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Andy



Joined: 17 Oct 2011
Posts: 55
Andy 02 Dec 2023, 23:51
I usually use fasm to generate binary code that will be called from a high level language but I am tired to write one version of code to compiled as 32 bit application and then to rewrite the code to compiled as 64 bit application. To illustrate what I mean I have the code below that is a basic function that adds two integers.

Code:
mov ax, cs
cmp ax, 33h     ; cs = 0x23/0x33 -> x86/x64 mode
je x64

use32
mov eax, [esp+4]
add eax, [esp+8]
ret 8

use64
x64:
mov rax, rcx
add rax, rdx
ret    


I read here that windows use a call to fs:[0xC0] to switch into 64 bit mode when running a 32 bit application on 64 bit CPU. I wonder if I can use this method to switch immediately into 64 bit mode when my function it's called so I don't have to write the same code for 32 bit also? If the answer it's yes, can I have an example?
Post 02 Dec 2023, 23:51
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bitRAKE



Joined: 21 Jul 2003
Posts: 3957
Location: vpcmipstrm
bitRAKE 03 Dec 2023, 04:41
I think this still works:
https://board.flatassembler.net/topic.php?p=97551#97551
(There is other code on the board - I just couldn't find it, atm.)

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Post 03 Dec 2023, 04:41
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Andy



Joined: 17 Oct 2011
Posts: 55
Andy 03 Dec 2023, 13:30
Thanks. I tried the example that you suggested but it crash the application. I suppose I should switch back to 64 bit mode before exit.

Let's take my example above with Add function. If the code it's called from a 32 bit application I want to jump directly to function and call it, that's straightforward.

But if the code it's called from a 64 bit application when I jump into 32 bit mode there should be some additional steps:
- since the fastcall send the first two parameters in rcx and rdx probably I should push them to stack according to stdcall calling convention
- run the 32 bit code
- switch back to 64 bit mode

Am I right?
Post 03 Dec 2023, 13:30
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bitRAKE



Joined: 21 Jul 2003
Posts: 3957
Location: vpcmipstrm
bitRAKE 03 Dec 2023, 17:02
From the high-level language (HLL) perspective there is no 32-bit/64-bit common solution for Windows. From the low-level perspective a general adaptor layer is non-trivial.

If the goal is to have your functions accessible from many high-level languages, both 32-bit and 64-bit execution environments, then my recommendation is to use macros to abstract the parameter passing on your functions - to meet the needs of the execution environment.

This will allow a single assembly function to be compiled for a large array of environments -- WITHOUT a runtime adaptor layer and getting the full benefit of assembly. Also, it requires no change on the HLL side.

https://en.wikipedia.org/wiki/X86_calling_conventions#List_of_x86_calling_conventions
Also, see the PROC macros in fasm[g].
Post 03 Dec 2023, 17:02
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Andy



Joined: 17 Oct 2011
Posts: 55
Andy 03 Dec 2023, 18:15
bitRAKE wrote:
If the goal is to have your functions accessible from many high-level languages, both 32-bit and 64-bit execution environments, then my recommendation is to use macros to abstract the parameter passing on your functions - to meet the needs of the execution environment.


This is exactly my goal but I still don't fully understand how this can be done with proc macros. Do I have to write a macro for stdcall procedure that will be called from a 32 bit environment and another one for fastcall when the procedure it's called from 64 bit environment?

I would be greateful if you can show me a basic example. If you want take the Add function from the first post as example so I can understand better. I am a beginner so it would be easier for me to understand.
Post 03 Dec 2023, 18:15
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bitRAKE



Joined: 21 Jul 2003
Posts: 3957
Location: vpcmipstrm
bitRAKE 03 Dec 2023, 19:05
With fasmg we don't need to get terribly complex - I like all the information for a function in one place = fewer surprises. Here is a terse example:
Code:
; set this on the command line for fasmg (example: -I "ABI := 2"):
;ABI := 1       ; 32-bit, __cdecl
ABI := 2        ; 64-bit, Microsoft x64 calling convention
;ABI := 3       ; 64-bit, System V AMD64 ABI

MyAdd:
        iterate <parm0,         parm1,          return>,\
                [esp+4],        [esp+8],        ret 8,\
                ecx,            edx,            retn,\
                edi,            esi,            retn

                indx ABI ; table selection based on convention
                mov eax, parm0
                add eax, parm1
                return
                break ; don't loop
        end iterate    
... obviously, there are many ways to accomplish this.

At the other extreme, we could build a custom syntax:
Code:
func MyAdd parm0:dword, parm1:dword
        mov eax, parm0
        add eax, parm1
        return
endf    
... I would use the PROC macros as a guide for creating such an abstraction layer.

edit: for fasm JohnFound has done a lot of work on creating a common framework for using x86 in multiple environments with Fresh IDE and its supporting code library.

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Post 03 Dec 2023, 19:05
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