Monitor program, exec function

The monitor program's state machine now has an exec branch that stores the 16 bit length argument in memory locations 0 and 1 and then resets the cpu. This will case the cpu to start executing at the address specified in the exec command. The monitor stays in the running state until the cpu signals that is has been halted (by the halt instruction). While in the running state the monitor also transfers bytes to and from the input and output fifo's.