loop x times in pasm

Comments

• mynet43 Posts: 644

  2011-03-30 12:48 edited 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 Posts: 1,930

  2011-03-30 12:48 edited 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 Posts: 6,347

  2011-03-30 12:49 edited 2011-03-30 12:49

          mov count,#NUM_LOOPS
  loop    ....
          djnz count,#loop
  
• doggiedoc Posts: 2,246

  2011-03-30 12:49 edited 2011-03-30 12:49

  I'd use a decrement loop and jump on zero.
• doggiedoc Posts: 2,246

  2011-03-30 12:52 edited 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 Posts: 6,347

  2011-03-30 12:57 edited 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 Posts: 614

  2011-03-30 17:49 edited 2011-03-30 17:49

  Doogiedoc,Dave Hein and JasonDorie thanks for the tips!
• JasonDorie Posts: 1,930

  2011-03-30 18:15 edited 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 Posts: 1,951

  2011-03-30 18:54 edited 2011-03-30 18:54

  if you want higher than 511, say 1000
  
  mov counter, #500
  shl counter, #1 ' double the value.
• Ariba Posts: 2,693

  2011-03-30 20:02 edited 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 Posts: 614

  2011-03-30 20:16 edited 2011-03-30 20:16

  tonyp12 and Andy thanks I actually need to loop 1024 times.
• kuroneko Posts: 3,623

  2011-03-30 20:20 edited 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 Posts: 2,693

  2011-03-30 23:02 edited 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