Sudo code for SMULT64X64:

let result be split into result0 and result1 where they correspond to the 64 LSBs and MSBs of result respectively.

void SMULT64X64(num0, num1, result) {

	previousBit = 0;
	result0 = num0;

	for(int i = 0; i < 64; i++) {
		currentBit = num0 & 1;

		if(currentBit == 0 && previousBit == 1) {
			result1 += num1;
		} else if(currentBit == 1 && previousBit == 0) {
			result1 -= num1;
		}

		result = result >> 1;
		previousBit = currentBit;
	}
}