Simple assembler runtime error

[vraifreud]

I'm new to 32-bit assembler and when I wrote a simple program as follows:

.386
.model flat,stdcall
.stack 100h
.code
start:
mov ah,1
int 21h
end start


The compilation and linkage all passed well, but at runtime a dialogbox displayed: "filename.exe has encountered a problem and needs to close. We are sorry for the inconvenience."

The code "mov ah,1
int 21h" makes no sense, but when it is changed to any other code the runtime error occurs all the same.

Can anybody help me?

[Rockoon]

The sequence..

mov ah, 1
int 21h

..is from the days of 16-bit programming. Specifically, this is an MS-DOS system call used to terminate the program.

You are however assembling a 32-bit windows program and because of that, you cannot call 16-bit MS-DOS, and that is why you get the error when you leave this code in. The code raises an exception and windows forcibly terminates your program because you have not provided your own exception handler.

The reason you get an error when you remove it, is that your program never ends. The processor just keeps fetching memory trying to execute it until it find an illegal instruction, at which point an exception is raised and windows forcibly terminates your program because you have not provided your own exception handler.

Clear?

[dsouza123]

The following is 32 bit code that has standard program exit code.

Code Select Expand


.386
.model flat,stdcall
option casemap:none

include \masm32\include\windows.inc
include \masm32\include\user32.inc
include \masm32\include\kernel32.inc
includelib \masm32\lib\user32.lib
includelib \masm32\lib\kernel32.lib

.code
start:
  invoke ExitProcess,0
end start


Code Select Expand


  invoke ExitProcess,0

is treated as

Code Select Expand


  push 0
  call ExitProcess


[vraifreud]

Thank you all. I know now what the problem is and I've succeeded in running my first 32-bit assembler program by invoking ExitProcess.

[sinsi]

The sequence..

mov ah, 1
int 21h

..is from the days of 16-bit programming. Specifically, this is an MS-DOS system call used to terminate the program.

Nope, it's "READ CHARACTER FROM STANDARD INPUT, WITH ECHO"
AH=0 is the (non-recommended) way to terminate a DOS program.

[hutch--]

Common problem when first migrating rom DOS assembler. DOS interrupt driven os code is not available in 32 bit protected mode.

Code Select Expand


.386                    ; ok for 32 bit
.model flat,stdcall     ; model and calling convention OK
.stack 100h             ; incorrect in 32 bit PE, stack is set in linker
.code
start:
mov ah,1
int 21h                 ; < crash is here, interrupt fatal in protected mode windows
end start               ; address range for interrupts not available