May I make a first request?

Comments

• Mike Green Posts: 23,101

  2006-09-07 14:09 edited 2006-09-07 14:09

  I know you sent this to Paul, but this code fragment is straightforward if you take it one instruction at a time. What's missing is the value of :d_inc, but I assume it's a long with a one value shifted to the destination field of an instruction (1 << 9).
  
  

  :hptr   mov     :hptr,par           ' This instruction just copies the value in par over top of itself.  It's a "one time only" thing
                                                '  just to save a location and make use of a local label rather than a global assembly label.
            mov     :i,#CogBufSz2   '  This initializes the loop counter with the number of longs to transfer (excluding the start flag)
            wrlong  :d_inc,:hptr      '  When the first long of the buffer area (the start flag) is set to a non-zero, this signals that
                                                '  the transfer has started.  PAR contains the address of this buffer (and here is in :hptr).
            add       :hptr,#1          '  This increments the buffer pointer.  It probably should increment the pointer by 4, not by 1
                                               '  since there was a long stored here.  I'm not sure what the HUB does with an incorrect address
                                               '  (one that's not a multiple of 4 with a wrlong instruction).  It probably ignores the low order bits.
  :writeloop wrlong fbufstart,:hptr  ' The starting address in the cog buffer is "fbufstart".  This copies the long to HUB RAM.
            add       :writeloop,:d_inc   ' Modify the previous instruction by adding a one to the destination address field (to get
                                                '  "fbufstart+1", "fbufstart+2", etc.)  Note that cog addresses are always long addresses.
            add       :hptr,#4          ' Increment the buffer pointer to the next long in HUB RAM.
            djnz      :i,#:writeloop  '  Continue until all longs have been transferred
  
  

  
  
  This would be clearer if the HUB RAM pointer were stored in a separate location and if the initialization code at the beginning
  also initialized :writeloop like with a "movd :writeloop,#fbufstart" with the instruction at :writeloop written as
  "wrlong 0-0,:hptr". That makes it clearer that this instruction is modified, but takes an extra instruction. Unfortunately,
  doing any kind of indexing of the destination or source fields requires instruction modification on the Propeller. It works great, but can be confusing if you don't document it carefully.
  
  Post Edited (Mike Green) : 9/7/2006 2:16:28 PM GMT
• Paul Baker Posts: 6,351

  2006-09-07 16:48 edited 2006-09-07 16:48

  Mike has a good understanding of the code, d_inc is indeed defined as 1<<9 which is the first bit of the destination field within the instruction.
  
  I·used the code in a digital storage scope object (which should be one of the first objects I release, 1 mode works, I'm working on the other two modes).·So I was trying to squeeze everything into as small of a memory footprint as possible to provide as many memory locations for the data buffer as possible.
  
  Also the code is designed to be executed through once then stop. I employed several tricks such as using an initalization instruction for a working variable as the variable's location as well. This is what I was doing with the :hptr location.
  
  Thanks for pointing out the adding 1 to the hub pointer after first use, I intended it to be declared as a byte variable followed by an array of longs, but looking at the full code, I noticed this wasn't the case. I will have to modify the code either to declare it that way (and use wrbyte) or add 4.
  
  The comments are the working comments and will be expanded before the object's release.
  
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  Paul Baker
  Propeller Applications Engineer
  
  Parallax, Inc.
  
  Post Edited (Paul Baker (Parallax)) : 9/7/2006 4:52:55 PM GMT