Re: Woodstock Design: Why so many problem 27's? Message #8 Posted by Andrés C. Rodríguez on 12 Aug 2000, 12:06 p.m., in response to message #1 by Katie Wasserman
In first place let me say Thank you! for all these details about Woodstocks. Many years ago, I tried to learn more about them, with little luck.
Just to add a detail: At least some, if not all, HP 29C lose some of their memory registers when turned off, as a standard feature, so stated on the manuals.
As the 25C keeps the memories, but lose the stack. I assumed that the 25C just replaced the PMOS RAM on the regular 25 with a CMOS chip, hence losing the stack because it was placed on the CPU. And the CPU, being the same chip than in the HP 21, only has the normal stack and one storage location.
The HP 25 memory should have 16 x 56 bit regsiters, which amounts to 8 56 bit memories + 49 1-byte program steps + Last X. These are the registers that the 25C preserves.
My assumption was that the 29C had some volatile registers, and some CMOS (non-volatile); but the stack was "phantomed" on part of the non-volatile memory by software means, so to give the impression of a non-volatile stack. The actual stack is, of course, on the CPU.
(A disgression: That last sentence makes me remember that the TI 9900 was the only 16 bit processor at its time which kept its registers and accumulators in RAM and not on-chip. Context switching was a breeze on such processor!)
Is it possible from your observations, to deduce if the volatile / non-volatile assignment is so, or if my "assumptions" are wrong?
I also suspected that the 16 x 56 bir CMOS RAM used on the 25C and 29C may be a close relative to the 16 x 56 bit CMOS RAM used on the first HP 41C and memory modules, before the advent of a 64 x 56 bit CMOS RAM for the HP 41 CV and Quad RAM.
On the HP 41C (not CV) there were five of those 16 x 56 CMOS RAM chips: four made the 64 register user memory, which could be extended up to 320 registers with four memory modules (64 registers each) or one Quad module (256 registers). The fifth internal CMOS RAM was used for Alpha, Flags, and internal housekeeping.
As many of these ideas came from limited experiments made some 20 years ago, I may well be "off the road", indeed!
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