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indirect addressing
Posted: Thu Aug 20, 2020 4:57 am
by DanielS
I'm coming along with my walk into C64 assembly. I've hit another block though.
Take this code...
Code: Select all
position_1 .byte $51, $04 ;Points to an address in screen memory
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.
Code: Select all
lda #$20
sta ??position_1??,y ; should resolve to "sta $0451,y"
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.
Re: indirect addressing
Posted: Thu Aug 20, 2020 5:21 am
by DerTrueForce
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.
Re: indirect addressing
Posted: Thu Aug 20, 2020 5:23 am
by barrym95838
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).
Re: indirect addressing
Posted: Thu Aug 20, 2020 5:29 am
by DanielS
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.
Re: indirect addressing
Posted: Thu Aug 20, 2020 6:00 am
by DerTrueForce
Oh. I had a braino moment... I'll see myself out...
Re: indirect addressing
Posted: Thu Aug 20, 2020 6:03 am
by barrym95838
Found something that said I can safely use $02-$7f.
Depends on your definition of "safe".
https://sta.c64.org/cbm64mem.html
$02-$06 and $fb-$fe seem quite safe, by my personal definition.
Re: indirect addressing
Posted: Thu Aug 20, 2020 12:19 pm
by IamRob
Sorry! Posted to the wrong forum.
Totally unrelated!
Re: indirect addressing
Posted: Thu Aug 20, 2020 12:47 pm
by DanielS
Using...
Code: Select all
lda #$51
sta position_1
lda #$04
sta position_1 + 1
lda square_data,x
sta (position_1),y
position_1 .byte $02 ;Location in zero page
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.
Re: indirect addressing
Posted: Thu Aug 20, 2020 1:54 pm
by BigEd
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.
Re: indirect addressing
Posted: Thu Aug 20, 2020 2:12 pm
by DanielS
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?
Re: indirect addressing
Posted: Thu Aug 20, 2020 2:28 pm
by BigEd
If I read
the manual right, you'd use
Code: Select all
position_1 = $02 ;Location in zero page
and it might need to be up at the left edge. And it should be before the first point you use it, I think.
Re: indirect addressing
Posted: Thu Aug 20, 2020 2:30 pm
by BigEd
Or, an example from the manual, ahead of all of your code:
Code: Select all
* = $02
position_1 .addr ? ;a zero page pointer
(But that's very 64tass specific.)
Re: indirect addressing
Posted: Thu Aug 20, 2020 2:32 pm
by DanielS
If I read the manual right, you'd use
position_1 = $02 ;Location in zero page
and it might need to be up at the left edge. And it should be before the first point you use it, I think.
Looks like that did it. Thanks!
Re: indirect addressing
Posted: Thu Aug 20, 2020 2:39 pm
by BillG
I'm coming along with my walk into C64 assembly. I've hit another block though.
Take this code...
Code: Select all
position_1 .byte $51, $04 ;Points to an address in screen memory
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.
Code: Select all
lda #$20
sta ??position_1??,y ; should resolve to "sta $0451,y"
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.
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
Code: Select all
Screen = $0451
ldy #Offset
sta Screen,Y
which is saying store A at Screen+Offset provided you stay within the range $0451..$0550.
Re: indirect addressing
Posted: Thu Aug 20, 2020 2:52 pm
by DanielS
If you will be writing to other locations nearby, you can do
Code: Select all
Screen = $0451
ldy #Offset
sta Screen,Y
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!