Re: What about that 2nd disc under "Enter"?
Message #10 Posted by Eric Smith on 5 Feb 2004, 2:06 a.m.,
in response to message #9 by Paul Brogger
All HP handheld calculators from the HP-35 through
the Voyagers have an unused switch contact under the
"other" half of the ENTER key, with the possible
exceptions of the 10 and 19C. I'm not sure whether the
Topcat family (desktop printing, 91, 92, 95C, and 97) or any of the Saturn based calculators also have it.
On the HP-45 the extra switch was famous as the way to
enter stopwatch mode. This was done by pressing RCL
followed by the right half of ENTER. The trick was that
the molded ENTER key did not press the right half. You
could either press CHS, 7, and 8 (or certain other combinations) simultanously to get the right half as a "phantom key" due to the way the key matrix is wired and the scan order, or you could add a shim under the key.
On most if not all of these calculators, the extra key does something. However, what happens is that the calculator executes a "KEYS->ROM" instruction to do a computed branch based on the key code. The addresses between those of valid keycodes were usually used for short code snippets, so unless the programmer made specific plans for the extra key, it's going to most likely do something entirely useless.
After the first generation (Classics), they added an instruction that reads the keycode into the C register. Some calculators use that instead of "KEYS->ROM", so they may not jump into the arbitrary code location, but they most likely still do not do anything particularly useful.
However, I have not made a study of exactly what they do, so it's possible that there could be something interesting in one or more of the other models. Probably nothing nearly as exciting as the HP-45 stopwatch, which was written because the hardware design provided 512 words more ROM than the calculator needed.
|
Go back to the main exhibit hall