x32 x86 x64 - HOW TO DETECT PROCESS BITNESS

Detection Matrix

The general idea is to check the following environment variables:

    PROCESSOR_ARCHITECTURE - reports the native processor architecture EXCEPT for WOW64, where it reports x86.
    PROCESSOR_ARCHITEW6432 - not used EXCEPT for WOW64, where it reports the original native processor architecture.

Visually, it looks like:

Environment Variable \ Program Bitness ----- 32bit Native ---- 64bit Native ------ WOW64
PROCESSOR_ARCHITECTURE --------------------- x86 ------------- AMD64 ------------- x86
PROCESSOR_ARCHITEW6432 --------------------- undefined ------- undefined --------- AMD64

    WOW64 = 32bit Program on 64bit OS
    Replace AMD64 with IA64 for Itaniums

Detection Logic

The logic that I use from a program to detect whether the OS is 32bit or 64bit looks like this:

IF PROCESSOR_ARCHITECTURE == amd64 OR
   PROCESSOR_ARCHITEW6432 == amd64 THEN
   // OS is 64bit
ELSE
   // OS is 32bit
END IF

Another way to test for the same thing is:

IF PROCESSOR_ARCHITECTURE == x86 AND
   PROCESSOR_ARCHITEW6432 NOT DEFINED THEN
   // OS is 32bit
ELSE
   // OS is 64bit
END IF

While detection for whether a process is in WOW64 mode is to simply check for the presence of the PROCESSOR_ARCHITEW6432 environment variable.