As I get more into my project, I am running across some issues where I am not understanding some of the idiosyncrasies of the 6502.

So, I think I understand how

Code:

LDA ($ADDR), y

works. But when would you use

Code:

LDA ($ADDR, X)

Every time I google it, I get specs on the commands and not real examples of when to use it. Using it in a game situation might be helpful too.

Anyone want to give me a 1 minute explanation?

Thanks!

There hardly is any use of the 2nd one, that's why you hardly see any examples.

AFAIK, the first one takes the value at address $ADDR, then adds Y to that value and loads it into the accumulator.

The second one takes the value at address $ADDR plus X, does not add anything to it then loads it into the accumulator.

Ergo, in the first case, Y is added to the obtained value, in the second case, X is added to the target address.

Also, I believe there is a bug with the second one should the sum of $ADDR plus X result in going to the next 256 bytes page.

Since the bug has apparently been fixed in CMOS versions of the 6502 (not that it matters too much since the 64 uses the 6510 which is NMOS only) then you can get completely different results in a 1541 drive whose CPU has been replaced during repairs.

Its also a complete bugger with VIC-20s that have a 65C02 or with CMD drives.
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oh dear...
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Both these commands are 2-byte dealies meant for use with zero page locations. To use a precise example:

LDA ($FB),Y

If location $FB is 0, $FC is 4, and Y is 40, then you'll be loading the first character on the 2nd row of the screen. ($FB) points to a specific address (in this case the start of the screen) and from there, you can use Y to reach anywhere within a page's distance from that pointer. VERY useful. Simply modify $FB and/or $FC when the need arises.

LDA ($FB,X)

If location $FB is 40, $FC is 4, and X is 0, then you'll ALSO be loading the first character on the 2nd row of the screen. ($FB) still points to a specific address (in this case the start of the 2ND screen row) but that can ONLY be accessed when you're using an X index of 0.

If X=2 above, then the value fetched would be wherever ($FD) pointed to. The X is "indexed" BEFORE the pointer is looked up. This relatively useless command exists so a table of pointers can exist in zero page, with the X register used to specify which pointer we want.

This command is best left forgotten, but does have one good use: If the Y register is in use, yet you need to directly check something like ($ZP),Y assuming Y=0 (but isn't) you can use ($ZP,X) when X=0 instead.

hint:

Quote:

I am running across some issues where I am not understanding some of the idiosyncrasies of the 6502.

... the 65816 has JMP(abs,x), the 6502 doesnt.
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http://www.hitmen-console.org http://rr.pokefinder.org
http://magicdisk.untergrund.net http://ftp.pokefinder.org

congrats for rephrasing Lee73s post =P
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Thanks to everyone for posting information. Reading posts here on Lemon for the last few days has helped me reevaluate how I am coding my project. Which means back to the drawing board for some things but at least I am understanding it more.