Subject: [nesdev] Re: the skinny on nes scrolling Date: Tue, 13 Apr 1999 17:48:54 -0600 From: loopy Reply-To: nesdev@onelist.com To: nesdev@onelist.com From: loopy (more notes on ppu logic) you can think of bits 0,1,2,3,4 of the vram address as the "x scroll"(*8) that the ppu increments as it draws. as it wraps from 31 to 0, bit 10 is switched. you should see how this causes horizontal wrapping between name tables (0,1) and (2,3). you can think of bits 5,6,7,8,9 as the "y scroll"(*8). this functions slightly different from the X. it wraps to 0 and bit 11 is switched when it's incremented from _29_ instead of 31. there are some odd side effects from this.. if you manually set the value above 29 (from either 2005 or 2006), the wrapping from 29 obviously won't happen, and attrib data will be used as name table data. the "y scroll" still wraps to 0 from 31, but without switching bit 11. this explains why writing 240+ to 'Y' in 2005 appeared as a negative scroll value. ________________________________________________________ NetZero - We believe in a FREE Internet. Shouldn't you? Get your FREE Internet Access and Email at http://www.netzero.net/download.html ------------------------------------------------------------------------ Looking for a new hobby? Want to make a new friend? http://www.ONElist.com Come join one of the 115,000 e-mail communities at ONElist!