Re: Can an apple II take a C64 microprocessor?

["Spiro Trikaliotis" <spiro.trikaliotis@gmd.de>]

Hi Robert,

Robert Collet wrote in message <80amq6$i6t$1@bgtnsc02.worldnet.att.net>...

>Huh, I didn't realize the 6510 was such a crappy chip :) How similar would a
>microprocessor have to be to the 6502 to just swap it in? I assume most
>later 65xx chips are slightly more compatible than the 6510.

The 6510 was never intended to be fully HW compatible to the 6502; in fact,
it's only purpose was to get the C64 running. So, IMHO it's not a 
"crappy" :-( chip!

The timing modification and the (only) externel clock generation has been
made, because the 6510 has got to share the bus with a DMA source, the 
6567 (NTSC) / 6569 (PAL) video controller, which itself has very severy
clocking constraints (it must generate the PAL or NTSC signals), so that
chip generates the phi2 for the 6502. For the sharing of the bus, the 6510
is only allowed to use the bus half of the clock cycle (instead of a bit
more than that on the 6502), and it even has tri-state buffers for the 
address- and datalines to allow that sharing.

Since the 6502 is not able to replace a 6510, there was no need to make
it pin-compatible (I forgot to mention this difference, but others did).

Since the 6510 uses the addresses 0 and 1 internally, there is also no
need to make them externally available. (AFAIK, when reading or writing
to/from 0 or 1, the address lines are valid, but the data lines are not
recognized when reading and not driven when writing).

BTW: The timing modification is not the big problem as it seems; I have
seen 6522 or 6821 connected to the C64 expansion port without problems.
But there is the chance that these chips will not work on a C64 (AFAIR,
some 'industrial' extension cards had exactly that problem.
I've seen a circuitry that would change the pulse ratio of phi2 from
1:1 to 3:5, utilizing the 8 MHz dot clock of the C64. With
this modification on an expansion card, even problematic 6522 (and other
chips of the 65xx) will work correctly.

[C64]

>The
>problem with it is that when I turn it on, nothing happens on the screen
>(just static) although the little light on the c64 does turn on.  I opened
>it up and found that if I shorted it out in certain places it would display
>a black screen, but I'm still kind of stumped.


The little light always turns on if you have the 5V from your power supply
on the C64. So, that's not a real indicator.

There are many possible causes for your problem: Defective PLA, defetive 
KERNEL ROM, defective VIC, ... Especially the PLA is defective very often.

I'm sure the people at news:comp.sys.cbm can help you with that problem.

Spiro.