loop x times in pasm

bennettdan · 2011-03-30 12:46

Hello how would you go about looping a certain number of time in a pasm loop?

mynet43 · 2011-03-30 12:48

Try looking up the djnz command. It's pretty simple. If you need an example, just ask.

Try it, it's fun!

Jim

JasonDorie · 2011-03-30 12:48

  mov LoopCount, #16

:Loop
  ' do something useful here
  djnz LoopCount, #:Loop

djnz = Decrement <variable> and jump if not zero.

For any loop count greater than 511 you need to generate the value another way, as an immediate can't hold a value that large. Most people just create a variable in the DAT section and set it to the value needed, then copy it out, like this:

  mov LoopCount, NumberOfLoopsToRun

:Loop
  ' do something useful here
  djnz LoopCount, #:Loop

DAT
  NumberOfLoopsToRun   long    1000

Dave Hein · 2011-03-30 12:49

        mov count,#NUM_LOOPS
loop    ....
        djnz count,#loop

doggiedoc · 2011-03-30 12:49

I'd use a decrement loop and jump on zero.

doggiedoc · 2011-03-30 12:52

doggiedoc wrote: »

I'd use a decrement loop and jump on zero.

Uh... I'm a little slower than Dave Hein and JasonDorie.

Dave Hein · 2011-03-30 12:57

Or you could do it the hard way

        mov count,#(NUM_LOOP>>27)
        shl count,#9
        or count,#(NUM_LOOPS>>18)
        shl count,#9
        or count,#(NUM_LOOP>>9)
        shl count,#9
        or count,#NUM_LOOP
loop    ....
        sub count,#1 wz
  if_nz jmp #loop

bennettdan · 2011-03-30 17:49

Doogiedoc,Dave Hein and JasonDorie thanks for the tips!

JasonDorie · 2011-03-30 18:15

Dave Hein wrote: »

Or you could do it the hard way

        mov count,#(NUM_LOOP>>27)
        shl count,#9
        or count,#(NUM_LOOPS>>18)
        shl count,#9
        or count,#(NUM_LOOP>>9)
        shl count,#9
        or count,#NUM_LOOP
loop    ....
        sub count,#1 wz
  if_nz jmp #loop

For clarity, each of the preceding instructions takes a long value in cog memory, so doing it this way works, but it's very wasteful compared to the single MOV instruction and DAT statement version, which is 2 longs total compared to Dave's 7.

The djnz instruction is also one long, compared to the sub + conditional jmp of Dave's method. I'm quite sure Dave knows all this and is just being cheeky, but I thought it was worth pointing out because someone just starting out isn't likely to know the difference.

And hence, I have out-cheeky'd Dave.

tonyp12 · 2011-03-30 18:54

if you want higher than 511, say 1000

mov counter, #500
shl counter, #1 ' double the value.

Ariba · 2011-03-30 20:02

Here is how I load constants > 511 but max 18 bits:

mov  counter,#1477
  movd counter,#1477>>9

It's very readable and needs not more longs than with a long in a constant pool

Andy

bennettdan · 2011-03-30 20:16

tonyp12 and Andy thanks I actually need to loop 1024 times.

kuroneko · 2011-03-30 20:20

[COLOR="red"]mov  counter,#1477[/COLOR]
  movd counter,#1477>>9

That doesn't even compile. Besides, you get extra runtime as opposed to using a constant pool.

Ariba · 2011-03-30 23:02

You' re right, it does not compile (I thought PropTool does not test it). So you need to write it like that:

mov  counter,#1477 & $1FF
  movd counter,#1477 >> 9

Andy

loop x times in pasm

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