29C/25C question.


Hello all.

I've both the 29C and 25C. The 29C starts up with my X register preserved. When I turn on the 25C the X register is zero. Is this natural?




Yes. If you want to know why, see a detailed explanation by Andrés C. Rodríguez in this old thread from 1999 (Message #4)


Thank you Gerson, I'm glad you bring that old posting to the table! In fact, I was about to write it all again from memory, so you save me a lot of time! As I have no 25C nor 29C with me, I'm not 100% sure about the 25C keeping LastX contents in continuous memory. There are two unassigned registers: the "memory" register of the CPU chip (as used in the HP 21) and the 16th CMOS RAM register (the only one remaining after 8 data- and 7 program-assigned registers). One of them was to be used for LastX, I suppose the other one was used to keep angular mode, display mode, and program counter value. On the 29C, as more non-volatile registers were available, I think that nested subroutine return addresses may have been kept on the available registers.


You are correct that the contents of LAST x is maintained in continuous memory in the 25C when the calculator is turned off. Funny thing is I just checked this on my 25C and it was acting weird even though the battery voltage was not low. I couldn't see any corrosion on the contacts, but I swabbed them with white vinegar anyway, and now it is working fine. So it just goes to show that even a very slightly bad contact between the battery and contacts can affect these calculators.

Edited: 2 Apr 2012, 11:30 p.m.


Funny you should mention the vinegar swab treatment. I just tried this as a low-daredevil attempt to save my 29C from it's frantic behaviour. And you know what? That seems to have geen the rescue operation for my 29C.


Grazias & obrigado for that old post and pointing to it.

BTW, the QRC of HP-29C contains a note on its very first page telling 'Primary storage registers, program memory, displayed X-register, and display format are maintained by continouous memory.'

And on page 9 of Getting the most from continuoous memory you find

'When you tum off your HP-25C, the calculator retains
information stored in the:

• 49-step program memory,
• 8 addressable storage registers,
• LAST X register.

Numbers in the stack are not saved. However, if you
wish to save the number in the display (X-register),
simply press a [ + ] or a [ - ] before switching your calculator
off. When you tum your HP-25C on again, press [ LAST X ] to recall that number to the display .'

Sometimes RTFM really helps, but also small printed matters contain valuable information ;-)

Edited: 2 Apr 2012, 11:56 p.m.


Thanks for this. As my 25C acquisition didn't come with a QRC but my 29C did, I was unclear that the 25C's Continuous Memory behaviour was different than the 29C's.

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