Re: TED badlines, how do they work?

From: Segher Boessenkool <>
Date: Fri, 2 Sep 2011 19:20:25 +0200
Message-Id: <>
> Thanks for the info. So, if I get that correctly, the temporary  
> space is
> in fact sram, yet, the active row is selected by a shift register  
> (ie. a
> single bit in a shift register).

Yeah.  You could instead do a binary counter and decoder, but this is a
lot simpler eh :-)

> Also, as this is really sram, I suppose, only one cell ("address") of
> the bank can be accessed at a time, whether written or read (or both).

Only one row is accessed at a time, yes.  In principle you could write
all rows at the same time; reading won't work, of course.

> BTW, what do you know about how the VIC-II handles attribute data? I
> mean: the character pointer fetch / character mask fetch / display
> mechanism looks to work straightforward (current character pointer  
> byte
> is taken from the internal ram bank, whether the cell is being
> overwritten or not --> do graphic mask fetch --> mask data is put on
> some temporary space, from which it is displayed bit by bit  
> (optionally
> delayed by 0..7 dot clocks, as defined by bit0..3 of $d016).

It's not so much delayed as that those three bits say at what X  
the whole thing starts each line.

> From the
> other hand, the attribute info needs to be delayed by one cycle (the
> cycle of graphic data fetch), until it can be displayed. Similarly,  
> even
> the character pointer data needs to be delayed by one cycle, when the
> VIC-II is in bitmap mode (ie. the character pointer data is no  
> longer a
> pointer to the character rom, it's used as attribute info instead,  
> which
> needs no additional memory fetch). Does the VIC-II have temporary
> registers for this 1-cycle delay, or does it handle this problem by  
> some
> other (smart) trick?...

Yes.  It's the (quite big) structure at the center top in the picture;
it's 12 bits.

 From there it is moved to immediately left of the colour registers,  
of like virtual colour registers (they are used exactly the same).

When the character # is used to fetch bitmap data (in text mode), it is
directly put on the address bus (immediately when read from the video

>> I have no idea about TED badlines, never seen one or programmed one.
>> Where do I read about it?
> I'm afraid there aren't any precise docs that describe them in good
> detail. In principle, they work similarly to those of the VIC-II. In
> practice, there are a lot of little details, which make the state
> machine a lot more complicated... probably all due to the fact that  
> the
> TED has no dedicated color ram, ie. it needs to do two DMA fetches to
> display a single character row.

Well, so does the VIC (6560/1), and that was simple ;-)

I don't think there are any complicated state machines really; just a
bunch of simple ones, that work together in intricate ways.  I.e. just
like VIC-II :-)


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Received on 2011-09-02 18:00:08

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