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WOW64

WOW64 = Windows on Windows 64 bit.
Purpose: WOW64 is an emulator that allows 32 bit applications to be run on a 64 bit Windows system.
This is necessary because a 64-bit kernel expects 64-bit pointers when its syscalls are called, but a 32-bit application can only provide 32-bit pointers.
The emulator runs in user mode and provides an interface between the 32-bit version of Ntdll.dll and the kernel of the processor, and it intercepts kernel calls.

Involved DLLs

DLLs:
comment platform
wow64.dll Core infrastructure for the emulation / thunks for the ntoskrnl.exe entry-point functions. all
Wow64win.dll Entry points for win32k.sys for 32 bit applications all
wow64cpu.dll supports running x86 executables on x64 / switch CPU between 32 and 64 bit x64
IA32Exec.bin x86 software emulator Intel Itanium
Wowia32x.dll Interface between IA32Exec.bin and WOW64 Intel Itanium only
xtajit.dll x86 software emulator ARM64
wowarmw.dll Running ARM32 programs on ARM64 ARM64
Ntdll.dll
etc. ?
These DLLs as well as the 64-bit version of ntdll.dll are the only 64-bit binaries that can be loaded into a 32-bit process.

Environment variables

The values of some environment variables are dependent on the bitness of process that uses them.
environment variable 32 bit process 64 bit process
PROCESSOR_ARCHITECTURE x86 AMD64 or IA64 or ARM64
PROCESSOR_ARCHITEW6432 %PROCESSOR_ARCHITECTURE% n/a
ProgramFiles %ProgramFiles(x86)% %ProgramFiles%
ProgramW6432 %ProgramFiles% %CommonProgramFiles%
CommonProgramFiles %CommonProgramFiles(x86)% %CommonProgramFiles%
CommonProgramW6432 %CommonProgramFiles% %CommonProgramFiles%
Here are two cmd.exe batch files that try to demonstrate the differences between these values.

See also

The Registry keys HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node and HKEY_CLASSES_ROOT\WOW6432Node.
Determine if a Portable Executable is 32 or 64-bit.
cmd.exe: Determine if running in a 32 or 64 bit environment
The WinAPI function IsWow64Process (Windows 10: IsWow64Process2) allows a 32-bit application to check if it runs under WOW64.
%SystemRoot%\SysWOW64
VBA: 32-bit vs 64-bit

Index

Fatal error: Uncaught PDOException: SQLSTATE[HY000]: General error: 8 attempt to write a readonly database in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php:78 Stack trace: #0 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(78): PDOStatement->execute(Array) #1 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(30): insert_webrequest_('/notes/Windows/...', 1788383571, '216.73.217.21', 'Mozilla/5.0 App...', NULL) #2 /home/httpd/vhosts/renenyffenegger.ch/httpsdocs/notes/Windows/WOW64/index(103): insert_webrequest() #3 {main} thrown in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php on line 78