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Martin
Asked: 2023-03-03 00:08:59 +0800 CST

How does Windows set the %SystemRoot% variable?

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The %SystemRoot% environment variable contains the path to the Windows directory. (e.g. c:\windows)

SystemRoot is not configured in Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment, it is automagically set.

Now, typically on Windows 10 this directory has the caseing C:\Windows on NTFS disk.

After a script test(*) broke, we noticed:

  • With same OS version Windows 10 (LTSC 1809 17763.3887)
  • Some boxes use %SystemRoot% == C:\WINDOWS (all uppercase, despite the directory on-disk being C:\Windows)
  • Some boxes use %SystemRoot% == C:\Windows (case of dir on disk)
  • We cross checked with ProcessExplorer: The casing difference is already present in the environment block of the wininit.exe process, so it's not overridden somewhere.

Q:

Is the base for the value of SystemRoot read from any specific registry key?

What could explain this difference?

(*): To be clear, the test was broken anyway - thou shall not rely on path casing on Windows.

windows

1 Answers

  1. Best Answer

    It's set in the boot process. It's not set in the registry, because the OS boot process needs to know where to find the registry files (and other OS files) before it can read them from the registry.

    https://learn.microsoft.com/en-us/windows-hardware/drivers/devtest/boot-options-in-windows

    Windows boot options are stored in the Boot Configuration Data (BCD) store on BIOS-based and EFI-based computers.

    PS C:\> bcdedit.exe /enum | select-string "device|root"

device                  partition=\Device\HarddiskVolume1
device                  partition=C:
osdevice                partition=C:
systemroot              \WINDOWS

PS C:\> $env:systemroot
C:\WINDOWS

    On older Windows OS, it's probably in boot.ini

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