Holy cow, I wrote a book!
If the system directory is always %windir%\SYSTEM32,
why is there a special function to get it?
Because it wasn't always that.
For 16-bit programs on Windows NT, the system directory
is %windir%\SYSTEM. That's also the name of the
system directory for Windows 95-based systems and all the 16-bit
versions of Windows.
But even in the 16-bit world, if it was always
%windir%\SYSTEM, why have a function for it?
Because even in the 16-bit world, it wasn't always
Back in the old days, you could run Windows
directly over the network. All the system files were
kept on the network server, and only the user's files were kept
on the local machine. What's more, every single computer
on the network used the same system directory on the server.
There was only one copy of USER.EXE, for example, which
Under this network-based Windows configuration, the
system directory was a directory on a server somewhere
and the Windows directory was a directory on the local
machine (C:\WINDOWS). Clients did not have write permission into
the shared system directory, but they did have permission to write
into the Windows directory.
That's why GetSystemDirectory is a separate function.