S\->N -- Converts SPICE-scaled value string into
         a real number
Version 1.1 1993/10/19
(C) Copyright 1993 by James Unterburger.
All rights reserved.
         295 bytes
         #6F2Bh checksum

Converts SPICE-scaled value strings into real numbers
INPUT:
1:  string of SPICE-scaled number, e.g., "-1.234MEG",
    OR
    real number (which is just passed through as-is
    with no processing)

OUTPUT:
1:  real number representation of value,
    e.g., -1.234E6

This string-to-real-number converter recognizes
the following SPICE scale factors:

    F   = 1E-15
    P   = 1E-12
    N   = 1E-09
    U   = 1E-06
    M   = 1E-03
    K   = 1E+03
    MEG = 1E+06      (same as 'ME', just like SPICE)
    G   = 1E+09
    T   = 1E+12

Note that the SPICE2G.6-supported token
    MIL = 2.54E-02
is NOT SUPPORTED.

This routine uses the first 15 significant digits
of the manitissa when constructing its output number
which is rounded to 12-digits, a standard HP48SX 
real number.  Numbers with more than 15 digits can
be used as input, but only the first 15 significant 
digits will be used (and then rounded to 12 digits).

This routine is case-insensitive.

Scientific notation is legal and will produce
expected results.
Only the first 3 digits of the exponent will be used,
and they should not exceed the maximum of 499;
else an Overflow condition will exist.  Exponents
less than -499 will the cause corresponding Positive
or Negative Underflow condition to exist.  Both
Overflow and Underflow conditions are handled according 
to the system flags for Math exceptions; see the 
user's manual for details about which flags these are.

######## THIS ROUTINE ALWAYS RETURNS A VALUE #########

This routine differs from SPICE in several
unimportant ways which relate to the handling of
non-standard characters in the input string.  This
routine will return the value of as much of the input
string as can be parsed as a number, returning zero for
non-parseable numbers, whereas SPICE will sometimes
generate an error for non-parseable numbers.
Furthermore, this routine ignores all leading
non-numeric characters whose ASCII code less is than
or equal to 2Ch, the comma character ","), whereas
SPICE will treat leading commas differently.  Since
leading plus characters ("+") are ignored, numbers
with 2 or more leading "+" characters (which cause an
error in SPICE?) will parse OK in this routine.
Stick to generally recognizable numbers and you
will be OK.

Mixing scientific notation and SPICE-scale
factors has the same affect as in SPICE,
i.e., the scale factor is ignored.

Examples:
    "12345.t"                ==> 1.2345E16
    "1.2345e-034"            ==> 1.2345E-34
    "-1.234E+0234"           ==> -1.234E23
    "(,++-.0023x8"           ==> -2.3E-3 
    "-12345678901234567890K" ==> -1.23456789012E22
    "-123.4e2K"              ==> -1.234E4
    "  BEAVIS "              ==> 0

This routine was tested on a HP48SX-E machine, but
since it uses only supported entry points and 
involves no graphics and other fancy doings, 
it is anticipated that it will work fine on 
the HP48G/X series.

Finally, I wrote and rewrote this routine trying
to minimize the size, so there are certain
inefficiencies in the processing as far as speed goes.
So don't flame me for these; rather, praise me for 
how compact and robust the code is!
    
    (C) Copyright 1993 by James Unterburger.
    All rights reserved.

    Noncommercial distribution allowed, provided that
    this copyright message is preserved, and any
    modified versions are clearly marked as such.

    This routine may not be used in any commercial 
    product without my permission.  "Shareware" and
    "Giftware" are both included in this restriction.

    This program makes use of undocumented low-level
    features of the HP48SX calculator, and may or may
    not cause loss of data, excessive battery drainage,
    and/or damage to the calculator hardware.
    The Author takes no responsibility whatsoever for
    any damage caused by the use of or inability to use
    this program.

    THIS SOFTWARE IS PROVIDED "AS IS" AND WITHOUT ANY
    EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF
    MERCHANTABILITY AND FITNESS FOR A PARTICULAR
    PURPOSE.