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