Yep, one way or another, X must be zero.

Welp, that seems to have worked!



I still think I should get that book that Garth recommended. Because if this exercise has proven anything, it's that I REALLY need to brush up on how addressing modes work. If only so I'll stop trying to use addressing modes that don't exist. lol

So what you're saying here is that BNE and BEQ also have a kind of dual function as hypothetical Branch if Zero Flag Clear and Branch If Zero Flag Set opcodes if I ever need that?

Neat!

_________________
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?

So really all the compare opcodes do is to kind of force a zero or non zero value to trip the Zero flag to go the way you want?

Think of compare like a subtract operation (for that's what is really is), but with the result thrown away.

It's more useful to compare with a value, then you can branch equal (zero flag), greater than or less than.

See here: http://6502.org/tutorials/compare_instructions.html

-Gordon

_________________
--
Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/

_________________
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?

If you're referring to the Programming the 65816 book, I already have it ordered. From a excerpts I read, it looks like a stellar resource. Thanks for the recommendation.

I've also been reading through the primers you wrote in the meantime. They're clearing up a LOT of things. Until then, thanks for putting up with all the rookie questions.

I'm starting to feel like a three year old that's always asking "What's that do?!"

Not directly relevant, but there's a book and website called "But how do it know?" about the workings of computers.

Okay. Based on all the helpful feedback here, I've tried to refine my screen fill program to be as small as possible while using as few clock cycles as possible. This is as tiny and fast as I can make it for now.



I rearranged the code to remove the need for the JMP command and the only registers being used are the Accumulator and X, so I can take advantage of Y always equaling 0 and use Indirect Indexed Addressing instead to eliminate the need to load 0 into X every loop.

Whittled it down to 30 bytes and the second half of Main is only ever used after the high byte is equal to 05!

Well, here's another baker's routine to fill a memory area from $0200 - $05FF with a byte value:



23 bytes in all with an RTS, else 22 bytes without it, assuming inline code with something else. Note CMOS processor required.

_________________
Regards, KM
https://github.com/floobydust

Nice one EvilSandwich. Be sure to initialise Y!

I think there might be another notch of improvement available. As you are counting up, and filling whole pages, INY can probably be part of an inner loop.

You may well then also find that you can BNE or BEQ (as appropriate) without needing to compare against a constant: the Z flag will already be what you want, in the inner loop.

The more and more I see of the 65C02, the more I realize that I don't think there's much point in using the NMOS unless you really like using illegal opcodes. I would give my right arm to be about to set an address to 0 with a single opcode and being able to use indirect addressing for anything other than JMP opcodes lol

Its all so nice.



I took your advice and got it all the way down to 26 bytes:

I even shaved a little more off by initializing Y and using the 0 in Y for the indirect addressing AND setting the pointer's low byte at the same time.

That's the spirit! The 65C02's extra opcodes are handy if you have them, but by no means necessary - and of course if you use only the originals, your code will run on either flavour.

A very true statement, however, I decided to use nothing but the CMOS version decades ago. I still have the pair of Rockwell R65C02P2 processors I purchased back in the 80's and the original datasheet. One of those chips is still in my original Vic-20 and the other in my original 1541 diskette drive. I haven't written any NMOS specific code since then. Now that you can still get Rockwell CMOS chips at fairly cheap prices (for older hardware replacements) and new WDC chips, I can't see any reason to use the older NMOS chips. That's just my personal view of course, so all of my code since then requires the CMOS processor.

_________________
Regards, KM
https://github.com/floobydust