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.

System path redirection is applied to the values of certain parameters that take paths — see Entry Bitness for the full list. Paths starting with {sys} (or other value expanding to x:\Windows\System32) are automatically rewritten if needed so that on 64-bit entries, it is the 64-bit System directory that is accessed, and on 32-bit entries, it is the 32-bit System directory that is accessed.

For example, on 64-bit Windows, in this case:

[Files]
Source: "MyLibrary.dll"; DestDir: "{sys}"; Flags: 32bit

the file is actually installed to SysWOW64 (32-bit), not System32 (64-bit). But in this case:

[Files]
Source: "MyLibrary.dll"; DestDir: "{sys}"; Flags: 64bit

the file is installed to System32 (64-bit). Behind the scenes, however, 32-bit installers access the directory under the alias Sysnative (see table below).

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:

Entry 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 entry 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 entries, `Sysnative` should only be present in 32-bit entries.

^[1] With the ApplyPathRedirRules support function, the A64Bit parameter specifies the "entry bitness".

^[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).