HP-25 Subroutines possible?

[Dieter]

(06-23-2017 06:13 PM)Dave Britten Wrote:  They could have gotten away with only having GSB 01, GSB 02, and GSB 03, with the programmer expected to place an appropriate GTO in those three locations (and another GTO at step 00 to branch to the proper entry point). But as others have pointed out, it would have been of limited use with only 49 program steps to work with.

I beg to differ. I think subroutines even make sense with 49 (merged) steps, just as they do on the SR56 with 99 unmerged ones. There are essentially two reasons.

1. Subroutines can make programs shorter. Program parts that occur multiple times need to be coded only once. With line addressing even a short sequence of five steps that appears twice in a program would require ten steps regularly and eight with subroutine calls. With only 49 available steps this can make a difference.
2. A substantial number of programs for programmable calculators handles user defined functions, e.g. root solvers, integration programs, differential equations and many more. Doing this without subroutines is quite cumbersome and requires some tricks – as well as additional memory for implementing these instead of a simple subroutine call.
   

That's why I think that subroutines would have been very useful on the HP25. Just as they were later on the 49 step HP33E/C or the 50 step TI57. There would have been enough space for the required 50 additional opcodes, but since memory was limited this feature had to be omitted.

Dieter