user extensions private data in TCB question

[Till Straumann]

How does a user extension determine it's slot/pointer
in the TCB extension area -- is 'rtems_index(extension_id)'
the standard method?

If so:
How can an 'initial' user extension (i.e., one that was
created during rtems_initialize_executive_early())
determine the slot number it owns in the 'extensions'
array of the TCB?

 From scanning through the sources, it seems that
initial extensions don't even have a slot for private
data in the TCB -- is that true? [if so: bummer!].

Thanks
-- Till