HP-25C Eumulator
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04-21-2017, 07:23 AM
Post: #327
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RE: HP-25C Eumulator
(04-20-2017 10:24 PM)Thomas Okken Wrote:(04-20-2017 07:02 PM)PANAMATIK Wrote: The transition from HP-25 to HP-25C by HP was ingenious and made a very fast time to market possible at these competitive days. The firmware remained bit by bit identical, even the "Clear Memory" instruction, which cleared the external RAM chips at initialization, was left in place. There were only two hardware changes made: the two PMOS RAM chips were replaced by two pin compatible CMOS RAMs, and battery plus was connected to a previously unused pin of these RAMs. As the PMOS RAM chips itself (and not the ACT processor) decoded and performed the "Clear Memory" instruction, the CMOS RAMs decode unit was modified and just didn't execute this instruction. Good question! The HP-25C firmware does not check the RAM contents at cold start and there is no checksum test. Therefore it does not display "Error" like the HP-29C does when RAM contents is lost, because the more advanced HP-29C firmware reads a RAM check word and clears the RAM if it is not correct. I assume that there could be a hardware "Clear Memory" inside the HP-25C CMOS RAMs at power up, that resets all RAM bits to zero, which clears all registers and program steps. Bernhard That's one small step for a man - one giant leap for mankind. |
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