The INX, DEX, INY, and DEY instructions not affecting the overflow flag makes sense to me - the index registers are not designed for doing arithmetic.

But what about INC and DEC? Why don't they affect the overflow flag? Is this an oversight? These are what I refer to as strict New Zealand instructions - they only clobber the n and z flags. Is the overflow flag in a place conveniently accessible by the ALU, but not the accumulator incrementing circuit?

Taking a look at viewtopic.php?p=15729, it does appear that the ALU is adjacent to the accumulator, but the the incrementing circuit is not.

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What's an additional VIA among friends, anyhow?

Seems like a design choice. The E&L manual provides some interesting clues: INX and INY are
andThis seems to imply that using the ALU circuitry would require doing something with the carry flag, either internally or by the user.

Is that the meaning of this cryptic quote from E&L?I'll have to test this out. My emulator doesn't handle that.

_________________
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?

Ah. That's clearer.

For me it does seem like a design choice. In a sense, incrementing is doing something other than arithmetic - if you needed to increment a counter or index during a multibyte sequence, then it's handy to leave the carry bit out of it. Similarly overflow - one might plausibly need to increment and test a counter after a final arithmetic operation and still want to be able to see whether the overall calculation overflowed.

But note that increment does use the ALU: the carry machinery and the BCD machinery is already close by. The only incrementer on the 6502 is for the program counter, and only for that.

I can't see that it would be good to have BCD in use for increment and decrement: it would make counters and address offsets be treated as decimal. Very often this would be confusing or a source of bugs

_________________
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 would have guessed that INC and DEC were sensitive to decimal mode and that even INX, DEX, INY and DEY might be affected. My reasoning is quite simple. 6502 was covered by an elegant decimal mode patent, all computation goes via ALU and 6502 was conceived as a minimal microcontroller where decimal output is important. Decimal inhibit requires additional transistors which is contrary to other goals while precluding useful functionality.

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Unlike all of the other 8-bitters I've encountered over the last 40 years, the 65c02's flag behavior has always coincided 100% with my personal programmer's intuition. 65c816 addressing mode behavior ... not nearly as much.

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