2011-11-17



Currently, I read ``Compilerbau'' by Niklaus Wirth  (in  German).
The  book's  very  old  (1977), but still worthwhile, even though
it's all Pascal syntax.

The book shows the following multiplication function:

     procedure multiply;
             var a, b;
     begin   a := x; b := y; z := 0;
             while b > 0 do
             begin   if odd b then z := z + a;
                     a := 2*a; b := b/2;
             end
     end;

(The global variables x and y are the factors, z is the result.)

My first though had been: ``Strange!''

But let's get rid of the Pascal syntax and convert it to C style:

     int
     multiply(int a, int b)
     {
             int z = 0;

             while (b > 0) {
                     if (b % 2 == 1) {
                             z += a;
                     }
                     a *= 2;
                     b /= 2;
             }
             return z;
     }

Well, it's still strange ... but wait,  remember  assembler  pro-
gramming:

     int
     multiply(int a, int b)
     {
             int z = 0;

             while (b > 0) {
                     if (b & 1) {
                             z += a;
                     }
                     a <<= 1;
                     b >>= 1;
             }
             return z;
     }

All those operations are fast on the machine level.   That's  the
reason for doing it this way.


It had been interesing to find this out, but  I'm  too  young  to
have lived in this world myself.



http://marmaro.de/lue/        markus schnalke <meillo@marmaro.de>