A destination operand is specified by just a pointer. It is the responsibility of the caller to ensure that the destination has enough space for storing the result. With this way of specifying operands, it is possible to perform computations on subranges of an argument, and store the result into a subrange of a destination. A common requirement for all functions is that each source area needs at least one limb. No size argument may be zero.
All areas have size limbs.
In the notation used here, a source operand is identified by the pointer to the least significant limb, and the limb count in braces. Return carry, either 0 or 1. This is the lowest-level function for addition. It is the preferred function for addition, since it is written in assembly for most targets. Return carry, either 0 or 1.