Machine Level Architecture (Part 1 of 2)

In my computing lesson today I learned about some of the important concepts relating to the CPU (Central Processor Unit), what it is comprised of and what its functions are.  The basics of the CPU can be summarised in von Neumann's 'Three Box Model' from 1945 (shown below).

The three main components:
-The Processor
-The Main Memory
-Inputs/Outputs (to the processor)
These three components are connected by a bus.

The processor interprets and executes programs which are sets of instructions.  Each microprocessor may contain up to one billion transistors/logic gates.  The main memory is comprised of the ROM where bootstrap (load-up) information is kept, and the RAM which is a volatile storage medium used by the processor to temporarily store information and as such this information is lost after the computer is turned off.  Together these two make up what is known as Immediate Access Memory, because it can be quickly accessed by the processor.  The I/O allows the processor to communicate with other devices (external peripherals), although they may not understand how these devices function.  What are known as I/O devices (peripherals) are connected to the processor through an I/O port from an I/O controller, which results in the processor being compatible with new peripherals.  All of these are connected by a bus, which passes information down microscopic wires, each bit passed down a different wire.

At a more fundamental level there are a greater number of components:
-Microprocessor and System Clock
-Hardware Devices and I/O
-RAM and ROM
-Control, Address, and Data Buses

In this form each bus performs a specific function.  The Control Bus is generally comprised of 8 wires, and is used to send read, write, timing and reset instructions, and is two way.  Secondly, the Address Bus is a one way bus that is used by the processor to specify which location it wants to read/write from.  Each location has a different numeric address, meaning that it can be accessed individually.  On the other hand, the data bus is used to transfer information between the processor, main memory and inputs/outputs.  Running programs and storing data involves reading from and writing to the main memory (to fetch and obtain instructions in machine code), and the data bus handles this.  The microprocessor executes instructions, whilst the speed of the system clock determines how many instructions are given per second.  Its main functions are calculation, execution of instructions, locating components (addresses) and obtaining/sending data.
  Inside the processor itself however there are even smaller components, which have even more specific functions.  The PCU (Program Control Unit) executes programs separately, whilst the ALU (Arithmetic Logic Unit) carries out the instructions executed by the PCU, operating on data, and the Accumulator retains the data which is currently being operated on.  Similar to their equivalents in the overall CPU, the processor contains a clock that also has a purpose relating to speed, and an internal bus allowing the components to communicate.  Registers are also contained in the processor, and these are special memory for the processor's use.  They may be 'dedicated' (and as such have a single, defined purpose in their design) or 'general purpose' and as such can be used to store data provisionally.