multiplication-bitwise.c

#include <stdio.h>

unsigned long add(unsigned long a, unsigned long b)
{
	unsigned long xor = a ^ b;
	unsigned long and = a & b;

	while (and) {
		a = xor;
		b = and << 1;
		xor = a ^ b;
		and = a & b;
	}

	return xor;
}

unsigned long mul(unsigned long a, unsigned long b)
{
	unsigned long n = 0;
	unsigned long sum = 0;

	while (a >> n) {
		if ((a >> n) & 1) {
			sum = add(sum, b << n);
		}
		n = add(n, 1);
	}

	return sum;
}

int main(int argc, char** argv)
{
	int x, y;

	for (x=0; x<10; ++x) {
		for (y=0; y<10; ++y) {
			printf("%d * %d = %d\n", x, y, mul(x, y));
		}
	}

	return 0;
}