coding questions re: loops

[Heaven/TQA]

stupid question but i need a solution for this...

 

normally i would do a loop like this:

 

ldx #100

loop lda tab,x

sta destination,x

dex

bpl loop

 

but what if x>=128?

 

f.e. i need above routine for 160 bytes... my question seems little bit stupid but i want to avoid a inx, cpx... maybe bvs etc?

 

any ideas?

[Heaven/TQA]

oh... i forgot to mention... of course it should perform the x=0 as well...

[vdub_bobby]

oh... i forgot to mention... of course it should perform the x=0 as well...

 

Well, this is ugly, but I think it will work...

   ldx #100

Loop 

  lda tab,X

  sta destination,X

  txa

  beq Done

  dex

  bne Loop

  beq Loop

Done 

Heh.

[Paul Slocum]

I'd say just use BNE and add an extra STA destination,x after the loop. Only uses 3 extra bytes and no extra cycles.

 

Although I always just use BNE and STA destination-1,x. I can't think of many situations where that won't work except maybe where you have some tricky page alignment issues.

 

-paul

[Paul Slocum]

correction, LDA/STA after the loop. 6 extra bytes (or less if zero page) Still not a significant cost unless you have a ton of these (which I'm guessing is not the case).

[Thomas Jentzsch]

  ldx #160 

.loop :

 lda tab-1,x 

 sta destination-1,x 

 dex 

 bne .loop 

[Heaven/TQA]

thanks guys...

 

thomas... i came up with one of yours suggestions in meantime... sometimes small things seems more complicated than they are...

 

thanks...

 

(btw.... it's for boinxx and it's screenwrapper scroll routine...)

[carlsson]

Here's an extra ugly variant, if you want to avoid offset-1 :

 

      lda #159; will be 160 bytes including index #0

loop: tax

     lda src,x

     sta dest,x

     txa

     sec

     sbc #$01

     bcs loop

done:

 

No, I'm not really serious that this is a good alternative. The CPX solution is two bytes shorter (and the offset-1 BNE is four bytes shorter). And that is without counting all the cycles wasted in each turn of the loop.