Binary Multipy (Binary opp'ds)

[kevin]

This PlayBASIC example / theory function shows how integer multiplies actually work.  The multiply is only 16bit ..  So it can mult two 16bit values and get a 32bit result, without overflow.  

 I only posted this there was chat how shifts (those << and >> things) could be used to mult values..

PlayBASIC Code: [Select]

do
  A=rnd(64000)
  B=rnd(64000)

  print A*B
  print BinaryMult16(A,B)

  inc c
  if c >10 then c=0: cls 0
  sync
loop


Function BinaryMult16(A,B)
    Result=0
    Shiftoffset =1

    for Bitlp=0 to 15
        if (ShiftOffset and B)<>0
            result=result+(A << bitlp)
        endif
        ShiftOffset=ShiftOffset << 1
    next bitlp
EndFunction Result

[BinaryMoon]

What's that written in? DBP?

Are the << and >> the same as shr and shl in Blitz (I have a feeling they are). I have never understodd these things although I use them to get inividual colour values from hex(?) colours. eg

Code Select Expand



rcol=(pix Shr 16) And $FF
gcol=(pix Shr 8) And $FF
bcol=pix And $FF



[kevin]

Yeah it's Dbpro.. The << and >> are bit shift left and right. Same as shr, shl in BB

Shifting as well as bitwise and/or/xor are binary (base 2) level operations.. it's prolly easier to see what they do by looking at them like this

Code Select Expand


` Shift Left..
v=rnd(65535)
  print "Orig Value:";bin$(v);"  ";v
   for Bitlp=0 to 15
        result=(V << bitlp)
      print bin$(result);"  ";result
  next bitlp
wait key