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From: billw@hpcvra.CV.HP.COM (William C Wickes)
Newsgroups: comp.sys.handhelds
Subject: Re: HP48 units bug
Message-ID: <25590059@hpcvra.CV.HP.COM>
Date: 20 Sep 90 00:33:56 GMT
References: <1990Sep19.024034.15518@mintaka.lcs.mit.edu>
Organization: Hewlett-Packard Co., Corvallis, OR, USA
Lines: 44

Christopher Moore writes:

>  I have discovered what appears to be a previously unreported bug in my 
>  revision D ROMs.  The units application fails to properly handle units
>  with fractional powers.  

The bug here is that the HP 48SX manual fails to document the unit management
system properly; it should have specified that while 48 units may
have fractional powers, those powers are converted to signed 8-bit
integers for the sake of dimension checking.  Thus while you
can successfully add 1_m^.5 +1_cm^.5, you can also add 1_m^.5 + 1_cm^.4
without the 48 reporting an error.  The dimension checking limitation
also carries over to UBASE, and UFACT, which executes UBASE; these return
meaningless results for fractional exponents.

The HP customer support representative was correct in stating that
this design was intentional.  There is a considerable amount of processing
involved in unit operations, and representing the base unit dimensions
with small integers provides a considerable efficiency compared with
using floating-point numbers.  The loss of ability to handle fractional
exponents in all circumstances was taken as a modest price to pay
for the speed gain, given that the great majority of physical problems
involves integer powers of units.  There are many cases where fractional
powers may appear then disappear in the course of calculations, but
these can be generally be handled correctly within the 48 system with a little
care in the order of operations.  

The HP 48SX unit management system is a valuable tool that can be
applied to a wide range of physical problems (in 10 years of
university physics research and teaching, I don't recall any unit
manipulations that would have been intractable on a 48, although my
memory may be biased).  This is little consolation if it doesn't work
for your particular problem, but there are other things you can't do
on a 48.  The key is proper documentation that lets you know the
reach and limitations of your tools.  We fell short in this case, but
documenting the 48 for a very wide range of customers was and is
a massive task.

It might be helpful to the user community if Mr. Moore and others
could to post some example problems involving fractional units, so
that we can study how they might be dealt with on the 48.

Bill Wickes
billw@cv.hp.com