As noted previously, the following relation is always true because the difference between any two adjacent known angles is always four.
0 <= THETA - first known angle less than THETA) <= 3
Consequently, the difference between THETA and the first known angle less than THETA is simply THETA logically ANDed with three.
With the two terms that comprise DELTA now calculated, DELTA itself can be calculated by taking their product. The unsigned product returned by the MUL instruction is stored with the MSB in the index register and the LSB in the accumulator.
Because the maximum difference between any two cosine terms is nine, and is subsequently divided by four and multiplied by a number no greater than three, the DELTA product will always reside only in the accumulator.
Knowing this, THETA is again read to calculate the offset into the look up table of the first known angle less than THETA. DELTA is negated and added to (effectively subtracting it from) the cosine of the first known angle less than THETA. This is the interpolated cosine of THETA.