HP-25 purpose

[Valentin Albillo]

 
Hi, Gene:

Gene Wrote:I for one have not ever thought that subroutines were ** that ** useful on a 49 step machine given the overhead required to call and return from one.

You're wrong, it's precisely the opposite: the less program steps you have available, the more useful is a subroutine capability. The number of steps saved by replacing a sequence of #instructions repeated #times is given by the formula:

      #Saved steps = (#instructions - 1) * (#times - 1) - 2

For instance, if you have a 7-step sequence that is repeated 5 times then replacing them by 5 subroutine calls will save:

      #Saved steps = (7 - 1) * (5 - 1) - 2 = 22 steps saved

A few values are given in the following table:
\begin{array}{|c|c|}
\hline Instructions & 1  time & 2  times & 3  times & 4  times & 5  times \\\hline
1 & -2 & -2 & -2 & -2 & -2 \\\hline
2 & -2 & -1 & 0 & 1 & 2 \\\hline
3 & -2 & 0 & 2 & 4 & 6 \\\hline
4 & -2 & 1 & 4 & 7 & 10 \\\hline
5 & -2 & 2 & 6 & 10 & 14 \\\hline
6 & -2 & 3 & 8 & 13 & 18 \\\hline
7 & -2 & 4 & 10 & 16 & 22 \\\hline
\end{array}
As you can see from the table above, even just 2 instructions repeated 4 or more times will save steps if converted to a subroutine. Same with 3 instructions repeated 3 or more times and 4 or more instructions repeated at least twice.

Also, there are times when you can save an additional step, for instance if you can have the last call immediately before the n-step subroutine, in which case you can omit it altogether and save one extra step.

Best regards.
V.