Programming Forums > Application Development > Assembly

Klarre · Jan 29th, 2006, 11:44 AM

I am wondering how to do if I want to write functions in asm (not inline) and make calls to them from within a C++ program. The problem is how I shall tell g++ where the asmAdder() function definition is. The code looks like this:

C++ program

(Toggle Plain Text)

void asmAdd();

int main()
{
    asmAdd();

    return 0;
}

Asm function

(Toggle Plain Text)

.386
.model flat, STDCALL

.code
asmAdd proc
    push eax
    pop eax

    ret
endp asmAdd

end

I got the asm function as an obj file. How do I make my C++ program compile?

Thanks for your help!

/Klarre

Klarre · Jan 29th, 2006, 12:48 PM

I solved the problem by myself, by using VC++ and MASM!

/Klarre

DaWei · Jan 29th, 2006, 3:51 PM

Good. You merely have to link the two object files together. You must, additionally, make sure that the asm function treats the stack in the way that the C++ function expects it to. This is a matter of calling convention. In this case, with no arguments, you should be perfectly safe.

lectricpharaoh · Feb 14th, 2006, 8:42 PM

A few other things you should note: First, in addition to parameter order as DaWei pointed out, you will need to pay attention to symbol naming conventions. Many C/C++ compilers for Windows and DOS prepend an underscore on non-local symbols (function names, global variables, etc), and this is what the linker looks for. If you get a message that it cannot find _asmAdd(), you should just add this underscore in your assembly source. Second, make sure your assembler is configured to generate case-sensitive symbol names, since many assemblers, by default, use case-insensitive names, or export them as all uppercase. This of course causes havoc when trying to mix code with C/C++, which are case-sensitive. Lastly, you should check your compiler docs to see what registers you can safely use. If you clobber a register that your compiler expects will hold its value from one line to the next, it can cause nasty problems in your code that can be very hard to debug. The safest approach is to push any registers onto the stack if you plan to modify them, and pop them off after, but of course this is inefficient for registers the compiler considers 'scratch registers'. However, ones like ESP, EBP, and any segment registers should generally be considered off limits. The general registers are usually safe to use (but I believe this varies with some gcc ports, so check the docs), and ESI/EDI are often used for 'register variables', so they usually need to be preserved, too.

Klarre · Feb 15th, 2006, 1:28 AM

Thanks for your thoughts "lectricpharaoh"! I have not got any problem using the MSVC compiler and MASM. But I will remeber this when using other compilers and assmblers The code I wrote was this:

(Toggle Plain Text)

.586
.model flat, C

.code

asmAdder proc x:DWORD, y:DWORD
	fld x
	fmul y

	ret
asmAdder endp

end

(Toggle Plain Text)

#include <iostream>

extern "C"
{
	float asmAdder(float x, float y);
}

int main()
{
	std::cout << asmAdder(2.0f, 5.0f) << std::endl;

	return 0;
}

Jan 29th, 2006, 11:44 AM	#1
Klarre Game engine designer Join Date: May 2005 Location: Sweden Posts: 301 Rep Power: 4	Function call from C++ program I am wondering how to do if I want to write functions in asm (not inline) and make calls to them from within a C++ program. The problem is how I shall tell g++ where the asmAdder() function definition is. The code looks like this: C++ program (Toggle Plain Text) void asmAdd(); int main() { asmAdd(); return 0; } void asmAdd(); int main() { asmAdd(); return 0; } Asm function (Toggle Plain Text) .386 .model flat, STDCALL .code asmAdd proc push eax pop eax ret endp asmAdd end .386 .model flat, STDCALL .code asmAdd proc push eax pop eax ret endp asmAdd end I got the asm function as an obj file. How do I make my C++ program compile? Thanks for your help! /Klarre

Jan 29th, 2006, 3:51 PM	#3
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Good. You merely have to link the two object files together. You must, additionally, make sure that the asm function treats the stack in the way that the C++ function expects it to. This is a matter of calling convention. In this case, with no arguments, you should be perfectly safe. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Feb 14th, 2006, 8:42 PM	#4
lectricpharaoh Caffeinated Neural Net Join Date: Jun 2005 Location: Dry west coast of Canada Posts: 1,031 Rep Power: 5	A few other things you should note: First, in addition to parameter order as DaWei pointed out, you will need to pay attention to symbol naming conventions. Many C/C++ compilers for Windows and DOS prepend an underscore on non-local symbols (function names, global variables, etc), and this is what the linker looks for. If you get a message that it cannot find _asmAdd(), you should just add this underscore in your assembly source. Second, make sure your assembler is configured to generate case-sensitive symbol names, since many assemblers, by default, use case-insensitive names, or export them as all uppercase. This of course causes havoc when trying to mix code with C/C++, which are case-sensitive. Lastly, you should check your compiler docs to see what registers you can safely use. If you clobber a register that your compiler expects will hold its value from one line to the next, it can cause nasty problems in your code that can be very hard to debug. The safest approach is to push any registers onto the stack if you plan to modify them, and pop them off after, but of course this is inefficient for registers the compiler considers 'scratch registers'. However, ones like ESP, EBP, and any segment registers should generally be considered off limits. The general registers are usually safe to use (but I believe this varies with some gcc ports, so check the docs), and ESI/EDI are often used for 'register variables', so they usually need to be preserved, too. __________________ And once again, Probability proves itself willing to sneak into a back alley and service Drama as would a copper-piece harlot. - Vaarsuvius, Order of the Stick

Feb 15th, 2006, 1:28 AM	#5
Klarre Game engine designer Join Date: May 2005 Location: Sweden Posts: 301 Rep Power: 4	Thanks for your thoughts "lectricpharaoh"! I have not got any problem using the MSVC compiler and MASM. But I will remeber this when using other compilers and assmblers The code I wrote was this: (Toggle Plain Text) .586 .model flat, C .code asmAdder proc x:DWORD, y:DWORD fld x fmul y ret asmAdder endp end .586 .model flat, C .code asmAdder proc x:DWORD, y:DWORD fld x fmul y ret asmAdder endp end (Toggle Plain Text) #include <iostream> extern "C" { float asmAdder(float x, float y); } int main() { std::cout << asmAdder(2.0f, 5.0f) << std::endl; return 0; } #include <iostream> extern "C" { float asmAdder(float x, float y); } int main() { std::cout << asmAdder(2.0f, 5.0f) << std::endl; return 0; }

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Jan 29th, 2006, 12:48 PM	#2
Klarre Game engine designer Join Date: May 2005 Location: Sweden Posts: 301 Rep Power: 4	I solved the problem by myself, by using VC++ and MASM! /Klarre

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Programming Forums > Application Development > Assembly

Function call from C++ program