Back Lectures
The CPU's organization has a number functional structures.
     
   **** Visible to program (datapath)****
   ALU - arithmetic logic unit - performs most program calculations and
     decisions.

   Work Registers (data path) - high speed scratch area holding working data 
     and code.
     Some of these are tied to the ALU, others serve different functions.
     Varies by cpu design and tightly coupled to instruction design.
 
   I/O - Address and Data interface used to move program/data into/out of CPU.
     Modern Personal Computers use memory mapped I/O
       Standard memory address that connects to an interface circuit that does
         actual I/O. 

     Some CPUs are designed to directly interface with external circuitry 
       and use dedicated instructions for this.

   **** Limited or behind the scene ****
   Decoder/control unit - interprets and processes each instruction.
     Using a program counter to track location commands in memory.
     Various control signals.

   Buses - CPU has sereral bus structures, Power, Address, Data, and Control.
     
     These connects the various functional units together and have a strong 
       influence on the performance of the CPU.

   Work registers - like visible work registers except these are not 
     addressable by user/program.