Clear Stack (CLST, CLSTK) and LastX

[PANAMATIK]

In the HP-25/HP-25C the LastX register is physically located in an external RAM chip and occupies 1/16 of total external RAM, whereas the stack is located inside the ACT processor. This cannot resolve whether LastX belongs to the stack or not. But it explains why in HP-25C the LastX is "Continuous", whereas the stack is lost after switching off. In later model HP-29C, where more external memory chips were used, the firmware used also the X-Register to store in external RAM and made it "Continuous" too, but still Y,Z,T were lost. The todays LCD calculators like 35S are "Continuous" thoroughly and even remember the blinking cursor state.

In 35S the UNDO key could have been used to include the LastX functionality, but the designers missed to implement that, they decided that it should do nothing when used directly after an operation. But I don't consider that as a bug?

Bernhard