Why 2 GB memory limit when running in 64 bit Windows?

Question

I\'m a member in a team that develop a Delphi application. The memory requirements are huge. 500 MB is normal but in some cases it got out of memory exception. The memory al

Answer 1

If you compile the Delphi application using the /LARGEADDRESSAWARE flag, it will be able to address the full 4GB on a 64-bit OS. Otherwise, when running in a WOW32, the OS assumes that the app is expecting the same environment that it would have on a 32-bit OS which means that of the 4GB of address space, 2GB is dedicated for the OS and 2GB is allocated to the application.

Answer 2

http://msdn.microsoft.com/en-us/library/aa366778(VS.85).aspx

User-mode virtual address space for each 32-bit process: 2 GB

Answer 3

The syntax in Delphi to set the LARGEADDRESSAWARE flag in the PE executable is:

{$SetPEFlags IMAGE_FILE_LARGE_ADDRESS_AWARE}

Put that in your .dpr file.

Answer 4

As long as you opt into receiving 32-bit pointers with the high-bit set (by including the LARGE_ADDRESS_AWARE PE flag), there is no 2GB limit.

Direct Observation

var
   p: Pointer;
   n: Int64;
begin
   p := Pointer($D0000000); //Above the 2GB line; and the 3GB line!

   p := VirtualAlloc(p, 1024, MEM_COMMIT or MEM_RESERVE, PAGE_READWRITE);
   if p = nil then
      RaiseLastWin32Error;

   n := Cardinal(p);
   ShowMessage(IntToHex(n, 16));
end;

Conclusion

There is no 2GB limit, on 64-bit Windows, as long as you swear you can handle pointers above $7FFFFFFF.

Note: Any code is released into the public domain. No attribution required.