NES Scrolling 2

nes scrolling

Date: Tue, 13 Apr 1999 17:48:54 -0600

From: loopy <zxcvzxcv@netzero.net>

Reply-To: nesdev@onelist.com

To: nesdev@onelist.com

From: loopy <zxcvzxcv@netzero.net>

(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.

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