34 replies to this topic

[Bryan]

But the links that have been posted are already correct:

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[Cybergoth]

Hi there!

bryede said:

But the links that have been posted are already correct:    

That's what I meant when I was repeating Eriks quote

Greetings,
Manuel

[Thomas Jentzsch]

Hm, I suppose I shouldn't post while being on a Java training.

[Nukey Shay]

Turns out I was wrong about my earlier post...as I could only find LDA-BPL/BMI combos in that other program.


But just as a test, I'd altered Adventure's routine...
;original
LDA Jstick ;Get joystick record.
AND #$C0 ;Merge out previous presses.
CMP #$40 ;Was it previously pressed?
BNE PickupPutdown_2 ;If not branch.


;works
BIT Jstick
BVS PickupPutdown_2


;fails
LDA Jstick
AND #$40
BVS PickupPutdown_2


;fails
LDA Jstick
AND #$40
CMP #$40
BVS PickupPutdown_2

[ErikM]

I wonder if there might be different behavior in different versions of the 6502 architecture? I know the 65C02 has additional instructions and addressing modes that the 6502 and 6507 don't. Perhaps later versions of the chip do update the V flag on a compare instruction...?

[Thomas Jentzsch]

ErikM said:

Perhaps later versions of the chip do update the V flag on a compare instruction...?

I don't know, but I suppose not, else that would have broken backward compatibility for legal (or documented) opcodes.

[EricBall]

Sorry, I was wrong about the Compare instructions. (I've modified my post.) I assumed that since CMP is so close to SBC that it would also affect the oVerflow flag (and didn't check my own refs ). Wierd. I wonder why the designers decided to explicitly exclude it. Maybe there is a well known routine which needs that behaviour.

Sorry Erik, we were both wrong.

[Bryan]

I think the reason CMP's are excluded is because the purpose of V is to guarantee accuracy when doing extended calculations with signed numbers. Since CMP doesn't affect any values, V wasn't deemed necessary.

-Bry

[EricBall]

bryede said:

I think the reason CMP's are excluded is because the purpose of V is to guarantee accuracy when doing extended calculations with signed numbers. Since CMP doesn't affect any values, V wasn't deemed necessary.

My point is it probably required extra transistors for CMx to not update V, and all indications are the 6502 design team didn't waste transistors without a good reason.

[Bryan]

It's probably part of the same logic that suppresses the result from being written to A.

-Bry