W65C02S to 6502

How much do you want for one of your spare boards?  Interested!  Thanks in advance.

In post #1, "saint" wrote:

"Will you see a difference? Nope!"

Uncle Bernie is not so sure if the Disk II controller card will be able to format and write disks with the 65C02 in lieu of the NMOS 6502.

IIRC, the Apple IIc was the first Apple II family member using the 65C02 and they had to use the IWM to avoid problems with the floppy disk timing.

This would strike only during writes to disks, not during read. Disk read would work normally.

So I would be curious if the Disk II controller and a normal DOS 3.3 would be able to do disk writes if the 65C02 is used.

I would appreciate if you could do a test.

The outcome would confirm or debunk a myth about the IIc and the IWM.

Comments invited !

Apple sold the Apple //e enhanced using the 65C02 - so apparently they saw no issue with this CPU. And I have never seen issues with mine.

And AFAIK the newer W65C02 only has two additional instructions, but otherwise unchanged behaviour and unchanged instruction timings compared to the 65C02. So the machine should behave just like any other Apple //e enhanced.

But I like Saint's dedication: he shows a photo with a stash of more than a dozen W65C02 adapter PCBs, yet, he still needs a bit more time to ponder whether he has a single to spare... :))

appreciated !

In post #6, "saint" wrote:

"As soon as I get one of the processors I ordered I'll test."

Uncle Bernie cheers !

This is the spirit, "saint" ! You should know I'm no naysayer, I just want to know of the 65C02 has some differences in instruction cycle timings which could affect the Disk II controller card (in write mode). This hinges on a subtle quirk in the NMOS 6502 which other than the datasheets say, in its STA abs,x instruction execution always does a "phantom read" in cycle 4 followed by the write in cycle 5.

The datasheets from back in the day claim this only happens on case of page crossings but I was told it happens always. In the NMOS 6502, there is no 4 cycle STA abs,x despite the dataheets claim so (I was told).

The Woz machine on the Disk II card is controlled by the address lines (which set its mode) and exploits this "phantom read" to switch from SHIFT to LOAD mode during floppy disk write operations so in the CPU cycle immediately following the phantom read, which is the write cycle, to the same address as the phantom read (assuming no page crossing this time), the Woz machine will be in LOAD mode and grab the disk byte ("nibble") from the 6502 data bus. The Woz machine does a blind grab of that byte, it does not look at the R/W signal to determine if it's a  write cycle. If the phantom read is missing, it just won't work because the data byte will be gone before the mode changes to LOAD.

I am quite sure that this is how the Disk II works. But this is where my own hands-on experience ends.

I was told that "some" CMOS 6502 "fixed" that quirk and this would mean the Disk II would not be able to do proper disk write operations if this CPU is used. The question is if the W65C02 you intend to use is "fixed" or not.

If it was "fixed", my money is on the Disk II system not to work anymore. But I'm not sure. The later Apple II had some quite interesting LSI customs chips in it and maybe they put some tricks inside them to make a fake phantom read to keep the 65C02 from derailing the Disk II system.

If the 65C02 in question is "not fixed" and has the same 5 cycle STA abs,x with the same old phantom read, it will work anyways, which would be good, and I'll go ahead and buy a bunch of them for myself. So far the fear of the possible lack of the "phantom read" kept me from doing that. But then, who cooked up the misinformation (?) that Apple had to do the IWM just to make the Disk II work again with those CMOS 6502. Who knows. Even back in the day there was a lot of misinformation about the Apple II around and especially about the inner workings of the Woz Machine. It was a kind of mystery to most.

Mysteries upon mysteries ! And, of course, fading and unreliable memories contribute. Has been a few decades in the past.

Seems to me that these mysteries soon will be solved thanks to your 65C02 adapter cards !

Comments invited ! (Maybe somebody reading this knows more about the IWM story and why it was used in the IIc)

About 20 years ago I tried a GTE and a Rockwell 65c02 in a II+ without running into any problems. Occasionally I use a transwarp with A 65816 in a II+.

Wayne wrote:

About 20 years ago I tried a GTE and a Rockwell 65c02 in a II+ without running into any problems. Occasionally I use a transwarp with A 65816 in a II+.

Ditto that...  I've also run both Rockwell and GTE 65C02s in a ][+.  Also a Transwarp and MCT SpeedDemon cards.  And more recently Ian Kim's A2 Turbo which comes with a 65C802.  Actually I haven't tried the A2 Turbo in an actual ][+ but it works in a Franklin ACE 1200 and a generic Taiwan/Hong Kong type ][+ clone so I'd assume it will in a real ][+.

Just non-functional chips or re-marked chips that aren't what they claim to be?