Most efficient method of adding 16-bit numbers, lookup table, binary addition or simple addition?

Question

I am attempting to code the ADD function for an MSP-430 emulator I am attempting to build.

I was wondering if a lookup table for the addition of two 16-bit numbers would be a viable solution for efficient retrieval of my results. Alternative methods I have thought of implementing are a binary addition loop, which adds bits one by one or a simple decimal addition. I need to be able to determine the sign and the carry (if there is one).

Any specs on the micro-controller can be found here .

Answer 1

You can use simple 32-bit, two's-complement arithmetic. Assuming src and dst are stored in the lower 16-bits of a uint32_t , the following code sequence emulates the ADD instruction:

uint32_t xor = src ^ dst;
dst += src;
flags.N = (dst >> 15) & 1;
flags.C = (dst >> 16) & 1;
flags.V = ((~xor & (src ^ dst)) >> 15) & 1;
dst &= 0xFFFF;
flags.Z = (dst == 0);

The result of the addition itself can be obtained by simply taking the lower 16 bits of a 32-bit addition. The N, Z, and C flags are also easy to compute. The (signed) overflow flag V is more tricky. The code basically tests whether the signs of src and dst were equal before the addition and are different afterwards.