汇编语言关于2号功能函数的坑点

KevinBruce

发布于 2020-03-12 08:07:07

55700

代码可运行

文章被收录于专栏：CTF及算法学习CTF及算法学习

运行总次数：0

代码可运行

汇编语言的2号功能函数有小小的坑点，需要格外注意一下。

mov ah,2
int 21h

某些dos版本下，该功能函数会返回一个值并存在al中。

如果不注意这一点，有时候就会出错。比如如下代码：

DATAS SEGMENT
    ;此处输入数据段代码  
DATAS ENDS

STACKS SEGMENT
    ;此处输入堆栈段代码
STACKS ENDS

CODES SEGMENT
    ASSUME CS:CODES,DS:DATAS,SS:STACKS
START:
    MOV AX,DATAS
    MOV DS,AX
    mov ax,3633H
    call print
    MOV AH,4CH
    INT 21H
print proc
    push dx
    mov dl,ah
    ;入口参数是ax
    mov ah,2
    int 21h
    mov dl,al
    mov ah,2
    int 21h
    pop dx
    ret
print endp
CODES ENDS
    END START

理想的输出结果应该是63，而实际输出为66。

以下是debug过程的部分截图，可以验证该说法。

也可以参考如下文章： http://www.delorie.com/djgpp/doc/rbinter/id/65/25.html

文章内容

Category: DOS kernel
INT 21 - DOS 1+ - WRITE CHARACTER TO STANDARD OUTPUT

    AH = 02h
    DL = character to write
    （某些版本存在返回值）
Return: AL = last character output (despite the official docs which state
        nothing is returned) (at least DOS 2.1-7.0)
Notes:  ^C/^Break are checked, and INT 23 executed if pressed
    standard output is always the screen under DOS 1.x, but may be
      redirected under DOS 2+
    the last character output will be the character in DL unless DL=09h
      on entry, in which case AL=20h as tabs are expanded to blanks
    if standard output is redirected to a file, no error checks (write-
      protected, full media, etc.) are performed
SeeAlso: AH=06h,AH=09h