS:
CALL INPUT
POP AX
ADD AX,BX
JMP NEXT

MINUS:
CALL INPUT
POP AX
CMP AX,BX
JL LESS
SUB AX,BX
JMP NEXT
LESS:
SUB AX,BX
NEG AX
PUSH AX
MOV DL,'-'
MOV AH,02H
INT 21H
POP AX
JMP NEXT

BY:
CALL INPUT
POP AX
MUL BX
JMP NEXT

DIVD:
CALL INPUT
POP AX
CMP AX,BX
JL LESS2
DIV BX
JMP NEXT
LESS2:
PUSH AX
MOV DL,30H
MOV AH,02H
INT 21H
MOV DL,'~'
MOV AH,02H
INT 21H
POP AX
JMP NEXT
;---------------------------
NEXT:

CALL OUTPUT
JMP NOW
MOV AH,4CH
INT 21H
RET
MAIN ENDP
;---------------------
INPUT PROC NEAR
MOV BX,0
NUM:
MOV AH,1
INT 21H
CMP AL,'C'
JZ CLEAR
CMP AL,'+'
JZ EXIT
CMP AL,'-'
JZ EXIT
CMP AL,'*'
JZ EXIT
CMP AL,'/'
JZ EXIT

SUB AL,30H
JL EXIT
CMP AL,9
JG EXIT
CBW
XCHG AX,BX
MOV CX,10
MUL CX
XCHG AX,BX
ADD BX,AX
JMP NUM

CLEAR:
MOV DL,0DH
MOV AH,2
INT 21H
MOV DL,0AH
MOV AH,2
INT 21H
MOV AX,0
MOV BX,0
JMP NUM

EXIT:RET
INPUT ENDP
;----------------------
OUTPUT PROC NEAR
mov si, offset divisors
mov di, offset results 
mov cx,5 
CAL:
mov dx,0 
div word ptr [si]
add al,30H
mov byte ptr [di],al 
inc di 
add si,2 
mov ax,dx 
LOOP CAL
mov cx,4 
mov di, offset results 
NZ:
cmp byte ptr [di],'0' 
jne print
inc di 
loop NZ
print:
mov dx,di 
MOV AH,9
INT 21H
RET
OUTPUT ENDP
;---------------------
CODE ENDS
END MAIN