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1553 Views 2 Replies Latest reply: Feb 6, 2010 8:54 PM by audioartillery RSS
audioartillery Just Startin'
audioartillery
80 posts since
Apr 18, 2008
Currently Being Moderated

Feb 6, 2010 1:02 PM

integer multiplication - stupid assembly question

Using the standard mpy instruction works great for fixed-point multiplication.  But for regular integer multiplication (e.g., 5*4 = 20) you can't just do this:

 

move #$5,x0

move #$4,x1

mpy x0,x1,b

 

Because $000005 and $000004 are really small fractions.  You end up getting 0 in b.  Is there an equivalent integer multiplication instruction?  I haven't found one.  Performing the multiplication by a loop of additions is really, really slow.

 

Edit:  ok, I'm confused.  The below actually results in the value 0x001e00.

 

        move #$5,x0
        move #$3,x1
        mpy x0,x1,b

 

What is going on here?  If I take 0x001e00 and shift it down 9 bits I get the answer I'm looking for.  Also, $5 comes out as 0x050000 in the GUI.  I'm not sure how I got this far without understanding the multiplication (I've got a semi-complicated effect sorta-working).

  • RedPandaCurt Just Startin'
    RedPandaCurt
    23 posts since
    Oct 30, 2009
    Currently Being Moderated
    Feb 6, 2010 6:37 PM (in response to audioartillery)
    Re: integer multiplication - stupid assembly question

    Think of the integers being scaled by some factor so that all numbers lie in the range [-1,+1).  The scaling factor is a tradeoff between magnitude and precision.  If the maximum possible value is 20 you could use 1/32 as the scaling factor, but I will use 1/128 in this example so that it is easier to see in the ToneCore UI hex display (not 1/256 - one bit is the sign bit).  You need to rescale the multiplication results, which can be done using arithmetic shifts if the scaling factor is a power of 2.

     

    Integer: 5 * 4 = 20

    Fixed point: 5/128  *4/128 = 20/16384

    multiply by 128 to get 20/128, which is the number you are looking for.

     

    Here is some code:

            move #>$050000,x0

            move #>$040000,x1

            mpy x0,x1,b

            asl #7,b,b

            move b,x:Debug_Read_from_DSP_4 ; displays 140000

     

    • audioartillery Just Startin'
      audioartillery
      80 posts since
      Apr 18, 2008
      Currently Being Moderated
      Feb 6, 2010 8:54 PM (in response to RedPandaCurt)
      Re: integer multiplication - stupid assembly question

      Thanks, RedPanda.  It makes sense, I was just hoping for something more automatic.  The accumulator shifter part of the processor is somewhat configurable from my read of the 56300 family manual, but I didn't see any option or instruction to prevent it from shifting.

       

      For the data I need to handle I need to spend some time analyzing my algorithm to determine out the right precision to use, but it will work.  I guess I could dynamically choose the precision based on the data, but that could burn a lot of cycles.

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