Newbie here, I want to allocate a very large amount of data at the beginning for a hash map, about 4 megabytes, however with:



The exe file rises by 4 megabytes as well, this isn't desirable. What is the more reasonable way of doing this?

Depends from EXE type and OS. Put it into EXE's reserved area (AKA "B.S.S." ) or hog the memory with appropriate allocation features of the OS.

If this is for Windows, use one of the following APIs: GlobalAlloc, HeapAlloc, LocalAlloc, VirtualAlloc.
And the thread should be moved to the Windows forum.

If it is really Windows (in which case I'll move this to the Windows forum), then what you did is correct already, BUT you must make sure that "HashSpace rd 1048576" is the last thing in the source or PE section, if it is not then depending on whether the next lines of code define initialized data (or CPU instructions) or uninitialized data on the section the "rd 1048576" will act as "dd 1048576 dup (0)" or "rd 1048576" respectively.

I had moved it around and found that the big file size problem was magically solved but not understood why, thanks for your explanation Loco.

bot,

No magic here, addition of uninitialized bytes in the PE section does increase VirtualSize but not necessarily increase SizeOfRawData for that section, hence file size (increase can occur when the section has initialized bytes somewhere after the aforementioned uninitialized bytes, as LocoDelAssembly mentioned).