In my high-school homework I have to write a program, that uses DOS Interrupts to input and output strings instead of std printf/scanf But when I attempting to run this program:
format ELF
use16
section '.data' writeable
msg db 'Hello, world!', 0
section '.text' executable
public _main
_main:
mov ebp, esp; for correct debugging
mov ah, 1
int 21h
mov ah,4Ch
int 21h
xor eax, eax
ret
It's just crashing. I attached debugger and found out that it crashes on this line: int 21h. I absolutely have no ideas about why it happens.
I use FASM, SASM IDE and Windows XP SP3 x32
When using SASM IDE and you use format ELFin your assembly code, FASM will assemble the file to an ELF object (.o file) and then use (by default) a MinGW version of GCC and LD to link that ELF object to a Windows executable (PE32). These executables run as native Windows programs, not DOS. You can not use DOS interrupts inside a Windows PE32 executable since the DOS interrupts do not exist in that environment. The end result is that it crashes on the int 21h.
If you want to create a DOS executable that can run in 32-bit Windows XP you could do this:
format MZ ; DOS executable format
stack 100h
entry code:main ; Entry point is label main in code segment
segment text
msg db 'Hello, world!$' ; DOS needs $ terminated string
segment code
main:
mov ax, text
mov ds, ax ; set up the DS register and point it at
; text segment containing our data
mov dx, msg
mov ah, 9
int 21h ; Write msg to standard output
mov ah, 4Ch
int 21h ; Exit DOS program
This will generate a DOS program with an exe extension. Unfortunately you can not use SASM IDE to debug or run a DOS program. you can run the generated program from the 32-bit Windows XP command line. 32-bit versions of Windows run DOS programs inside the NTVDM (virtual DOS machine).