On Tue, Sep 13, 2016 at 1:55 PM, Klemens Krause <kra...@informatik.uni-stuttgart.de> wrote: > Next question: my hp35 has 36 keys. The 36th key is not accessible > for normal users. It's hidden below the "ENTER" key. What does this > key do? If I remember right, it's a kind of debug-key. I believe it > shifts the whole register including exponent and signs digit by digit > to the left.
The keyboard dispatch is done using a "goto keys" instruction, which replaces the low eight bits of the program counter with a hardware keycode. On the HP-35, the "right half of ENTER" switch doesn't do anything useful. The location it jumps to is just an arbitrary instruction in the code. On the HP-45, the "right half of ENTER" key was carefully planned to be useful to enter the undocumented stopwatch mode, by a STO right-ENTER key sequence. If you put a shim under the right half of the enter key, then STO ENTER will do it. Otherwise you can get it as a phantom key by pressing STO, then a simultaneous press of CHS, 7, and 8. It takes some practice to get that reliably by hand; some people used the "penny trick" to press the three keys together.
