Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-pcd!hpcvra!bruce
From: bruce@hpcvra.CV.HP.COM (Bruce Stephens)
Newsgroups: comp.sys.handhelds
Subject: Re: 'New' Functions for hp42s
Message-ID: <21580042@hpcvra.CV.HP.COM>
Date: 27 Mar 90 17:30:16 GMT
References: <3842@plains.UUCP>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 19

Let me dispell any misconceptions about the use of XFCN on the HP-42S.
It is totally useless in any machine covered under HP's warranty.  It
could possibly be used if some machine happened to have a static RAM
or an EPROM accidently stuck onto its circuit board and a trace or two
broken or solder-bridged on the circuit board.

If such a calculator happened to have this configuration, it would
have memory at address 20000 (hex).  In a really bizzare twist of
fate, if the first five nibbles at that address happen to be F3CA5,
the HP-42S will not give the usual "Nonexistent" error.  Instead, the
CPU will do a subroutine jump to address 20005 (hex) and execute
whatever assembly level code happens to be there.  It is possible that
this code could even look at the value of the mysterious parameter to
XFCN and do different things based on its value.

All of this would be fairly difficult given that this is a "closed"
machine, whose internals are not documented or supported by HP.

Bruce Stephens