Code Associated with Jack Crenshaw's Programmer's Toolbox, "More on Multiplication" April 1997, pp 19-26

listing 1
Multiplying longs.

typedef struct{
   unsigned short lo;
   unsigned short hi;
} wordpair;

typedef union{
   unsigned long lng;
   wordpair pr;
} Long;

longlong & longlong::mult(unsigned long &x,
 const unsigned long &y){
   // break input up into 16-bit chunks
   Long X, Y, high, low, mid, temp;
   unsigned short a, b, c, d;
   X.lng = x;
   Y.lng = y;
   a = X.pr.hi;
   b = X.pr.lo;
   c = Y.pr.hi;
   d = Y.pr.lo;

   //multiply each pair of chunks
   mid.lng = (unsigned long)a * d;
   high.lng = (unsigned long)a * c;
   low.lng = (unsigned long)b * d;

   // add the pieces
   temp.lng = mid.lng + (unsigned long)
    low.pr.hi;
   if(temp.lng < mid.lng)
     ++high.pr.hi;
   mid.lng = temp.lng+(unsigned long)b*c;
   if(mid.lng < temp.lng)
     ++high.pr.hi;

   // build the result
   low.pr.hi = mid.pr.lo;
   high.lng += mid.pr.hi;
   lo = low.lng;
   hi = high.lng;
   return *this;
}


listing 2
Low half only.

unsigned long mult31(const unsigned long
 &x, const unsigned long &y){
   // break input up into 16-bit chunks
   Long X, Y, low;
   unsigned short b, d, mid;
   X.lng = x;
   Y.lng = y;
   b = X.pr.lo;
   d = Y.pr.lo;

   //multiply each pair of chunks
   low.lng = (unsigned long)b * d;
   mid = X.pr.hi * d + Y.pr.hi * b;

   // add the pieces
   low.pr.hi += mid;
   return low.lng;
}



listing 3
High half only.

unsigned long mult_1(const unsigned long
 &x, const unsigned long &y){
   // break input up into 16-bit chunks
   Long X, Y, high, low, mid, temp;
   unsigned short a, b, c, d;
   X.lng = x;
   Y.lng = y;
   a = X.pr.hi;
   b = X.pr.lo;
   c = Y.pr.hi;
   d = Y.pr.lo;

   //multiply each pair of chunks
   mid.lng = (unsigned long)a * d;
   high.lng = (unsigned long)a * c;
   low.lng = (unsigned long)b * d;

   // add the pieces
   temp.lng = mid.lng + (unsigned
    long)low.pr.hi;
   if(temp.lng < mid.lng)
     ++high.pr.hi;
   mid.lng = temp.lng+(unsigned long)b*c;
   if(mid.lng < temp.lng)
     ++high.pr.hi;

   // build the result
   high.lng += mid.pr.hi;
   return high.lng;
}

listing 4
Multiplying, scale L0.

unsigned long mult_L0(const unsigned long
 &x, const unsigned long &y){
   // break input up into 16-bit chunks
   Long X, Y, high, low, mid, temp;
   unsigned short a, b, c, d;
   X.lng = x;
   Y.lng = y;
   a = X.pr.hi;
   b = X.pr.lo;
   c = Y.pr.hi;
   d = Y.pr.lo;

   //multiply each pair of chunks
   mid.lng = (unsigned long)a * d;
   high.lng = (unsigned long)a * c;
   low.lng = (unsigned long)b * d;

   // add the pieces
   temp.lng = mid.lng + (unsigned
    long)low.pr.hi;
   if(temp.lng < mid.lng)
     ++high.pr.hi;
   mid.lng = temp.lng+(unsigned long)b*c;
   if(mid.lng < temp.lng)
     ++high.pr.hi;

   // build the result
   high.lng += mid.pr.hi;
   high.lng <<=1;
   high.lng += ((long)mid.lng <0);
   return high.lng;
}

listing 5
Multiplying, scale L15.

unsigned long mult_L15(const unsigned long
 &x, const unsigned long &y){
   // break input up into 16-bit chunks
   Long X, Y, high, low, mid, temp;
   unsigned short a, b, c, d;
   X.lng = x;
   Y.lng = y;
   a = X.pr.hi;
   b = X.pr.lo;
   c = Y.pr.hi;
   d = Y.pr.lo;

   //multiply each pair of chunks
   high.lng = (unsigned long)a * c;
   mid.lng = (unsigned long)a * d;
   temp.lng = (unsigned long)b * c;
   low.lng = (unsigned long)b * d;

   // add the pieces
   mid.lng += temp.lng + low.pr.hi;
   mid.pr.hi += high.pr.lo;
   return mid.lng;
}