HP-29C: 98 Program Steps, So, why not an even 100?

[rprosperi]

(08-28-2019 04:39 AM)teenix Wrote:  The HP-38C does the same thing, but register 30 is used for the first step instead. The other programmable Spice models all have multiples of 7 program steps.

Thanks Tony. I'm curious how these machines can use register storage to also (simultaneously?) store program steps, so I looked at the manual, which I've not seen in many years.

This confirms that the 38E/38C can actually use as many as 100 program steps (0-99, while step 00 is really just 'before' active steps begin, so effectively 99 steps of merged instructions).

Out of the box it defaults to 20 storage registers (R0-R9 and R.0-R.9) and 9 program steps, but as you say registers can be 'traded' for program steps at 7 steps each (7 bytes -> 56 bits) as needed, using up to 13 registers, so 13*7+9 => 100.

That said, I've 2 questions:

A. There seems to be no register 30, so is that some internal register numbering scheme, and

B. How can both a value and program steps be stored in the same register? The 38C with Continuous memory stores both program and register storage even when off.