HP42S making value comparisons

[Valentin Albillo]

(07-13-2020 08:39 PM)Davidy Wrote:  What's the easiest way to compare the x value with a constant during a running calculation without upsetting the stack? You can easily compare to y or to 0 but how about to a stored value?

On a side note, how many different ways did HP use for their programming? It seems that every programmable calculator does it differently.

I don't have a 42S at hand but something like this might work:

Let's assume that the stored value is in R13 and if the value in the X stack register equals that value then execution should branch to LBL 07. To achieve what you want, do this:

RCL- 13
X=0?
GTO 07
X<> ST L

This leaves the stack as it was before the comparison, except for the LastX register (ST L).

If you don't need the value of X after the comparison, omit the X<> ST L step. If you need the value of X (and Y,Z,T) at the LBL 07 destination, put the X<> ST L there as well, like this:

LBL 07
X<>ST L

This won't work for alpha data, of course.

V.