Gavin after me after Stan after me (starting to sound like a rap song):

>>>    GETPRIVMODE ();   /* unless caller was already in priv mode */
>>>
>>>>   return get_control_reg(10) & 0xff;
>>
>>Stan is, of course, correct about the need for GETPRIVMODE () if these
>>routines don't exist already exist in a privileged XL.

>Control registers (other than SAR, CR26, CR27, and the Interval Timer if
>the S bit in the PWS is off) can only be read at the "most privileged
>level", i.e. ring zero, so the GETPRIVMODE() call here isn't going to
>do you any good, as get_control_reg is not a not a function but a
>compiler construct that expands inline to an MFCTL instruction which
>will trap due to the code not executing at a sufficient privilege level
>(GETPRIVMODE() promotes a process from ring 3 to ring 2).

Actually, notice the language - it is C and I can personally attest that it
doesn't expand internally to a mfctl! :-)

Using AVATAR from Lund (written by Stan and one of the most useful tools
around), we see that:

AVATAR: open nl.pub.sys
Assuming space $A for NL.PUB.SYS
----> LST Module Directory <----    Starts @   #Length
1  HP31900                          $0012e000 21143704

... [snip]

Select a module number > 1

      Module #   1: HP31900
      Found 17051 unwind entries.
      Searching 91598 symbol dictionary entries
      Sorting 57379 symbols
FILE TYPE : sharable, executable SOM

AVATAR[nl.pub.sys]: look get_control_reg
symbol name         : get_control_reg
  address           : fcaab8
  symbol_type       : unspecified code
  symbol_scope      : exported symbol for other SOMs
  check_level       : 0
  must_qualify      : 0
  initially_frozen  : 0
  memory_resident   : 1
  is_common         : 0
  duplicate_common  : 0
  xleast            : 2
  privilege level   : 0
  code offset       : 185a98  -  185bb4    (72 instructions)

AVATAR[nl.pub.sys]:

For those unfamiliar with AVATAR, I'll explain the above output. We see
that get_control_reg is in fact a procedure in the NL. Its xleast is 2
indicating that it must be called at ring 2, and its privilege level is 0
indicating that it runs at ring 0. Hence, we need to be in privileged mode
(GETPRIVMODE() switches to ring 2 given correct permissions) to call it,
and since it runs at ring 0, all control registers can be returned by it.

>Nyeah. :-)

Same to ya! ;-) (We're starting to sound like my kids)