|
One of the first things that you might notice about an old HP calculator,
or the free calculator that is distributed on this
site, is that there is no "=" on the keyboard. That's because those
calculators don't use the AOS system that was invented by Texas Instruments
in the mid 70s; they use a differrent system, called RPN.
RPN is an acronym that was created by Hewlett-Packard in the early
70s. The acronym stands for Reverse Polish Notation, HP's name for
the postfix form of Polish Notation, first proposed by Jan Lukasiewicz in
the 1920s.
Hewlett-Packard invented the first handheld portable calculator in the
early 70s (I saw my first in 1974), before anybody else had any ideas about
how a calculator was supposed to work. The guys who invented that first
calculator were engineers and they wanted a calculator that was easy to
program and was able to handle complex, nested calculations. They were
familiar with some stack-oriented computer programming languages, like
Forth, and that may have influenced their design.
Arithmetic expressions are usually written using a sytem that is called
in-fix notation. This means that the symbols that represent binary operations
are placed between the values that are to operated upon. Like,
"2+2", or
"4.7/3". But, post-fix notation
moves the operator to the end, like
"2 2 +", or
"4.7 3 /".
Postfix notation might look pretty confusing when it's written, but you
won't be writing it. You don't need to write an expression before you
enter it in your calculator.
|
The way that you do arithmetic on an RPN calculator is
- key in the first value
- press the <Enter> key
- key in the second value
- press the appropriate operator key
.
|
The free calculator that is distributed on this
site does not have any graphic buttons; you just use your stylus (or
fingernail) to tap the digit or function name.
The free calculator has five binary operators:
| / | Division |
| X | Multiplication |
| - | Subtraction |
| + | Addition, and |
| y^x | Exponentiation |
That last one requires a little explanation. "y^x" is notation
that means "take the value in the Y register and raise it to the power
of the value that is in the X register." So what is the Y register, and
what is the X register?
X register is HP's name for the number that is displayed in the view
window of the calculator. This is usually the last number that you keyed
into the calculator or the result of the last calculation.
Y register is HP's name for the value in the top of the calculator's
internal stack. The stack is where the calculator stores the X register
when you press the Enter key. The stack in the free
calculator can hold up to 20 numbers and works like those spring-loaded
things that dispense plates at the cafeteria. When you press the Enter key
several things happen: 1) The current content of the X register
(remember that's the number in the display) is moved to the Y register
(another name for the top of the stack) and all other numbers in the stack
move down to make room; 2) The X register (the display) is cleared to
be ready to accept the next number that you will key into the calculator.
OK, that's enough explanation for now. What you need is
some practice
| 
