I'm coming along with my walk into C64 assembly. I've hit another block though.

Take this code...

The target address is $0451. I need to store the contents of the accum into that memory location.
I've tried different examples from online docs but none work.


Y is the indexer in this case (ie. $0451 + Y).
I can't figure out how to get the correct address that's pointed to by the high/low byte pair.

C64 with 64tass assembler.

Thanks for helping an old guy realize a childhood dream.

What you're doing in that first snippet is putting a label at the assembler's current address, then laying down two bytes starting at that position.
The second snippet is correct, if you remove the question marks.
Your assembler should tell you how to assign a value to a label somewhere in its documentation.

position_1 and position_1+1 must both be in zero page, then should store a $20 in address $0451+y (assuming 64tass uses classic 65xx assembly notation).

_________________
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)

I can put them in zero page. Can I just pick an arbitrary location or... ?

[Edit] Found something that said I can safely use $02-$7f.

Oh. I had a braino moment... I'll see myself out...

Depends on your definition of "safe".

https://sta.c64.org/cbm64mem.html

$02-$06 and $fb-$fe seem quite safe, by my personal definition.

_________________
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)

Sorry! Posted to the wrong forum.

Totally unrelated!

Using...


I get the error
square.asm:26:20: error: not a direct page address address '($18be),y'
sta (position_1),y


That being said... the file that has this code assembles fine by itself. I get the error when I assemble the file that includes (.include "foo.asm") this file.

So I don't know if I'm declaring something wrong or if it's a 64tass thing.

You need position_1 to be the value $02 - but that's not what you have. You need a directive more like an EQU to set the value of position_1.

What you have is position_1 as a label, and then you've placed $02 at that location. That's the sort of thing you do when you want a constant, or some constants, embedded in your program.

Could I bother you for a small example?

If I read the manual right, you'd use
and it might need to be up at the left edge. And it should be before the first point you use it, I think.

Or, an example from the manual, ahead of all of your code:

(But that's very 64tass specific.)

Looks like that did it. Thanks!

Is $0451 a constant or will it be different as your program runs?

If constant, you can do

or better,


If you will be writing to other locations nearby, you can do


which is saying store A at Screen+Offset provided you stay within the range $0451..$0550.

BOOM! Why didn't I think of it this way? This is exactly what I needed. The high/low byte pairs was just complicating something that should be simple. Thanks!