Re: Dating a C-64 board

From: William Levak (wlevak_at_cyberspace.org)
Date: 1999-04-24 03:40:56

On Fri, 23 Apr 1999, [ISO-8859-1] Marko Mäkelä wrote:

> On Fri, 23 Apr 1999, William Levak wrote:
> 
> > Four sprites on the same line cause the disk drive to hang up when
> > loading.
> > 
> > They can be the first four, the last four, or every other one, it doesn't
> > make any difference.
> 
> Actually 2 or 3 sprites should be enough, if they are not successive ones.
> As you can see from the timing diagrams in the document
> <URL:http://www.funet.fi/pub/cbm/documents/chipdata/VIC-Article.gz>, the
> timing of a bad line without sprites will be like this:
> 
> rrrrrXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX++0-1-2-3-4-5-6-7-rrrrr
> 
> The diagram is for the 6569.  On the 6567R56A, replace "++" with "++++".
> On 6567R8 and later, replace it with "+++++".
> 
> The BA line will be made inactive 3 cycles before the DMA takes place.
> That'll happen after the 2nd "r" (refresh cycle) on the left.  If sprite 0
> is active, the processor won't get any cycles after the character fetch on
> PAL systems (the 40 cycles marked with X), or it'll get 1 or 2 cycles on
> NTSC systems.  Considering the 3-cycle rule, it won't make any difference
> whether you have sprites 0..3 active, or just sprites 0,2,3 or 0,1,3: the
> processor won't get any time unless the DMA will pause for longer than 3
> cycles.
> 
> I think that activating e.g. the sprites 0,2,4 or maybe even 0,3 should
> hang the protocol.
> 
You're right,  both those combinations cause the load to hang up.

Bill


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