Von Neumann Architecture (Cambridge (CIE) O Level Computer Science)

Revision Note

Author

Expertise

Computer Science

The Von Neumann Architecture is a design of the CPU which was proposed by Mathematician John Von Neumann in the 1940s, which most general-purpose computers are built upon
The Von Neumann Architecture outlines how the computer memory, input/output devices and processor all work together

It consists of 5 main registers
- The Program Counter (PC)
- The Memory Address Register (MAR)
- The Memory Data Register (MDR)
- The Accumulator (ACC)
- Current Instruction Register (CIR)
For each of the registers you must know
- The name of the register
- Its acronym
- The purpose of the register

Name	Acronym	Definition
Program Counter	PC	Holds the memory address of the next instructions to be executed Increments by 1 as the fetch-decode-execute cycle runs
Memory Address Registers	MAR	Holds the memory address of where data or instructions are to be fetched from memory
Memory Data Register	MDR	Stores the data or instruction which has been fetched from memory
Current Instruction Register	CIR	Stores the instruction the CPU is currently decoding or executing
Accumulator	ACC	Stores the results of any calculations that have taken place in the Arithmetic Logic Unit (ALU)

Components within the CPU and wider computer system are connected by buses
A bus is a set of parallel wires through which data/signals are transmitted from one component to another
There are 3 types of bus:
- Address - unidirectional, carries location data (addresses), data is written to/read from
- Data - bidirectional, carries data or instructions
- Control - bidirectional, carries commands and control signals to tell components when they should be receiving reads or writes etc..

If asked to describe the purpose of the PC, MAR or the MDR make sure you explain how the data is being fetched or written to memory

Describe the role of the control unit, the control bus, the data bus and the address bus when fetching an instruction from memory [4]

Answer

The address of memory (holding instruction) is placed on the address bus (1)
The control unit sends a signal (1) on the control bus (to start a read operation) (1)
The instruction is/the contents of the memory are placed on the data bus (1)

to absolutely everything:

the (exam) results speak for themselves:

Did this page help you?