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Matt,
HP has not been consistent in implementing RCL arithmetic in all of its calculators. This feature appears in some models but not others.
Namir
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Yes, as per the book, it mentions how earlier models have the feature but later versions do not.
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All HP-45's had Recall arithmetic like the 42S and 15C. The difference between early and later models was in Store arithmetic. In early models, STO-n would subtract the value in register n from the value in stack X, and store the result in register n. Similarly, STO/n would divide the value in register n into the value in stack X, storing the result in register n. In later models, STO-n would subtract the value in stack X from the value in register n, and STO/n would divide the value in stack X into the value in register n. As for when the change occurred, I have a version 1 (with the raised metal bar at the top of the keyboard instead of the silver trim), S/N 1301A04xxx which has the former behavior and another, S/N 1350A89xxx which has the latter. I know that there are serial numbers which begin 1349 that also have the later behavior. I have no knowledge of other S/N series numbers and do not know when the change-over occurred.
Edited: 6 Apr 2012, 11:38 p.m.