Interesting point, well worth mentioning. Still, do we conclude that "something had to have been done," or, "it would have been nice if something had been done"?

If the '816 and '802 are the same basic die, differing only in some of the metalization, it doesn't seem plausible that invalid-address cycles would appear (as we know they do) on the '816 but not on the '802. What seems more likely to me is that the '802 also exhibits the invalid-address cycles, and, since VPA and VDA outputs are absent, this is a hazard with no good solution. IOW, people using the '802 would need to be on guard against the possibility that an invalid address would read from an IO device, which, on some devices, can erroneously alter status.

Odd (and disappointing) as this seems, it doesn't contradict the definition of the '802 as a drop-in replacement for an NMOS 6502, since the latter exhibits the same hazardous behavior. (Or was the '802 touted as a drop-in replacement for the 'C02 as well?)

cheers,
Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Back in the mid 80's I did swap the 65c02 in an apple IIe for the 802 and it worked fine. The Merlin 8/16 assembler I used at that time had an option that would actually use the 802 instructions to enhance Merlin's performance.

Maybe the '802 uses the same trick that the 'C02 uses (do another opcode fetch instead of invalid address read). What does the '816 do during those cycles ?

Edit: to answer my own question, I tried it out, and the '816 does generate an invalid address cycle for abs,X (but with VDA not asserted, of course). Still, since they had the logic for the 'C02, it is plausible that they have a metal option to switch between the 'C02 and the '816 behavior.

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

The '816 has the same behavior as the NMOS 6502. First, it has a cycle with the address minus the carry in the upper byte, then follows a cycle with the correct address. Here's what I see on the address/data bus. I have X=$F8, and I do LDA abs, X from $206, $207, $208, $209:

I did some additional test on my 65816, with a read, a write and read-modify-write instructions, and I also captured some extra control signals. Here's the code:

And here's the trace. Again X = $F8.

This is as I already stated several posts ago, here. "The [NMOS 6502] exhibits the same hazardous behavior."

I think the waters got murky when, in the immediately following post, clockpulse reported no difficulty after replacing a 'C02 with an '802. But his success is not at odds with the content of my post, and I apologize for not clarifying at this point. He had no difficulty because read-sensitive IO devices were not present and/or because of the factors Garth mentioned -- ie; that the code and the device mapping have to be "just so" in order for the invalid addresses to actually result in a problem. Were the problem more prevalent, the NMOS 6502 would not have been a success.

It's always nice to get the info straight from the horse's mouth! But for those of us who don't have test gear and an '816 system handy, just refer to Table 5-7 of the '816 Data Sheet. There's a 7-page listing of what the '816 does on every cycle of every instruction. Far fom being mysterious, the invalid address states are, as Garth says, quite predictable.

cheers,
Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

I got your point, but I think there's still two possibilities: the '802 has the same problems as the NMOS 6502, or the '802 design inherits the 'C02 workarounds for the invalid accesses. The fact that there's only metal changes between the '802 and '816 does not necessarily imply that their behaviour in this regard needs to be the same.

The WDC programmer's manual comes close to making a verdict here:

Saying that the '802 resembles the NMOS 6502, rather than the 'C02. However, it still leaves open the possibility that the invalid address cycles are considered 'bugs', and therefore have been corrected. The manual also says:

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!