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

delphi delphi-2007 win64
Roland Bengtsson picture Roland Bengtsson · Apr 29, 2010 · Viewed 14.7k times · Source

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 allocated in that cases is typically between 1000 - 1700 MB.

We of course want 64-bits compiler but that won't happen now (and if it happens we also must convert to unicode, but that is another story...).

My question is why is there a 2 GB memory limit per process when running in a 64 bit environment. The pointer is 32 bit so I think 4 GB would be the right limit. I use Delphi 2007.

EDIT: So if I set the IMAGE_FILE_LARGE_ADDRESS_AWARE flag in Delphi by using:

{$SetPeFlags IMAGE_FILE_LARGE_ADDRESS_AWARE}

And running the resulting Exe-file on a Windows Server 2003 x64 then the application can address 4 GB ?

  • Should I set /3GB switch in boot.ini ?
  • We have tried this but on a 32 bit Windows Server 2003 and it seems to limit the windows resources. There was more exceptions for "Out of memory" with GDIError in the log. But maybe this disappear when running in a 64 bit OS ?

Answer

Thomas picture Thomas · Apr 29, 2010

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.

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