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.


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.


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.


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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