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I’ve been looking into Window’s PE format lately and I have noticed that in most examples,
people tend to set the ImageBase offset value in the optional header to something unreasonably high like 0x400000.
What could make it unfavorable not to map an image at offset 0x0?
First off, that’s not a default of Windows or the PE file format, it is the default for the linker’s /BASE option when you use it to link an EXE. The default for a DLL is 0x10000000.
Selecting /BASE:0 would be bad choice, no program can ever run at that base address. The first 64 KB of the address space is reserved and can never be mapped. Primarily to catch null pointer dereference bugs. And expanded to 64KB to catch pointer bugs in programs that started life in 16-bits and got recompiled to 32-bits.
Why 0x40000 and not 0x10000 is the default is a historical accident as well and goes back to at least Windows 95. Which reserved the first 4 megabytes of the address space for the “16-bit/MS-DOS Compatibility Arena”. I don’t remember much about it, Windows 9x had a very different 16-bit VM implementation from NT. You can read some more about it in this ancient KB article. It certainly isn’t relevant anymore these days, a 64-bit OS will readily allocate heap memory in the space between 0x010000 and 0x400000.
There isn’t any point in changing the /BASE option for an EXE. However, there’s lots of point changing it for a DLL. They are much more effective if they don’t overlap and thus don’t have to be relocated, they won’t take any space in the paging file and can be shared between processes. There’s even an SDK tool for it so you can change it after building, rebase.exe