Using shifts

/*peasant.c - an implementation of the Russian Peasant
  multiplication algorithm, which implements binary 
  multiplication as a series of shifts and additions.  
  Copyright (c) 1998 Matthew Belmonte.*/

int main()
  {
/*declarations*/
  int left_multiplicand, right_multiplicand;	/*inputs*/
  register int mL, mR;		/*multiplicand registers*/
  register int acc;		/*product accumulator*/
/*inputs*/
  left_multiplicand = 6;
  right_multiplicand = 9;
/*the Russian Peasant algorithm (short form!)*/
/*invariant: mL*mR+acc==left_multiplicand*right_multiplicand*/
  for(mL=left_multiplicand, mR=right_multiplicand, acc=0;
      mL != 0;
      mL&1? mL&=~1, acc+=mR: (mL>>=1, mR<<=1))
    ;
  printf("%d * %d = %d\n", left_multiplicand, right_multiplicand, acc);
  return 0;
  }