OSDev.org

Posted: **Sat Oct 10, 2020 1:24 am**

I have my OS and am currently working on paging. Hence I need some asm to write to cr3, but since MSVC doesn't allow inline assembly in x64 I have to use yasm to assemble my assembly 'support' file and then link it with my MSVC code.
However, the calling convention between the two does not work. I have tried calling a test function from my C file built with MSVC:

Code: Select all

printInt(asm_test(324));

Declared as:

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extern uint32_t asm_test(uint32_t test);

An in assembly:

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global asm_test
asm_test:
	mov eax, dword [rcx]
	add dword [eax], 200#
	ret

However this does not work. I get a 0 returned when I pass in 324.
I'm using the microsoft x64 calling convention here as per the page on calling conventions but evidently it doesn't work.
Could anyone tell me what the problem here is? Do I need to specify flags when assembling under yasm?

Thank you very much in advance,

Heemogoblin

Posted: **Sat Oct 10, 2020 1:29 am**

Your code doesn't match your function prototype.

Try something like this:

Code: Select all

mov eax, ecx
add eax, 200
ret

Posted: **Sat Oct 10, 2020 1:37 am**

Great, that works. Thank you very much!

Posted: **Sat Oct 10, 2020 2:37 pm**

but since MSVC doesn't allow inline assembly in x64 I have to use yasm to assemble my assembly 'support' file and then link it with my MSVC code.

just out of curiosity, but you are aware of 64 bit MASM (ml64), right?

Posted: **Mon Oct 12, 2020 10:47 am**

zaval wrote:just out of curiosity, but you are aware of 64 bit MASM (ml64), right?

Yes, I just prefer yasm.

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