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<p class=rvps2><span class=rvts10>NES Scrolling 1</span></p>
<p>Subject: [nesdev] the skinny on nes scrolling</p>
<p>Date: Tue, 13 Apr 1999 16:42:00 -0600</p>
<p>From: loopy <zxcvzxcv@netzero.net></p>
<p>Reply-To: nesdev@onelist.com</p>
<p>To: nesdev@onelist.com</p>
<p><br></p>
<p>From: loopy <zxcvzxcv@netzero.net></p>
<p><br></p>
<p>---------</p>
<p>the current information on background scrolling is sufficient for most games;</p>
<p>however, there are a few that require a more complete understanding.</p>
<p><br></p>
<p>here are the related registers:</p>
<p> (v) vram address, a.k.a. 2006 which we all know and love. (16 bits)</p>
<p> (t) another temp vram address (16 bits)</p>
<p> (you can really call them 15 bits, the last isn't used)</p>
<p> (x) tile X offset (3 bits)</p>
<p><br></p>
<p>the ppu uses the vram address for both reading/writing to vram thru 2007,</p>
<p>and for fetching nametable data to draw the background. as it's drawing the</p>
<p>background, it updates the address to point to the nametable data currently</p>
<p>being drawn. bits 0-11 hold the nametable address (-$2000). bits 12-14 are</p>
<p>the tile Y offset.</p>
<p><br></p>
<p>---------</p>
<p>stuff that affects register contents:</p>
<p>(sorry for the shorthand logic but i think it's easier to see this way)</p>
<p><br></p>
<p>2000 write:</p>
<p> t:0000110000000000=d:00000011</p>
<p>2005 first write:</p>
<p> t:0000000000011111=d:11111000</p>
<p> x=d:00000111</p>
<p>2005 second write:</p>
<p> t:0000001111100000=d:11111000</p>
<p> t:0111000000000000=d:00000111</p>
<p>2006 first write:</p>
<p> t:0011111100000000=d:00111111</p>
<p> t:1100000000000000=0</p>
<p>2006 second write:</p>
<p> t:0000000011111111=d:11111111</p>
<p> v=t</p>
<p>scanline start (if background and sprites are enabled):</p>
<p> v:0000010000011111=t:0000010000011111</p>
<p>frame start (line 0) (if background and sprites are enabled):</p>
<p> v=t</p>
<p><br></p>
<p>note! 2005 and 2006 share the toggle that selects between first/second</p>
<p>writes. reading 2002 will clear it.</p>
<p><br></p>
<p>note! all of this info agrees with the tests i've run on a real nes. BUT</p>
<p>if there's something you don't agree with, please let me know so i can verify</p>
<p>it.</p>
<p><br></p>
<p><br></p>
<p class=rvps3><span class=rvts11>2008</span></p>
<p class=rvps4><span class=rvts12>This help file has been generated by the freeware version of </span><a class=rvts13 href="http://www.ibe-software.com/products/software/helpndoc/" target="_blank">HelpNDoc</a></p>
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