Loops with two counters

[vdub_bobby]

Here's my situation:

 

I am writing a (2600) kernel that uses Y to count scanlines but also uses X to count down as well.

 

So, the looping code looks something like this:

   dey                 ;+2   50

  tya                 ;+2   52
  cmp BlockChange,X   ;+4   56
  bne Kernel1Loopa    ;+2   58
  dex                 ;+2   60
  bpl Kernel1Loopb    ;+3   63

That takes 15 cycles. Is there a faster way?

[cd-w]

Here's my situation:

 

I am writing a (2600) kernel that uses Y to count scanlines but also uses X to count down as well.

 

So, the looping code looks something like this:

   dey                ;+2   50

  tya                ;+2   52
  cmp BlockChange,X  ;+4   56
  bne Kernel1Loopa   ;+2   58
  dex                ;+2   60
  bpl Kernel1Loopb   ;+3   63

That takes 15 cycles.  Is there a faster way?

887790[/snapback]

 

I'm not sure exactly how BlockChange works, but if you could extend it to have a separate entry for every scanline then you could combine X & Y as follows to save 3 cycles:

 

dey  	+2
bmi EndKernel  +2
lda BlockChange,Y	+4
bne KernelLoopa  +2
beq KernelLoopb  +2
EndKernel

 

Chris

[Robert M]

Hello,

 

Without understanding the relationship of the loopa and loopb labels to the flow of the kernel code its hard to recommend an optimization. Can you post the complete loop? Also is the BlockChange table stored in RAM or ROM?

 

Cheers!

[cd-w]

This discussion moved to the Stella mailing list: http://www.biglist.com/lists/stella/archiv...7/msg00031.html

It looks like this is part of the PoP kernel, so I am hoping to see some more developments there soon!

 

Chris

[vdub_bobby]

Hello,

 

Without understanding the relationship of the loopa and loopb labels to the flow of the kernel code its hard to recommend an optimization.  Can you post the complete loop?  Also is the BlockChange table stored in RAM or ROM?

 

Cheers!

888337[/snapback]

I don't really want to post the complete loop at this time, but the BlockChange table is stored in ROM.

 

I have come to a solution, though; thanks to everyone here (and on [stella] who helped).

 

Sorry, though - it isn't for a 2600 Prince of Persia. It is for something else...