On 6502 undocumented opcodes in hardware-level detail...

[kc5tja]

Recently, I came across a positively fascinating article:

http://www.pagetable.com/?p=39

thought others here would really enjoy this.

[schidester]

Thank you, that is an interesting article, and pretty much affirms what I thought was happening (at a basic level at least). I'd suspect that the combination arithmetic/shift instructions are caused by the same phenomenon... that the PLA is generating control signals for the ALU and shift registers at the same time.

[BigEd]

That pagetable article is excellent. Somewhere I've seen a good description of the transistor-level (switch-level) design of the 6502 ALU, but I can't find it. But I did find this page (now here) with a reverse-engineered diagram of (some of) the NMOS 6502 ALU:




(Edit: I now see there's a similar diagram of this in patent 3991307 - pdf available here)

I haven't figured out the difference between the two bit-slices. (Edit: aha, it's the inversion of carry out in alternate bits, presumably to simplify the implementation.)

On the subject of implementations (and side-effects) this page goes into a little detail about how BRK, RTI, JSR and RTS share the same sequence.

There's more detail in Bill Mensch's patents - see the "description of the invention" in here for example. Surprisingly readable for a patent - even covering the design decisions.

Edit: updated image URL

[Nightmaretony]

Ah, Jed Margolin's website. Traipse around it, plenty of fun things. Had emailed with him for a bit concerning a game of his called Hard Drivin.

The internal emails of Atari in those years are fascinating to no end