Re: How to transform an HP-25 into an HP-25C ? Message #5 Posted by Andrés C. Rodríguez (Argentina) on 20 Mar 2010, 5:52 p.m., in response to message #4 by Michael de Estrada
If I recall correctly, the HP-25C saves R0-R7 (eight 56 bit registers), 49 program steps (seven 56 bit registers, or 7 x 7 x 1 Byte), and the LAST X register; all of them in a 16 register, 56 bit per register CMOS RAM chip. So the X register is not preserved, as you said.
In the HP-29C, there are 32 non-volatile registers (double than in the 25C). Not completely sure about this, but I think that 98 program steps are saved using 14 registers, 16 data registers are saved, and a backup copy of X is also saved (the "real" X is on the CPU). I think that LAST X is not preserved on the 29C, perhaps the remaining regsiters are used to save the program counter and pending return addresses for GSB instructions.
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