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leeeeee
PostPosted: Fri Jun 20, 2003 3:19 pm 
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Joined: Fri Aug 30, 2002 2:05 pm
Posts: 347
Location: UK
ISTR that the NMOS 6502 does a garbage read during indirect indexed memory access at the pre indexed address e.g.

Code:
;     $20,$21 contains $1000

      LDY      #$10
      LDA      ($20),Y



Would do a read from $1000 before it did the read from $1010.
Trouble is I can't find the reference, I know we've discussed this before, on Delpi I think, and I changed the code for PEEK and POKE in EhBASIC to avoid this problem.

Anyone enlighten me?

Cheers,
Lee.
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8BIT
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PostPosted: Sun Jun 22, 2003 7:40 pm 
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Joined: Fri Aug 30, 2002 9:02 pm
Posts: 1686
Location: Sacramento, CA
Lee,

I found two articles that discuss the NMOS 6502 always doing a Read or Write on every cycle. The first link is an overview of some of the "bugs".

http://nesdev.parodius.com/6502bugs.txt

The second had a detailed description of each cycle of all addressing modes. See Appendix E.
http://www.ping.be/kim-1__6502/6502/pm-apndx.html#g

Hope these help!

Daryl
http://65c02.tripod.com/
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