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< p > < / p >
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< p > nes scrolling< / p >
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< p > < br / > < / p >
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< p > Date: Tue, 13 Apr 1999 17:48:54 -0600< / p >
< p > From: loopy < zxcvzxcv@netzero.net> < / p >
< p > Reply-To: nesdev@onelist.com< / p >
< p > To: nesdev@onelist.com< / p >
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< p > < br / > < / p >
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< p > From: loopy < zxcvzxcv@netzero.net> < / p >
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< p > < br / > < / p >
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< p > (more notes on ppu logic)< / p >
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< p > < br / > < / p >
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< p > you can think of bits 0,1,2,3,4 of the vram address as the "x scroll"(*8)< / p >
< p > that the ppu increments as it draws. as it wraps from 31 to 0, bit 10 is< / p >
< p > switched. you should see how this causes horizontal wrapping between name< / p >
< p > tables (0,1) and (2,3).< / p >
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< p > < br / > < / p >
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< p > you can think of bits 5,6,7,8,9 as the "y scroll"(*8). this functions< / p >
< p > slightly different from the X. it wraps to 0 and bit 11 is switched when< / p >
< p > it's incremented from _29_ instead of 31. there are some odd side effects< / p >
< p > from this.. if you manually set the value above 29 (from either 2005 or< / p >
< p > 2006), the wrapping from 29 obviously won't happen, and attrib data will be< / p >
< p > used as name table data. the "y scroll" still wraps to 0 from 31, but< / p >
< p > without switching bit 11. this explains why writing 240+ to 'Y' in 2005< / p >
< p > appeared as a negative scroll value.< / p >
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< p > < br / > < / p >
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< p > < / p >
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