asm question

[danwinslow]

Hi folks. Well, I have a situation where I need to switch between, say, 2 banks of status variables.

 

status_reg_1: .res 2

count_reg_1: .res 2

...

status_reg_2: .res 2

count_reg_2: .res 2

.. etc

 

I need to be able to switch code accessing these variables from one to the other depending on another setting, without having to have separate code. The only way I could figure out was to set up a zero page 'base' register and use (indirect),y to access the variables. When I want block 1 I put block 1's address in there, when I want block 2 I put block 2's. Is this really the only choice?

 

[+JAC!]

Option 1: lda/sta (zp),y

Option 2: lda/sta abs,x with x=<what you want>+0 or x=<what you want>+offset, where offset is the difference [status_reg_2-status_reg_1].

Option 3: lda/sta abs,x with x=<what you want> and modify the "abs" address directly (like you would modify the zp address) directly in the code (self-modifying code) to point to the start of block1/block2.

Edited February 1, 2014 by JAC!

[Nukey Shay]

Might be helpful to know what the ultimate goal is. Quicker execution? There is nothing faster than 100% dedicated routines. Self-modifying code would only be slightly slower than that. The example above could be even faster if the self-modifying lines are using immediate mode rather than X/Y indexed (2 vs. 4 cycles in each instance)...but even that could be counter-productive if you are altering too many lines (effectively losing any time savings in the process).

[Nukey Shay]

What about a temp ram approach? Save $abs,X or ($ind),Y to scratch ram, deal with a block of code (or an entire frame), and throw the values back from scratch ram to their actual locations (if the routines are modifying them)? That's 3 cycles for each instance. But also counter-productive if the amount of scratch ram needed is too great.

[Rybags]

Just use indexing, by far the easiest way.

 

Then either have the 2 data structures interleaved byte by byte or bundled - if bundled then you have to restrict the size to 128 bytes or less.

 

So, e.g. for a game where 2 player's stats are kept, references would be like:

 

ldx #$80 ; reference player 2

lda energy,x

 

Most games will do such array processing in similar fashion. In some cases they use an interleaved approach where the index is 0 or 1. You can do it any way you want. Even mix it up a bit, it's not like everything has to be stored in the 1 block of data.

[danwinslow]

Ok. Indexed it is.

Self modifying stuff I hadn't thought of...thats pretty cool but I doubt I could pull it off. I had thought of the 'scratch ram' ( if i understand what you mean ) approach but I don't want to spend the time copying a bunch of bytes everytime I need to switch.

Thanks folks.

Edited February 2, 2014 by danwinslow