Re: Plus/4 CPU

From: Nicolas Welte (
Date: 2001-02-08 19:47:56

john/lori wrote:
> > the x501 might
> > need an additional gate on the R/W line that is controlled by the "gate in"
> > pin. Though my machines were working fine even with the gate in disconnected,
> > which affected the R/W timing only a little bit.
> Do you know or is it known exactly what gate in does?

The best information I ever read about this input is from the Plus/4
service manual: It gives the explanation "R/W gate", that's all. But
that's really what it does, I checked it with a DSO (digital sampling
oscilloscope). I think I still have the measurements somewhere, but I
don't remember the details. I think it is needed to synchronize the R/W
signal with the weird double clock timing in the 264 machines, but I
don't know for sure of course.

> > PS: I have headaches when I think about the recently discovered behaviour of
> > writes/reads to unconnected bits of adresses 0/1. IIRC, the data direction
> > bit is connected, but the data port bit itself is not.
> why would this be a problem?

If you write to an unconnected bit, the written value is returned on a
read for a certain time. It's about the same behaviour you get with the
write only registers in the SID, and those *have* been used in software.
It might be used as a seed for a random number generator, and who knows
what happens if it doesn't behave like a real 8501. I'm sure the number
of affected programs is near zero, but still... Even emulators are down
to this level already, so why cripple a real machine ;-)


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