[WP 34s] emulator ENTER key behaviour

[Thomas Klemm]

(05-28-2015 07:34 PM)Marcio Wrote:  My question however was more like centered around why the ENTER key will duplicate entries therefore taking 2 registers instead of 1 only.

You may first make yourself familiar with how the 3-level stack of the HP-9100 works.
There's no [ENTER] key but there are five control keys one of them is the [↑] key:

• Duplicates the contents of X in the Y register.
• Shifts the contents of Y to the Z register.
• The contents of the Z register are lost.
  

Here's an example of how to calculate \(\frac{20}{3+2}\):

Code:

Operation      x          y           z

CLEAR 20 ↑     20         20          0
3 ↑            3          3           20
2              2          3           20
+              2          5           20
↓              5          20          20
÷              5          4           20

You may notice that the register y contains the result of the calculations.
This is very similar to how mechanical calculators worked: You could enter a number (x) and add it to an accumulator (y).
But with more sophisticated operations you need a temporary register (z).
Otherwise you can't calculate even a simple computation as the one above.

But the HP-35 can display only one register. To show the result in the accumulator the register y has to be shifted to the register x.
Thus both + and ↓ are executed together.

On the other hand the display (x) has to be shifted to the accumulator (y) before executing the next operation. Thus ↑ is executed before entering a number.
But of course this isn't needed when ↑ has just been executed. That's why stack-lift is then disabled.

The [ENTER↑] key of the HP-35 still contais this arrow which got lost with the Voyager series.
To me it's fascinating to see how these things evolved over the years.

HTH
Thomas