s100 busses!

From: Derek Peschel <dpeschel_at_eskimo.com>
Date: Tue May 9 23:31:45 2000

> Does anyone still sell s100 based computers?
> Aren't they used still for process controll and automation?

I don't know the answer to this one.

> 2nd question
> How did the control panel of the IMSAI work, was it controlled by a rom program
> that lit the leds and read the switches?

No, it's all run by hardware. It uses the voltage and timing conventions
that are laid down in the 8080 manual. (They are the same conventions that
the CPU uses to communicate with memory and peripherals.) Essentially the
control panel intercepts the signals that would normally be sent
automatically by the CPU or memory. I don't think it lets you do input and
output, but theoretically you could do that too.

With enough hardware you can do some pretty fancy stuff. The Mark 8 lets
you give instructions to the CPU that don't come from anywhere in memory.
(The CPU thinks they do, but they don't.) The PDP-12 lets you stop the
computer if certain things happen (i.e., your program branches to a certain
part of memory). There are also limitations to the hardware approach. For
example, the 8080 in the IMSAI doesn't easily make the contents of its
internal registers available, so you can't manipulate them with the control
panel (except for the program counter). You could change that with enough
work, but I don't know if anyone has.

Newer CPUs have been designed to make this sort of hardware snooping easier,
but they're still limited (you can only check for a certain number of
conditions at the same time, for example).

Heath made a computer (the H-8) which is similar to the IMSAI but actually
does use a program to control the computer along with the usual hardware.
The H-8 is a lot newer than the IMSAI, however.

-- Derek
Received on Tue May 09 2000 - 23:31:45 BST