Why does my assembly program (x86) freezes in Dosbox when I try to run it?

问题

Things to note: Working in x86 assembly (16-bit); using Nasm; running program in DosBox.

When I try to run the program in DosBox, the emulator freezes (I'm not sure freezes is the right word since the cursor still blinks) and refuses to respond to input. The first time I tried running it DosBox actually crashed.

Here is my code:

;ASSIGNMENT 3

    org 100h

section.data:

prompt1 db  0dh, 0ah, 0dh, 0ah, "Please input a signed base-10 integer: $"
prompt2 db  0dh, 0ah, "Your number in binary is: $"
prompt3 db  0dh, 0ah, "Pretty sure that wasn't a number. Please enter a number value. $"

section.text:

START:
    mov ah, 9       ;Display input prompt
    mov dx, prompt1 
    int 21h

DEC_IN: 
    mov bx, 0       ;Get input
    mov ah, 1
    int 21h

    cmp al, 0dh     ;compare input to carriage return; check if user is finished
    je  DEC_OUT     ;if yes, go display the prompt

    cmp al, '0'     ;compare to '0'
    jg  IS_DIGIT    ;jump to IS_DIGIT to confirm that it is a number
    jl  NAN_ERROR   ;if below 0, print error prompt and start over

IS_DIGIT:
    cmp al, '9'     ;confirms digit value
    jl  BIN_CONV    ;if digit, convert to binary
    jg  NAN_ERROR   ;if not, print 'not a number' error message

BIN_CONV:
    and al, 0fh     ;converts ASCII to binary
    neg al      ;one's complement
    add al, 1       ;add 1 to make two's compliment
    jmp ADDIT       ;add to bx

ADDIT:
    or  bl, al      ;adds new digit to sum in bx
    int 21h
    jmp DEC_IN

NAN_ERROR:
    mov ah, 9       ;display error message
    mov dx, prompt3
    int 21h

    jmp START       ;Go back to beginning

DEC_OUT:
    mov ah, 9       ;Display the signed decimal value
    mov dx, prompt2
    int 21h

If it matters, the program is supposed to take input in the form of an unsigned decimal value and output it as a signed decimal value, and then as an octal value. I'm aware my program doesn't do that yet even it did run; it's still early in development.

Thanks in advance

回答1:

You should learn to use a debugger as "early in development" as possible.

That said, make sure your data is after the code, otherwise the cpu will try to execute it as instructions and that won't be very pretty.

Note that section.data: is doubly wrong, you must not use a colon since it's not a label, and you must put a space before the dot, as in section .data. Similarly for the section.text: of course. If you do that, nasm will be clever enough to place the data after the code.

Nevertheless, DOS .com files don't have sections, it's just a convenience feature of nasm that I don't recommend using.

来源：https://stackoverflow.com/questions/33356493/why-does-my-assembly-program-x86-freezes-in-dosbox-when-i-try-to-run-it

标签

assembly

x86

nasm