program Project1;

{$APPTYPE CONSOLE}
{$R *.res}

uses
  System.SysUtils;

function to_num(arr: array of byte; const i, j: integer): UInt64;
var
  k: integer;
begin
  Result := arr[i];
  for k := (i + 1) to j do
  begin
    Result := Result shl 8;
    Result := Result or arr[k];
  end;
end;

function Convert(arr: array of byte): UInt64;
asm
  {$IF Defined(CPUX86)}
  mov     edx, DWORD PTR [eax]
  mov     eax, DWORD PTR [eax+4]
  bswap   edx
  bswap   eax
  {$ELSEIF Defined(CPUX64)}
  mov rax, qword ptr [arr]
  bswap  rax
  {$ENDIF}
end;

begin
  try
    Writeln(to_num([1, 2, 3, 4, 5, 6, 7, 8], 0, 7));
    Writeln(Convert([1, 2, 3, 4, 5, 6, 7, 8]));
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  Readln;

end.

// Example program
#include <iostream>
#include <byteswap.h>

uint64_t Calc(unsigned char  *array, int i) {
void* p = reinterpret_cast<void*>(array + i);
return bswap_64(*static_cast<uint64_t*>(p) << 8 * i);
}

int main()
{
  unsigned char abc[8] = {1,2,3,4,5,6,7, 8};
  std::cout << Calc(abc, 5) << "!\n";
}

program Project1;

{$APPTYPE CONSOLE}
{$R *.res}

uses
  System.SysUtils;

function to_num(arr: array of byte; const i, j: integer): UInt64;
var
  k: integer;
begin
  Result := arr[i];
  for k := (i + 1) to j do
  begin
    Result := Result shl 8;
    Result := Result or arr[k];
  end;
end;

begin
  try
    Writeln(to_num([1, 2, 3, 4, 5, 6, 7, 8], 5, 7));
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  Readln;

end.

Calc(unsigned char*, int):                             # @Calc(unsigned char*, int)
        movsxd  rcx, esi
        mov     rax, qword ptr [rdi + rcx]
        shl     ecx, 3
        shl     rax, cl
        bswap   rax
        ret