16-bit ALU, Address Computations
This 16 bit ALU performs all of the math functions required for
computing addresses. Only five operations are needed. Increment
is used for normal program flow to increment the program counter,
to execute stack push and increment the data pointer registers.
Decrement is used for stack pop and data pointer decement. Two
special functions are needed for relative branches, and of course
a pass through without any change is used for ordinary data
transfers from one register to another.