2.4 The Central Processing Unit

2.4 The Central Processing Unit

1. The Purpose of the CPU is to process data.

   • The CPU has to deal with all the data and processing to make any task possible.

2. Von Neumann architecture

   • This was built for breaking secret war time codes.
   • This is a "stored program" computer. This is where both the program and the data where stored in memory.
   • In the past if a computer needed to do a different job it would have to be rewired which could take weeks, so Jon Von Neumann created a computer where the programs where stored in memory instead.
   • Data and programs are stored in the same block, this means there is only one bus which is very slow.

3. Harvard architecture

   • The Harvard system splits the memory into two parts; one part for the data and the other for the programs. This speeds up the CPU because it can fetch both instructions and data at the same time. This also decreases chance of corruption.
   • Due to having two buses rather than one, the Harvard system takes up a lot more space.
   • Handling two separate blocks of data at once is more complicated and expensive thus the Von Neumann architecture is used unless there is a significant speed difference.
   • This is the architecture the CPU uses.

4. Arithmetic Logic Unit

   • The ALU carries out calculations and makes decisions on the data sent to the processor. This is where the computer processes data by either manipulating it or acting upon it.
   • Arithmetic part: This performs calculations on the data.
   • Logic part: This deals with logic and comparisons.

5. Control Unit

   • This controls the data moving through the processor. It controls the timing of operations and the instructions sent to the processor and peripheral devices. This does the fetching and decoding and managing of instructions.
   • It controls and monitors the hardware - to make sure the commands that are given to it by the application software are used.
   • It controls the input and output of data - so that the the signals go to the right place at the right time.
   • It controls the flow of data within the CPU - which is the Fetch-Execute cycle.

6. Registers

   • Registers are also called internal memory or immediate access stores.
   • A register is a small amount of fast temporary memory within the processor where the ALU or CU can store and change values needed to execute instructions.
   • The CPU needs registers as it is too slow to be able run applications directly from main memory.
   • The Accumulator: this holds data while it is being processed.
   • The Program Counter: this stores the address of the next instruction.
   • The Memory Address Register: this stores the address of next instruction in main memory.
   • The Memory Data Register: after an address in main memory is specified by MAR is examined it is temporarily held by MDR.
   • The Instruction Register: this holds the current instruction ready for decoding and executing by the CU.

7. Buses

   • A bus is a collection of wires that carry signals or communication between between the various components in a computer system.
   • The control bus: Connects the CU with the other components of the CPU and devices in the computer system. The CU uses it to send information back to the CPU.
   • The Address bus: is used to request the location of data in RAM by the CPU.
   • The Data bus: is used to transfer data to the CPU.