When is each registry hive loaded/available?

Hello,

In case someone already has the info, I need to figure when the
following hives are available for writing (in case it differs from
just reading):

  • HKLM\Security
  • HKLM\Software
  • HKU
    I noticed in one of the earlier threads that HKLM\Software is not
    available during a Boot driver load, but is there any way a driver can
    tell when it is available? I.e. for a SYstem start driver DE,
    Automatic start driver’s Driver Entry? Or would a user mode signal
    (from a control app) be required?