System Path Redirection

On 64-bit Windows, there are two "System" directories: one containing 64-bit DLLs/EXEs, called System32 on disk, and one containing 32-bit DLLs/EXEs, called SysWOW64 on disk.

In certain contexts where paths are specified, such as the DestDir parameter of the [Files] section, system path redirection is applied. Paths starting with {sys} (or other value expanding to x:\Windows\System32) are automatically rewritten if needed so that in 64-bit install mode, it is the 64-bit System directory that is accessed, and in 32-bit install mode, it is the 32-bit System directory that is accessed.

No rewriting occurs on systems running 32-bit Windows, as System32 is the only System directory. SysWOW64 doesn't exist, and the Sysnative alias isn't supported.

Paths are rewritten according to the following rules:

Input path bitness^[1]	Target process bitness^[2]	Substitution made	Explanation
64-bit	32-bit	`System32` → `Sysnative`	32-bit processes have to use the `Sysnative` alias to reach the "real" 64-bit `System32` directory.
32-bit	64-bit	`System32` → `SysWOW64`	64-bit processes can only access the 32-bit System directory as `SysWOW64`. All subdirectories are redirected as well; there are no exemptions.
32-bit	32-bit	Usually: `System32` → `SysWOW64` In special cases: `SysWOW64` → `System32`	In 32-bit processes, WOW64 file system redirection already routes `System32` to `SysWOW64`, but certain subdirectories are exempt (one example: `System32\drivers\etc`). Explicitly changing `System32` → `SysWOW64` bypasses those exemptions, keeping the redirection consistent with the 64-bit target process rule above. In a few special cases, the rule is flipped to `SysWOW64` → `System32`. These cases are: when setting values in the 32-bit `SharedDLLs` registry key, because `SysWOW64` isn't used there when passing paths to the 32-bit `regsvr32.exe`, because DLLs are traditionally registered with a `System32` path when registering/unregistering type libraries in 32-bit Setup/Uninstall, because type libraries are traditionally registered with a `System32` path when the `ApplyPathRedirRules` support function is called with `tp32BitPreferSystem32` in the target process parameter
64-bit	64-bit	`System32` → no change	`System32` is correct as-is when the input path and target process are both 64-bit.
Any	64-bit	`Sysnative` → `System32`	64-bit processes cannot use the `Sysnative` alias; it is only available in 32-bit processes. Although this rule is applied for both 32-bit and 64-bit input paths, `Sysnative` should only be present in 32-bit input paths.

^[1] By default, the "input path bitness" is the same as Setup/Uninstall's install mode — 32-bit in 32-bit install mode, and 64-bit in 64-bit install mode. In the [Files] and [Run] sections, the path bitness can be overridden on a per-entry basis using the 32bit and 64bit flags.

^[2] "Target process" is the process where the rewritten path will ultimately be accessed. In most cases, paths are accessed by Setup/Uninstall itself, so the target process bitness is Setup/Uninstall's bitness (as set by SetupArchitecture).