CPU architectures

Computer Architectures:

Early computers were able to calculate an output using fixed instructions. In the 1940s, John von Neumann and Alan Turing both proposed the stored program concept. In this architecture a program must be loaded into main memory to be executed by the processor. The instructions are fetched one at a time, decoded and executed sequentially by the processor. The sequence of instructions can only be changed by a conditional or unconditional jump instruction.

Von Neumann Architecture:

The most common architecture is the Von Neumann architecture. In this architecture instructions and data are stored in a common main memory and transferred using a single shared bus, because of this there it experiences problems with bottlenecking. 

• Used in PC’s, laptops, servers and high performance computers.
• Data and instructions share the same memory. Both use the same word length.
• One bus makes the control unit design simpler and makes it much cheaper.

Harvard Architecture:

The other main model is the Harvard architecture which separates the data and instructions into separate memories using different busses. Instructions and data don't compete for the same bus. Different sized memories and word lengths can be used for data and instructions. Harvard principles are used with specialist embedded systems and digital signal processing, where speed takes priority over the complexities of design.

• Used in digital signal processing, microcontrollers and in embedded systems such as microwave ovens and watches.
• Instructions and data are held in separate memories which may have different word lengths. Free data memory can’t be used for instructions, and vice versa.
• Separate buses allow parallel access to data and instructions.
• Control unit for two buses is more complicated…