Inline Assembly with Borland/Embarcadero

Question

A quick question here:

I'm trying to experiment with inline assembly using Embarcadero C++ Builder XE3 on 32-bit Windows 7, specifically to start re-teaching myself assembly with the possibility of future pragmatic application.

So far I've been able to write a set of instructions within a simple console project which compiles fine using all C++ Builder syntax at my disposal:

int _tmain(int argc, _TCHAR* argv[]) {
    _asm{
        mov dl, 0x24
        mov ah, 2 // Print character
        int 0x21 // Dos Interrupt - Run operation in AH

        mov dl, 0x25
        mov ah, 2 // Print character
        int 0x21 // Dos Interrupt - Run operation in AH

        mov dl, 0x26
        mov ah, 2 // Print character
        int 0x21 // Dos Interrupt - Run operation in AH

        mov dl, 0x27
        mov ah, 2 // Print character
        int 0x21 // Dos Interrupt - Run operation in AH

        int 0x20 // terminate COM
    }

    return 0;
}

This compiles ok, and these instructions work straight in NASM and the Windows native DEBUG environment. What I expect is a simple write to the console: $%&' followed by a successful termination.

However, when I try to run Project1.exe via the command line I immediately get a standard Win7 runtime error popup which disappears after a few seconds and terminates the program.

Running with debugging, I seem to be successfully reaching the first int 0x21 before it hangs, at which point Borland gives me the message:

Project Project1.exe raised exception class $C00000005 with message 'access violation at 0x00401213: read of address 0xffffffff'.

My theory is that I'm not properly initializing the segment with a 256-byte offset (NASM would use something like org 0x100 at the beginning of the instruction set). But I'm not entirely sure that's the issue.

Any ideas?

Answer 1

Those int 0x21 instructions are for MS-DOS and not Windows. They mean something else in a Windows program, with the result that you go up in flames.