Posted By
Litwr
on 2016-10-18
13:05:32
|  Re: What would I do differently in the Plus/4
Sorry, I still have no time to read about 6516. 
So I can only say about some obvious to me things. The C64 acceleration looks like a kind of masterpiece for me. SID and VIC can't work at 2 or 4 (!) MHz. So this project should use the very tricky electronics to give the proper frequency to them. C+4 is much easier. We may just add controllable (by AEC, BA, ...) clock divider at TED output phi2 frequency. C64 project may work at 4 times higher frequency, C+4 with clock divider will use only 2 times higher frequency for CPU, RAM, ROM, ... TED will work at the same frequency as before. I also have to repeat, it is just a hypothesis.
EDIT.
I've just read the article about 6516. A lot of thanks for this interesting data. It is another mystery around 6502... What prevented to release this chip? IMHO the idea of user defined flag was great. It also might use faster instructions like 4510 which appeared only 10 years later. The idea of modes looks wrong for me. 65816 implements it and this makes it a bit *clumsy*.
EDIT.
I've just found this page about 6516...
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