3 Jan
2018
3 Jan
'18
4:30 p.m.
On Wed, 3 Jan 2018, Rob Jarratt via cctalk wrote:
The reason I say this is that when I set up the logic
analyser to *stop*
on a zero ROM address, it didn't. Which means it isn't resetting in the
way I thought it was. When I traced the instruction addresses during
boot up I could see lots of resets back to address zero, and assumed
that somehow it was getting further into the sequence to show the
copyright screen, but then resetting again, because the screen would
display the "snow" after showing the copyright screen.
As I recall with the ZX Spectrum there wasn't any "snow" on TV with a
subsequent reset (as when pressing the reset button on the Plus
variation).
Moreover DRAM refresh continued to work (presumably being driven by the R
register of the Z80 CPU according to its machine cycles, although at this
point I'd have to double check the details as my brain actually seems to
be missing the R register altogether), so unlike with a power-up reset,
where you'd initially get random rubbish on the screen, with a subsequent
reset you'd see a black rectangle (with thin vertical stripes IIRC,
meaning a pattern with a single bit set) instead as the part of memory
used for video data gets cleared in the DRAM initialisation process,
followed by the copyright notice you mention.
That you see "snow" instead is I believe indeed a strong indication that
something else goes wrong beyond (or instead of) just an intermittent
reset. And given that the ULA produces video output right away after
power-up with no need to program any device registers is I think an
indication that either the chip is broken or something else causes it to
malfunction, just as you have concluded.
FWIW,
Maciej