Re: [EE] New retro-computing project

MC6800 has a community named HCF instruction "Halt and Catch Fire" - op
codes = $DD 50+ year old (internal) memory says.

Processor goes into a counter mode with address lines increasing at I think
one quarter of clock speed. Exited only by power off.
A useful debugging aid.


Russell

On Wed, Mar 11, 2026, 2:43 PM Brent Brown <[email protected]> wrote:

> On 10 Mar 2026 at 15:42, Harold Hallikainen wrote:
>
> > One thing I really like about microcontrollers is that the address and
> > data buses stay inside the chip. With systems with external memory and
> > I/O, it is a real pain to find a problem when it affects one of the buses
> > since NOTHING works. With microcontrollers, I can put in a power on self
> > test that tests all the external devices and lets me know they are
> > working.
>
> I remember looking for such problems... drop in an EPROM filled with NOPs
> (0x00
> for 8051), or maybe a resistor network pulling the 8 x data lines low,
> scope the
> address lines to see a nice square wave on each, halving in frequency as
> you step
> up to the next address bit.
>
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