Re: Hardware emulation of 6509 using 6502?

From: Dr Jefyll <laughton_at_cyg.net>
Date: Fri, 23 Feb 2018 21:39:39 -0700 (MST)
Message-ID: <1519447179438-0.post@n4.nabble.com>
Jim Brain wrote
> I used the following schematic from Dr. Jefyll as my basis:
> 
> http://forum.6502.org/viewtopic.php?p=17597&sid=0966e1fa047d491a969a4693b5fed5fd#p17597
> 
> His/her explanation:

Hello, Jim.  The explanation is definitely from a "him." (It's Jeff Laughton
here; Dr Jefyll is just a goofy handle I came up with. You & I had a yack
just last month after you emailed to ask about a project of mine on
Anycpu.org. And in prior years there were group conversations with Rich,
Jack, Dave, Garth, Mike Stein and others. Hello, everyone!)

Anyway, I joined CBM Hackers to congratulate all of you for really getting
serious about a hardware emulation of 6509. It's terrific to see some
interest and some actual progress on this! Thank you, Mia, for creating the
thread, and thanks to all who contributed.

Various approaches are possible but the idea I published (see the 6502.org
link) avoids creating a whole new cpu (in FPGA, for instance). Instead I did
pretty much what the 6509 designers apparently did. I took a 6502 and added
/a bare minimum/ of extra logic!  

Barring some misunderstanding of the datasheet, I'm confident the simple
circuit I posted will reproduce 6509 behavior. Indeed, simple though it is,
my circuit  may be slightly more complex than necessary. I included variable
timing to account for the possibility of page crossings, but the doc I've
seen is unclear as to whether an /actual/ 6509 allows page crossings or
deems them illegal when using $B1 and $91 opcodes for cross-bank accesses.
It wouldn't be the only rough edge on what otherwise seems like a very
clever design. 

Thanks to Jim's efforts, and those of others, we'll soon get to the bottom
of this. Very exciting! If anything's unclear in my 6502.org explanation I
hope someone will call attention to my error. 6510 and 65816 are only very
distantly relevant; the circuit I posted is squarely focused as a 6502 or
65C02 add-on.

best regards,
Jeff
http://LaughtonElectronics.com



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Received on 2018-02-24 05:00:03

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