Article 1574 of comp.sys.handhelds:
Path: en.ecn.purdue.edu!noose.ecn.purdue.edu!samsung!cs.utexas.edu!helios!tamuts!n233dk
From: n233dk@tamuts.tamu.edu (Rick Grevelle)
Newsgroups: comp.sys.handhelds
Subject: More SYSEVALS
Message-ID: <8650@helios.TAMU.EDU>
Date: 30 Sep 90 12:00:23 GMT
Sender: usenet@helios.TAMU.EDU
Organization: Texas A&M University
Lines: 54

There is perhaps a slight possiblity some confusion might develope over 
this hidden object/directory business.  It seems that Eric Toonen was the
first to discover that the HOME directory contained a directory which is
normally hidden, and unavailable for use.

This usually hidden directory is always present in the HOME directory, and
can not be deleted by a reset.  As a matter of fact, if your knowledge is
such that you're able to purge this directory, either a system halt, or a
reset, will actually reinstall it.  In other words, the hidden directory is
there by default.  And also by default, the directory has been named by HP,
using the NULL NAME '' (no spaces).

This could constitute a potential problem in that either the user, or the 
calculator could become confused by multiple NULL NAMES of the same length.  
Generally, it is not a good idea to use names, local or global, which are 
already being used by the calculator.  But, as everyone in this news group
is already aware, whatever can be used, can be abused.

The actual SYSEVAL address for the NULL NAME is #15781h.  This returns the 
NULL NAME without quotes so that it would be immediatly evaluated if the
current directory were the HOME.  There exist an RPL SYSEVAL that contains
a prefixed machine routine which allows the NULL NAME to be place on the 
stack unevaluated.  Its address is #15777h.

As Toonen has already pointed out, aside from the fact that it is hidden,
you CAN use this directory as if it were any other.  It is a true directory
just as the HOME directory.  If this seems a bit unbelievable, it can be
proven simply by using a prefixed machine routine from the ROM to recall
the entire contents of the HOME directory as a directory.  This also is
something you can't do without the use of SYSEVALs, or user written machine
code.  

While in the HOME directory, SYSEVAL this address: #8D5Ah.  Essentially this
routine recalls the contents of the current directory to the stack just as if
the name of the directory had been RCLed.  The only difference is that the
routine already knows what directory to recall, and, unlike RCL, it works in
HOME.  Once the HOME directory is on the stack you should be able to see the
hidden directory and the file names it contains.  This is particularly evident
in a 48 whose memory has been cleared.  

One final note, which has nothing to do with the above; for reasons unknown,
I was recently asked about a programmable OFF.  Perhaps someone else also has
a use for this as well.  It seems as if there should be another way to do this;
the only one of which I know is to use the SYSEVAL for the [blue shift] [ON] 
key, #3A9CE.

Here is a quick summary of the four SYSEVALs covered above:

#15781h      recalls the null name '' unquoted
#15777h      recalls the null name '' quoted
#08D5Ah      recalls the current directory
#3A9CEh      turns the 48 off

Rick Grevelle