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<h2>NES Scrolling 1</h2>
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<p></p>
<p class="rvps2"><span class="rvts6">Subject: [nesdev] the skinny on nes scrolling</span></p>
<p class="rvps2"><span class="rvts6">Date: Tue, 13 Apr 1999 16:42:00 -0600</span></p>
<p class="rvps2"><span class="rvts6">From: loopy <zxcvzxcv@netzero.net></span></p>
<p class="rvps2"><span class="rvts6">Reply-To: nesdev@onelist.com</span></p>
<p class="rvps2"><span class="rvts6">To: nesdev@onelist.com</span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p class="rvps2"><span class="rvts6">From: loopy <zxcvzxcv@netzero.net></span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p class="rvps2"><span class="rvts6">---------</span></p>
<p class="rvps2"><span class="rvts6">the current information on background scrolling is sufficient for most games;</span></p>
<p class="rvps2"><span class="rvts6">however, there are a few that require a more complete understanding.</span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p class="rvps2"><span class="rvts6">here are the related registers:</span></p>
<p class="rvps2"><span class="rvts6"> (v) vram address, a.k.a. 2006 which we all know and love. (16 bits)</span></p>
<p class="rvps2"><span class="rvts6"> (t) another temp vram address (16 bits)</span></p>
<p class="rvps2"><span class="rvts6"> (you can really call them 15 bits, the last isn't used)</span></p>
<p class="rvps2"><span class="rvts6"> (x) tile X offset (3 bits)</span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p class="rvps2"><span class="rvts6">the ppu uses the vram address for both reading/writing to vram thru 2007,</span></p>
<p class="rvps2"><span class="rvts6">and for fetching nametable data to draw the background. as it's drawing the</span></p>
<p class="rvps2"><span class="rvts6">background, it updates the address to point to the nametable data currently</span></p>
<p class="rvps2"><span class="rvts6">being drawn. bits 0-11 hold the nametable address (-$2000). bits 12-14 are</span></p>
<p class="rvps2"><span class="rvts6">the tile Y offset.</span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p class="rvps2"><span class="rvts6">---------</span></p>
<p class="rvps2"><span class="rvts6">stuff that affects register contents:</span></p>
<p class="rvps2"><span class="rvts6">(sorry for the shorthand logic but i think it's easier to see this way)</span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p class="rvps2"><span class="rvts6">2000 write:</span></p>
<p class="rvps2"><span class="rvts6"> t:0000110000000000=d:00000011</span></p>
<p class="rvps2"><span class="rvts6">2005 first write:</span></p>
<p class="rvps2"><span class="rvts6"> t:0000000000011111=d:11111000</span></p>
<p class="rvps2"><span class="rvts6"> x=d:00000111</span></p>
<p class="rvps2"><span class="rvts6">2005 second write:</span></p>
<p class="rvps2"><span class="rvts6"> t:0000001111100000=d:11111000</span></p>
<p class="rvps2"><span class="rvts6"> t:0111000000000000=d:00000111</span></p>
<p class="rvps2"><span class="rvts6">2006 first write:</span></p>
<p class="rvps2"><span class="rvts6"> t:0011111100000000=d:00111111</span></p>
<p class="rvps2"><span class="rvts6"> t:1100000000000000=0</span></p>
<p class="rvps2"><span class="rvts6">2006 second write:</span></p>
<p class="rvps2"><span class="rvts6"> t:0000000011111111=d:11111111</span></p>
<p class="rvps2"><span class="rvts6"> v=t</span></p>
<p class="rvps2"><span class="rvts6">scanline start (if background and sprites are enabled):</span></p>
<p class="rvps2"><span class="rvts6"> v:0000010000011111=t:0000010000011111</span></p>
<p class="rvps2"><span class="rvts6">frame start (line 0) (if background and sprites are enabled):</span></p>
<p class="rvps2"><span class="rvts6"> v=t</span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p class="rvps2"><span class="rvts6">note! 2005 and 2006 share the toggle that selects between first/second</span></p>
<p class="rvps2"><span class="rvts6">writes. reading 2002 will clear it.</span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p class="rvps2"><span class="rvts6">note! all of this info agrees with the tests i've run on a real nes. BUT</span></p>
<p class="rvps2"><span class="rvts6">if there's something you don't agree with, please let me know so i can verify</span></p>
<p class="rvps2"><span class="rvts6">it.</span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p class="rvps2"><span class="rvts6"><br/></span></p>
<p></p>
<p class="rvps4" style="clear: both;"><span class="rvts18">Created with the Personal Edition of HelpNDoc: </span><a class="rvts19" href="https://www.helpndoc.com/feature-tour/create-help-files-for-the-qt-help-framework">Qt Help documentation made easy</a></p>
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