1 Nov
2015
1 Nov
'15
6:18 a.m.
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
-tony