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From: Bionic (all posts)
Date: 2001-03-13
Subject: Re: Strange TED 'effect'

> > >this when we use the characters from ROM area but we
> write
> > >an "illegal" value to the $FF13.
> > >So, does anyone know what could cause this strange
> effect?
> >
> > Your pointing the character data down into Zeropage. This
> changes all
> > the time, and so will the data for the characters. Its
> just chance it
> > seems to go diagonally, theres probably a few counters
> down there somehere
>
> Not quite that! I don't point to the zero page, 'coz I do
> NOT set the $FF12 register to read from RAM area! In theory
> it still reads from ROM! But, there's no underlying ROM
> area there for values <=0x80.

So you already explained the effect byself. What you see is appearantly the data which was read on the bus in the previous cycle. I assume that no new RAS/CAS load is done and therefore the old adress is still latched. So the TED is actually doing bank switching, but no adress muxing.
Well, i'd have to look at the schematics and a real Plus/4
to be sure.. ;)

The random stuff (six lines per charline) are the instruction reads by the cpu. The two static lines are the TED byself reading Color and Char data (lines 6 and 7 each each textline).

> Besides I think the 'scrolling' effect is there as well
> when I disable the ROM (although that needs to be checked).

I guess the CPU bank switching is seperated from the TED bank switching.
Otherwhise you would always see "sparks" caused by RAM accesses by the BASIC interpreter.