This was a good read, thank you to those who gave such well explained advice.

Allow me to ask a question: Like the OP, I dabble a bit with a simple project where I need to dynamically change memory addresses.

I want to print a series of characters on the screen starting with $0400 and ending with $07e7. To do that I have written four loops Right, down, left, up.

something like this



That is not too hard, the X index stops at 40, but I struggle with the "down" bit



It works until x ($1100) reaches 255, then the x index flips because it has 8 bits only, and my whole little thing comes crashung down.

Could I change the routine so that I don't address $0400,x but increment the address itself that the petscii circle is stored to?

Is that a typo or have you coined a neologism, a new word for flawed code, combining "crash" & "hung", much like hangry?

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"I am endeavoring, ma'am, to create a mnemonic memory circuit... using stone knives and bearskins." -- Spock to Edith Keeler

Typically you would store the address in two consecutive bytes of page zero, and adjust them there by doing 16-bit adds or subtracts. Then to read or write the target memory location you set X or Y to zero and use indexed indirect addressing, i.e. (zpaddr,x) or (zpaddr),y.

However you can certainly use self-modifying code instead, if your code is in RAM. Garth linked to some examples of that upthread.

The case where self-modifying code doesn't work so well is where there are two or more instructions that all need to use the same address. There are some more complex techniques to cater for that a bit but it's simpler to just use indirect indexed addressing.

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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?

It would be an excellent example of serendipity. Alas, it was a typo. But yes, why not? "It crashung"

Thanks, you put me on the track again. This is what I managed to write and test, I now have a nice column of petscii circles.

Yes that's the idea. Note Garth's point that with a modern 65C02 you can also do it without using an index register.

Even on older CPUs you can also speed things up a little by swapping the roles of the Y register and the low byte of the address, i.e. $60. So you'd store zero at $60 and put the low byte of your target address into Y instead. This doesn't affect the function of the STA line, but it allows you to save a few cycles in the addition as now you can use TYA instead of LDA $60, and TAY instead of STA $60.

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

Solid advice, every word. Shaved two bytes and ten cycles from the inner loop.

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Got a kilobyte lying fallow in your 65xx's memory map? Sprinkle some VTL02C on it and see how it grows on you!

Mike B. (about me) (learning how to github)

Oh, that was a clever thing. Thank you.

IS the 65C02 the same as the WDC 65C02 that is still manufactured? I've seen that it will be the heart of the new Commander X16.

Yeah, pretty much... especially software-wise. Compared with C02's from Rockwell and CMD, for example, the current WDC product has a couple of new instructions, but they're ones you're unlikely to use -- STP and WAI. (And are you concerned about the hardware side of things? Briefly, the WDC chips can be run faster and they have some slight but important changes to the pinout; also the inputs no longer accept TTL voltage levels.)

-- Jeff

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