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| __TOC__
| | #REDIRECT [[PPU scrolling]] |
| == Preface ==
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| "[http://nesdev.parodius.com/loopyppu.zip The skinny on NES scrolling]" was posted by loopy on 1999-04-13 to what eventually became the NESdev Yahoo! Group.
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| It was the first to publicly tell how exactly how the PPU uses addresses written to [[PPU registers|its ports]].
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| After over a decade, it is ''still believed accurate.''
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| Some people get turned off by the fact that it's provided as monospaced text inside a zipfile, that addresses have nothing to distinguish them from years, and that the diagrams of what bits get copied where are allegedly difficult to read.
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| What follows is this document, reformatted to web standards, with a few minor things made slightly clearer.
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| == PPU registers ==
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| Games using complex raster effects require a complete understanding of how the various address registers inside the PPU work.
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| Here are the related registers:
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| ;v: Current VRAM address (15 bits)
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| ;t: Temporary VRAM address (15 bits)
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| ;x: Fine X scroll (3 bits)
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| Registers ''v'' and ''t'' are 15 bits, but because emulators commonly store them in 16-bit machine words, they are shown with an extra bit that's never used.
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| The PPU uses the current VRAM address for both reading and writing PPU memory thru $2007, and for fetching nametable data to draw the background.
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| As it's drawing the background, it updates the address to point to the nametable data currently being drawn.
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| Bits 10-11 hold the nametable address minus $2000.
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| Bits 12-14 are the Y offset of a scanline within a tile.
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| == Stuff that affects register contents ==
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| In the following, ''d'' refers to the data written to the port, and ''A'' through ''H'' to individual bits of a value.
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| $2000 write:
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| t:....BA.. ........ = d:......BA
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| $2005 first write:
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| t:........ ...HGFED = d:HGFED...
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| x: CBA = d:.....CBA
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| $2005 second write:
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| t:......HG FED..... = d:HGFED...
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| t:.CBA.... ........ = d:.....CBA
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| $2006 first write:
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| t:..FEDCBA ........ = d:..FEDCBA
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| t:.G...... ........ = 0
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| $2006 second write:
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| t:........ HGFEDCBA = d:HGFEDCBA
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| v = t
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| At dot 256 of each scanline, if rendering is enabled, the PPU copies all bits related to horizontal position from ''t'' to ''v'':
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| v:.....H.. ...EDCBA = t:.....H.. ...EDCBA
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| And at dot 304 of the pre-render scanline, if rendering is enabled, the PPU copies all bits from ''t'' to ''v'':
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| v = t
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| Note: $2005 and $2006 share the toggle that selects between first/second writes.
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| Reading $2002 will clear it so that the next write is seen as a first write.
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| All of this info agrees with the tests Loopy has run on an NES console and Quietust's [[:File:Vramaddr.jpg|analysis of a micrograph of the PPU die]].
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| If there's something you don't agree with, please let [http://nesdev.parodius.com/bbs/viewtopic.php?t=8962 the BBS] know so that a member can verify it.
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| == Wrapping around ==
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| You can think of bits 4-0 of the VRAM address as the "coarse x scroll"(*8) that the PPU increments as it draws.
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| As it wraps from 31 to 0, bit 10 is switched.
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| You should see how this causes horizontal wrapping between nametables (0,1) and (2,3).
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| You can think of bits 9-5 as the "coarse y scroll"(*8).
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| This functions slightly different from the X.
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| It wraps to 0 and bit 11 is switched when it's incremented from 29 instead of 31.
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| There are some odd side effects from this.
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| If you manually set the value above 29 (from either $2005 or $2006), the wrapping from 29 obviously won't happen, and attribute data will be used as nametable data.
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| The "y scroll" still wraps to 0 from 31, but without switching bit 11.
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| This explains why writing 240+ to 'Y' in $2005 appeared as a negative scroll value.
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| == Examples ==
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| Below are examples using actual 6502 code, indicating (tracking) what happens to all relevant variables described above, both before and after the 6502 instructions are executed.
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| Individual bits written to a PPU register are colour-coded to reflect where they end up in ''t''.
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| Assume all 6502 code is run sequentially in the order shown, one instruction after the next.
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| Finally, note that the number of bits for ''v'' and ''t'' is 15, while further up the page the documentation shows them as 16-bit. The 16th bit is unused (always 0).
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| === Original skinny example ===
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| This is based completely off of the above [[#Stuff_that_affects_register_contents|Stuff that affects register contents]] section.
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| {| class="wikitable"
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| !colspan="3" style="background-color:palegreen" | Before
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| !rowspan="2" | Instructions
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| !colspan="3" style="background-color:lightcoral" | After
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| !rowspan="2" | Notes
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| |-
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| !style="background-color:palegreen" | t
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| !style="background-color:palegreen" | v
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| !style="background-color:palegreen" | x
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| !style="background-color:lightcoral" | t
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| !style="background-color:lightcoral" | v
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| !style="background-color:lightcoral" | x
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| |-
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ...
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| |style="font-family:monospace; white-space:nowrap;" | LDA #$00 (%000000<span style="background-color:lime">00</span>)<br />STA $2000
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| |style="font-family:monospace; white-space:nowrap;" | ...<span style="background-color:lime">00</span>.. ........
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ...
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| |-
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| |style="font-family:monospace; white-space:nowrap;" | ...00.. ........
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ...
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| |style="font-family:monospace; white-space:nowrap;" | LDA #$7D (%<span style="background-color:lime">01111</span><span style="background-color:yellow">101</span>)<br />STA $2005
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| |style="font-family:monospace; white-space:nowrap;" | ...00.. ...<span style="background-color:lime">01111</span>
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | <span style="background-color:yellow">101</span>
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| |-
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| |style="font-family:monospace; white-space:nowrap;" | ...00.. ...01111
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | 101
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| |style="font-family:monospace; white-space:nowrap;" | LDA #$5E (%<span style="background-color:lime">01</span><span style="background-color:yellow">011</span><span style="background-color:cyan">110</span>)<br />STA $2005
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| |style="font-family:monospace; white-space:nowrap;" | <span style="background-color:cyan">110</span>00<span style="background-color:lime">01</span> <span style="background-color:yellow">011</span>01111
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | 101
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| |-
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| |style="font-family:monospace; white-space:nowrap;" | 1100001 01101111
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | 101
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| |style="font-family:monospace; white-space:nowrap;" | LDA #$3D (%00<span style="background-color:lime">111101</span>)<br />STA $2006
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| |style="font-family:monospace; white-space:nowrap;" | 0<span style="background-color:lime">111101</span> 01101111
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | 101
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| |Bit 15 of ''t'' gets set to zero
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| |-
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| |style="font-family:monospace; white-space:nowrap;" | 0111101 01101111
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | 101
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| |style="font-family:monospace; white-space:nowrap;" | LDA #$F0 (%<span style="background-color:lime">11110000</span>)<br />STA $2006
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| |style="font-family:monospace; white-space:nowrap;" | 0111101 <span style="background-color:lime">11110000</span>
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| |style="font-family:monospace; white-space:nowrap;" | 0111101 11110000
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| |style="font-family:monospace; white-space:nowrap;" | 101
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| |After ''t'' is updated, contents of ''t'' copied into ''v''
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| |}
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| === 2006-2005-2005-2006 example ===
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| This is based on [http://nesdev.parodius.com/bbs/viewtopic.php?p=78593#78593 Drag's example on the nesdev forum] where writes to PPU registers are done in the order of $2006, $2005, $2005, $2006, and what the effect is.
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| {{mbox
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| | type = warning
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| | text = To understand this example, you need to keep in mind the following note from the above section:<br />'''Note: $2005 and $2006 share the toggle that selects between first/second writes. Reading $2002 will clear it.'''
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| }}
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| {| class="wikitable"
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| !colspan="3" style="background-color:palegreen" | Before
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| !rowspan="2" | Instructions
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| !colspan="3" style="background-color:lightcoral" | After
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| !rowspan="2" | Notes
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| |-
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| !style="background-color:palegreen" | t
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| !style="background-color:palegreen" | v
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| !style="background-color:palegreen" | x
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| !style="background-color:lightcoral" | t
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| !style="background-color:lightcoral" | v
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| !style="background-color:lightcoral" | x
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| |-
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ...
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| |style="font-family:monospace; white-space:nowrap;" | LDA #$27 (%00<span style="background-color:lime">100111</span>)<br />STA $2006
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| |style="font-family:monospace; white-space:nowrap;" | 0<span style="background-color:lime">100111</span> ........
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ...
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| | Bit 15 of ''t'' set to zero
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| |-
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| |style="font-family:monospace; white-space:nowrap;" | 0100111 ........
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ...
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| |style="font-family:monospace; white-space:nowrap;" | LDA #$3E (%<span style="background-color:lime">00</span><span style="background-color:yellow">111</span><span style="background-color:cyan">110</span>)<br />STA $2005
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| |style="font-family:monospace; white-space:nowrap;" | <span style="background-color:cyan">110</span>01<span style="background-color:lime">00</span> <span style="background-color:yellow">111</span>.....
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ...
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| | Behaviour of 2nd $2005 write
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| |-
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| |style="font-family:monospace; white-space:nowrap;" | 1100100 111.....
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | ...
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| |style="font-family:monospace; white-space:nowrap;" | LDA #$7D (%<span style="background-color:lime">01111</span><span style="background-color:yellow">101</span>)<br />STA $2005
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| |style="font-family:monospace; white-space:nowrap;" | 1100100 111<span style="background-color:lime">01111</span>
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | <span style="background-color:yellow">101</span>
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| | Behaviour of 1st $2005 write
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| |-
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| |style="font-family:monospace; white-space:nowrap;" | 1100100 11101111
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| |style="font-family:monospace; white-space:nowrap;" | ....... ........
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| |style="font-family:monospace; white-space:nowrap;" | 101
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| |style="font-family:monospace; white-space:nowrap;" | LDA #$99 (<span style="background-color:lime">%10011001</span>)<br />STA $2006
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| |style="font-family:monospace; white-space:nowrap;" | 1100100 <span style="background-color:lime">10011001</span>
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| |style="font-family:monospace; white-space:nowrap;" | 1100100 10011001
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| |style="font-family:monospace; white-space:nowrap;" | 101
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| |After ''t'' is updated, contents of ''t'' copied into ''v''
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| |}
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