HP-25/SR-56 Identical Conditionals

[Dave Britten]

(10-12-2021 12:44 AM)Gene Wrote:  I suspect not, since they were made by competitors... unless TI decided to copy the ideas of the HP-25 team.

But the SR-56 had the independent "t" or test register while the HP-25 had dedicated conditional tests of X against 0 and X against Y. The SR-56 simply compared the displayed value (X) using =, not =, >= and <, to the value of the "t" register, which may have held a zero or non-zero value.

The TI offers a bit of insight into the design of the 25 here. On the TI, you've got two tests and their inverses. Same on the 25, despite being spread over 4 keys instead of two (and having two comparison targets: Y or 0). So presumably this design saved ROM space, which there wasn't a whole lot of on the 25.