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