; DLL creation example

format PE64 GUI 4.0 DLL
entry DllEntryPoint

include 'win64a.inc'

var DD 1
var2   DQ 123
i DD 0
i2 DD 0
buf DD   4  DUP(?)

section '.code' code readable executable

proc DllEntryPoint hinstDLL,fdwReason,lpvReserved
        mov     eax,123456
        ret
endp




proc asm mode, i
       mov     eax,1
       ret
endp

; VOID ShowErrorMessage(HWND hWnd,DWORD dwError);

proc ShowErrorMessage hWnd,dwError
  local lpBuffer:DWORD
        lea     eax,[lpBuffer]
        invoke  FormatMessage,FORMAT_MESSAGE_ALLOCATE_BUFFER+FORMAT_MESSAGE_FROM_SYSTEM,0,[dwError],LANG_NEUTRAL,eax,0,0
        invoke  MessageBox,[hWnd],[lpBuffer],NULL,MB_ICONERROR+MB_OK
        invoke  LocalFree,[lpBuffer]
        ret
endp

; VOID ShowLastError(HWND hWnd);

proc ShowLastError hWnd
        invoke  GetLastError
        stdcall ShowErrorMessage,[hWnd],eax
        ret
endp

section '.idata' import data readable writeable

  library kernel,'KERNEL32.DLL',\
          user,'USER32.DLL'

  import kernel,\
         GetLastError,'GetLastError',\
         SetLastError,'SetLastError',\
         FormatMessage,'FormatMessageA',\
         LocalFree,'LocalFree'

  import user,\
         MessageBox,'MessageBoxA'

section '.edata' export data readable

  export 'ERRORMSG.DLL',\
         asm,'asm',\
         ShowErrorMessage,'ShowErrorMessage',\
         ShowLastError,'ShowLastError'

section '.reloc' fixups data discardable    

section '.reloc' fixups data readable discardable

    if $=$$
        dd 0,8          ; if there are no fixups, generate dummy entry
    end if
    

proc asm mode, inc
       xor eax,eax
       mov eax,dword [inc]
       ret
endp      

#include "App.h"
#include <windows.h>
#include <iostream>
#include "a.h"

using namespace std;

typedef int(__stdcall *f_funci)(unsigned char, unsigned int);

int main(int argc, char **argv) {
    
        HMODULE ll = LoadLibrary(L"asm.dll");

        if (ll == NULL) {
                cout << "asm.dll not loaded. Error code is " << GetLastError() << endl;
        }
        cout << "asm.dll loaded ok\n";



        f_funci assemblerMode = (f_funci)GetProcAddress(ll, "asm");

        if (!assemblerMode) {
                cout << "could not locate the function " << GetLastError() << endl;
        }
        cout << "assemblerMode() returned " << assemblerMode(1,143) << endl;
        
        App app(argc, argv);
    return app.exec();
}
    

Your code is 64-bit, but you are only reading 32-bits from the 'inc' parameter. Is that intentional?

proc asm mode, inc
       mov rax,rdx ;<--- 'inc' is in rdx
       ret
endp    

proc asm mode, inc, val
     ; rcx = mode
     ; rdx = inc
       mov rax,rcx
       ret
endp