assembly - 从汇编代码打印字符串时出错

Question

我是 assembly 新手，我学习的类(class)使用 EMU8086。我写这段代码来打印 Testing 和 50 的 ASCII 代码，即数字 2 但它只打印 Testing 并忽略其余部分。怎么了？

.model tiny
.code

org 100h


main proc

    mov ah, 09h
    mov dx, offset msg
    int 21h 
    mov ah, 4ch
    mov dx ,0  
    mov dl, 2
    add dl, 48
    int 21h      

endp

msg db "Testing$"

end main

阅读@500 - Internal Server Error 的评论后，我将代码更新为:

.model tiny
.code

org 100h


main proc

    mov ah, 09h
    mov dx, offset msg
    int 21h 
    mov dl, 2
    add dl, 48
    int 21h      

endp

msg db "Testing$"

end main

还是不行，怎么了？

Answer 1

DOS 中断 INT 21h/AH=9h不使用字符值进行打印，它使用内存偏移量到 $ 终止字符串的开头进行打印。

DOS 1+ - WRITE STRING TO STANDARD OUTPUT

AH = 09h

DS:DX -> '$'-terminated string

Return:

AL = 24h (the '$' terminating the string, despite official docs which state that nothing is returned) (at least DOS 2.1-7.0 and NWDOS)

如果您想使用 INT 21h/AH=9h 打印出单个字符，那么您需要将该值移动到以 $ 结尾的缓冲区中> 标志。然后将该缓冲区的地址传递给 INT 21h/AH=9h。根据你的第二个例子，这样的事情应该有效:

.model tiny
.code
org 100h

main proc
    mov ah, 09h             ; DOS Interrupt ah=9h print $ terminated string
    mov dx, offset msg      ; Address of msg 
    int 21h                 ; Int 21h/ah=9h Print msg 
    mov outchar, 48+2       ; Move ASCII val for `2` to outchar buffer
    mov dx, offset outchar  ; Address of the $ terminated outchar buffer in DX
    int 21h                 ; AH is still 9h, so this prints $ terminated string

    mov ax, 4c00h           ; Exit program with return value 0
    int 21h    
endp

msg db "Testing$"           ; msg string
outchar db ?, "$"           ; output buffer for single character terminated with $

end main

您可以像这样使用 ASCII 值而不是 mov outchar, 48+2:

mov outchar, '2'

---

或者，您可以通过一次调用 INT 21h/AH=9h 来完成此操作，方法是将您想要的字符放入输出缓冲区的中间:

main proc
    mov outchar, '2'    ; Place the ASCII value for '2' in the output buffer
    mov ah, 09h         
    mov dx, offset msg  
    int 21h             ; Print $ terminated string starting at `msg`
    mov ax, 4c00h
    int 21h             ; Exit with error code 0
endp

msg     db "Testing"
outchar db ?, "$"

之所以可行，是因为 INT 21h/AH=9h 会盲目打印它从偏移量 msg 开始找到的所有内容，直到找到 $ 终止字符。我们首先有效地替换了 outchar 处的字符，以便在执行 INT 21h/AH=9h 时它会在内存中遇到 Testing2$ .

一旦它到达 $ 它将停止打印所以输出看起来像:

Testing2

---

您还可以选择使用两个不同的 DOS (INT 21h) 中断。当 INT 21h/AH=9h 打印一个 $ 终止的字符串时，INT 21h/AH=2h显示单个字符:

DOS 1+ - WRITE CHARACTER TO STANDARD OUTPUT

AH = 02h

DL = character to write

Return: AL = last character output (despite the official docs which state nothing is returned) (at least DOS 2.1-7.0)

您可以编写您的程序以像您所做的那样显示 msg 字符串，但随后您可以使用 INT 21h/AH=2h 来显示单个字符。您的代码可能如下所示:

.model tiny
.code
org 100h

main proc
    mov ah, 09h             ; DOS Interrupt ah=9h print $ terminated string
    mov dx, offset msg      ; Address of msg 
    int 21h                 ; Int 21h/ah=9h Print msg 
    mov ah, 02h             ; DOS interrupt ah=2h print single character
    mov dl, '2'             ; DL = ASCII value of character to print
    int 21h                 ; Int 21h/ah=2h print single character in DL 

    mov ax, 4c00h           ; Exit program with return value 0
    int 21h    
endp

msg db "Testing$"           ; msg string

end main