The HP 20b RPN Arcus Cosine Bug


As I have already stated in the "First Impressions" thread, the HP 20b has a strange bug. I did some research and could narrow down the circumstances:

1) Select RPN mode

2) Select radian mode

3) Clear the stack (just to make things easier to check out)

4) Enter 1 +/-

5) Select Math/trig and scroll to COS-1

6) Press input

Now, the rounded value of PI is in the display and the argument -1 is in stacklevel Y. You can check it with swap ")" or roll down "(".

This is the only situation I could spot where the argument is not popped off the stack but pushed to Y. In degree mode, the bug is not present.


Edited: 17 Sept 2008, 8:22 a.m.


thanks, I will look at it.

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