LastXY Register for WP34S

[Bit]

(12-01-2014 11:07 AM)PANAMATIK Wrote:  

(12-01-2014 09:08 AM)Paul Dale Wrote:  There is some amount of code for a number of the older HP calculators that use an I register and (i) for indirection. Leaving the 34S's I register clear makes porting these a little bit easier. This is part of the reason there is an I register in the device.

- Pauli

I understand, that these older programs don't use complex arithmetik, therefore the I(ndirecton) register remains backward compatible.
Full Backward compatibility with LastXY feature can only be achieved, if there will be a new register for LastY.

But all RAM is already used, like 40 years ago. :-(

Bernhard

A quick look into the sources revealed that even an XROM routine (in orthopolys.wp34s) depends on the I register not being modified by dyadic operations, although it probably wouldn't be difficult to change that. Several library routines also make use of the I register. Such a change would need to be well tested against all XROM and library programs.

Since this would primarily benefit interactive operation rather than programs, another option is that the Y register could be saved only when not running any program, thus avoiding the compatibility issue.