initial fce ultra 0.81 import

[fceu.git] / Documentation / tech / ppu / loopy2.txt

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