View source for Using NASM with D

= Introduction =

Even though D has an [http://dlang.org/iasm.html inline assembler], you can use external assemblers and link with the assembled object files. This page describes using [http://www.nasm.us/ NASM] with D, but any other assembler should work as long as it has a compatible object file output.

Here is an example of a sum function implemented in assembly:

'''''sum.asm'''''
<syntaxhighlight lang="ASM">
global  sum

section .text
sum:
        mov     eax, [esp+4]
        add     eax, [esp+8]
        ret
</syntaxhighlight>


And here it is used from D:

'''''main.d:'''''
<syntaxhighlight lang="D">
import std.stdio;
extern(C) int sum(int, int);  // C linkage - pass parameters on the stack, return result in eax

void main()
{
    writeln(sum(2, 2));  // should print 4
    assert(sum(2, 2) == 4);
}
</syntaxhighlight>


The next sections describe how to compile and link the two on various operating systems.

= D and NASM on Posix =

Using NASM and D on Posix works out of the box. To compile and link the code use:

<syntaxhighlight lang="bash">
nasm -felf32 sum.asm
dmd main.d sum.o
</syntaxhighlight>


GDC users can simply replace '''dmd''' with '''gdc''' in the command.

= D and NASM on Windows =

== Using DMD ==

Using NASM and D on Windows is more difficult since DMD outputs OMF object files which are incompatible with NASM's COFF object files. By using the '''-f obj''' switch NASM can output compatible OMF object files, but you also need to add a '''USE32''' (or '''bits32''') command in your .text section to force NASM to output 32bit object files. Additionally C functions on Windows traditionally have an underscore prepended to their name.

The modified .asm file then reads:

'''''sum.asm'''''
<syntaxhighlight lang="ASM">
global  _sum

section .text USE32
_sum:
        mov     eax, [esp+4]
        add     eax, [esp+8]
        ret
</syntaxhighlight>


To compile and link use:

<syntaxhighlight lang="bash">
nasm -f obj sum.asm
dmd main.d sum.obj
</syntaxhighlight>


Note that the linker (Optlink) might complain if you compile your assembly file with debugging symbols ('''-g''' switch).

== Using GDC ==

Unlike DMD, GDC uses COFF object files and the LD linker. The '''USE32''' command is not necessary (but will not interfere). On Windows C functions need to have an underscore prepended to them. The .asm file then reads:

'''''sum.asm'''''
<syntaxhighlight lang="ASM">
global  _sum

section .text
_sum:
        mov     eax, [esp+4]
        add     eax, [esp+8]
        ret
</syntaxhighlight>

To compile and link use:

<syntaxhighlight lang="bash">
nasm -f win32 sum.asm
gdc -m32 -o main.exe main.d sum.obj
</syntaxhighlight>

[[Category:HowTo]]