Front Panels - Thoughts

tony duell ard at
Sun Nov 1 08:18:48 CST 2015

> One problem with a front panel on an S100 machine - or any microprocessor-based 
> machine - is getting access to the program counter so you can tell it to start running 
> at some arbitrary address. The Altair/IMSAI panels resolved this with a hack, jamming
> a jump instruction into the processor data lines (not the S100 bus data lines) via a 
> special connector to a special processor card.

Although not S100, the Intellec MCS8i was a 8080 machine with a front panel. There was
no hardware way to start the processor at a particular address (No logic to jam a jump
instruction onto the data bus, for example). You did have a reset switch. So what you did
was take the bus over from the processor (thus effectively halting the latter), toggle a
jump instruction with the appropriate address into the first 3 bytes of RAM, then reset, and
give the bus back to the processor. That was even the way to start the ROM monitor program


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