The HP-20S is an algebraic programmable, member of Pioneer series of HP calculators produced from 1988 to 2003. It is similar to HP-21S. It has ninety-nine steps of fully-merged program and ten memory registers.
Libraries
There are six libraries in the ROM, which could be loaded to RAM and used and edited as user programs.
A- [ROOT] finder, finds a root of f(x)=0
B- [INT] integral, calculates definite integral using Simpson method
C- [CPL] complex numbers manipulations
D- [3 by 3] matrix manipulations and line equations solver
E- [qUAD] quadratic equation solver
F- [fit] curve fitting, using exponential, logarithmic and power functions
Design peculiarities
The HP-20S is not a clean design. It shares the quirks with its close relative, HP-21S. Some of them are:
INPUT and SWAP keys, which are awkward and not clear solution to mimic RPN functionality.
Absence of "x<y" test, which enforces using two tests with additional labels and GOTOs in programs which need this functionality.
A real bug is "x<=y" test. First of all, its name is misleading. Really, it is "x>=y" test in terms of RPN calculators. The manual says that x is 'hidden' while y is 'visible'. But yx power key on the calculator's keyboards functions in the traditional fashion of other HP RPN calculators. The second problem here is due to algebraic design: in order to separate two arguments of a test you should divide them either with arithmetic operation or INPUT. If using arithmetic, like:
RCL 6
+
RCL 7
x<=y?
GTO A
you have "pend" announciator lighted, since calculator sees a pending addition operator. After the program stops, you can press "=" sign and get the result of pending operation. If you use INPUT instead of aritmetic operation, you'll see ":" anounciator. In order to hide this effect, you should place "C" clear command somewhere before program end.
Nevertheless, HP-20S is a functional and fast calculator, with very good LCD, keyboard, look, and feel. It uses the normal infix notation rather than RPN, which most HP scientific calculators use.
