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 Post subject: Re: PPU questions
PostPosted: Fri Jul 01, 2016 2:19 pm 
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Joined: Sat Jun 25, 2016 5:33 am
Posts: 66
yes I understand but I have a question

what is really the horizontal scroll ? I think it is counter ranging from 0 --> 255 for the 256 pxl on the screen , Is this true?

I will fully understand your comment , after answering this question.


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 Post subject: Re: PPU questions
PostPosted: Fri Jul 01, 2016 2:27 pm 
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Joined: Sun Sep 19, 2004 11:12 pm
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Location: NE Indiana, USA (NTSC)
If by "horizontal scroll" you mean the value written to $2005:

Bits 7-3 of the horizontal scroll are the "coarse horizontal scroll", which correspond to bits 4-0 of internal registers t and v in PPU registers

Bits 2-0 of the horizontal scroll are the "fine horizontal scroll", which select a line coming out of the SIPO.


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 Post subject: Re: PPU questions
PostPosted: Fri Jul 01, 2016 3:18 pm 
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Joined: Sat Jun 25, 2016 5:33 am
Posts: 66
Ok , you can continue to the SIPO , I understand


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 Post subject: Re: PPU questions
PostPosted: Fri Jul 01, 2016 3:48 pm 
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Joined: Sun Sep 19, 2004 11:12 pm
Posts: 19122
Location: NE Indiana, USA (NTSC)
So the coarse X bits of the starting address (t) scroll the background horizontally by 0, 8, 16, 24, 32, 40, ..., 240, or 248 pixels, and the SIPO adds between 0 and 7 pixels (x) to this scroll amount. Together, this adds up to a scroll between 0 and 255 pixels, the whole width of one nametable.


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