How to do an ASM signed multiply.

Comments

• Ale Posts: 2,363

  2010-01-12 06:14 edited 2010-01-12 06:14

  I used 15x15 plus signs in the FFT code below. Be awar that the signs are kept separated.
  
  Remember that you have to xor both signs and if you get a 1 then the result will be negative... in your code you are ignoring the sign of multiplyBuffer unless you know it will not be negative... You do not use the Z flag so you could use it to store the sign and then use negz, sorry it is 7:15 in the morning and I have to get ready to work and I cannot test your code right now... maybe later I can do it.
  
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• Ale Posts: 2,363

  2010-01-12 08:24 edited 2010-01-12 08:24

  This code works:
  
  In both cases the negative number has to be a 32 bit quantity for abs to work.
  
  

  DAT
                          abs     sampleBuffer,    sampleBuffer wc               ' Store sign.
                          muxc    multiplyTest,    #1                            '
                                                                                      
  multiplicand            'rdword  multiplyBuffer,  leftVolumeAddress             ' Setup inputs.
                          shl     sampleBuffer,    #16                           '
                          shr     multiplyBuffer,  #1 wc                         '
                          mov     multiplyCounter, #16                           ' Setup counter
                          
  multiplyLoop if_c       add     multiplyBuffer,  sampleBuffer wc               ' Preform multiplication.
                          rcr     multiplyBuffer,  #1 wc                         '
                         
                          djnz    multiplyCounter, #multiplyLoop                 ' Repeat.
   
  multiplySign            test    multiplyTest,    #1 wc                         ' Negate/swap 
                          negc    sampleBuffer,    multiplyBuffer            
  
  
  
  'local vars
  sampleBuffer     long $0000_0ffe
  multiplyTest     long 0
  multiplyBuffer     long $2
  multiplyCounter  long 0
  
  

  
  
  
  Depending on what you call optimizations... if you want speed or you prefer smaller code size... you cam either unroll the loop..
  
  Another piece of code could be this one. The loop is longer but if you do not have higher bits set in y it could be shorter... note the nice trick to use the Z flag to contain the sign of the number
  
  
  
  

  
  smul
          mov   result,#0
          abs   x, x   wc
          rcr   result, #1   wz, nr   
  mloop
          shr    y, #1     wc
     if_c add    result, x  wc
          shl    x, #1    wc
          tjnz   y,#mloop
          negnz  result, result
  
  ende2   jmp    #ende2
  
  
  'local vars
  y       long $5
  result  long 0
  x       long $5
  
  
  

  
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• Kye Posts: 2,200

  2010-01-12 15:31 edited 2010-01-12 15:31

  Thanks Ale, I was just looking to see if I was missing something when doing the multiply and if there was just a way to do the multiply with signed numbers·no problem... However the shift operations would be a red flag indicating that you cannot do stuff like that.
  
  Okay, I'll use this code variant.
  

         
          rdword  y,  leftVolumeAddress
   
          mov    result,#0
          abs    x, x   wc
          rcr    result, #1   wz, nr
     
  mloop   shr    y, #1     wc
  if_c    add    result, x  wc
          shl    x, #1    wc
          tjnz   y,#mloop
  
          negnz  result, result
  

  
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• Ale Posts: 2,363

  2010-01-12 15:37 edited 2010-01-12 15:37

  Do not forget that it is shorter but not necessarily faster. It depends of the values of Volume you have... just simulate some examples with typical values and see. OTOH your version with my 3 instruction sign-is-kept-in-Z is 2 instructions longer (counting the variable)
  
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• Kye Posts: 2,200

  2010-01-12 16:08 edited 2010-01-12 16:08

  I like your version becuase it has less instructions which was what I was going for.
  
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