WP31s error recovery

[BarryMead]

(03-02-2015 06:42 AM)Dieter Wrote:  Example:

Code:

1 [ENTER] 2 [ENTER] 3 [ENTER] 4

T   1        1        1
Z   2        1        2
Y   3        2        3
X   4        7        3   (!)
       [+]     [UNDO]

In the example you give above, the "Stack" does get restored by the UNDO command. The number 4 is not actually on the stack when the + key is pressed. It is in the "Input" register not the X register. If you do this sequence:

Code:

 1 [ENTER] 2 [ENTER] 3 [ENTER] 4 [X<>Y] [X<>Y]

T  1        1        1
Z  2        1        2
Y  3        2        3
X  4        7        4
       [+]    [UNDO]

You can see that the UNDO really does restore the actual "STACK" but in your example the number 4 was
not yet in the stack (only in the input register) so the UNDO couldn't restore it. In your example the actual X register
still had the 3 in it from [ENTER] duplication, and the input register (displayed in the lower line of the calculator) does not show
what is in the X register It shows the partially completed input register (4). If you run the QT emulator for the WP-31s you can see this in real time.

Modifying your original error report if you type: 3 [ENTER] 4 [+/-] [X<>Y] [X<>Y] [LN] [BACKSPACE] The -4 will be restored to the X register. But there is a difference in the behavior of the WP-34s and the WP-31s as follows:

WP-34s 3 [ENTER] 4 [+/-] [LN] [BACKSPACE] Shows -4 in X and 3 in Y
WP-31s 3 [ENTER] 4 [+/-] [LN] [BACKSPACE] Shows 3 in X and 3 in Y

So the difference is that on the WP-34s LN copies the input register to the X register first then computes the natural log so the backspace can restore the -4 but on the WP-31s this copy is not happening, or the error recovery (backspace) is restoring back too far, so what gets restored is the duplicate 3 from the previous [ENTER].