HP-35/45 Simulator for PDP-8

Eric Smith spacewar at gmail.com
Tue Sep 13 15:05:00 CDT 2016

On Tue, Sep 13, 2016 at 1:55 PM, Klemens Krause
<krause at 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

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.

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