1. Computer Systems 1Fundamentals of Computing The CPU & Von Neumann

2. The CPU & Von-Neumann • What’s in the box? – reminder • CPU Overview • Control Unit • ALU • Registers • Catching a Bus • Tying it all together • Mr Von Neumann • His legacy Computer Systems 1 (2004 - 2005)

3. Computer Architecture • CPU • Central processing Unit • Speed measure in clock cycles • Hertz (Hz) – usually MHz or GHz • How quickly the CPU can execute instructions • CPU often measured in ‘bits’ • 32-bit Processor / 64-bit processor • Confusion!! • Is this CPU memory word length? • Is this data bus width? usually the answer Computer Systems 1 (2004 - 2005)

4. ALU Control Unit CPU High Speed Registers Central Processing Unit (CPU) • CPU has three important parts: • ALU (Arithmetic and Logic Unit) • Control Unit • Registers Computer Systems 1 (2004 - 2005)

5. Central Processing Unit (CPU) • Arithmetic & Logic Unit • Handles mathematical and logical functions (numerical) • Deals with non-numerical logic operations • Control Unit • Handles all low-level hardware operations • Input & Output Devices and CPU • Carries out instruction handling • Fetch Execute Cycle Computer Systems 1 (2004 - 2005)

6. CPU Registers • Storage areas within the CPU • Used to temporarily store data read from memory • Accessible at High Speed • Anything for processing must be kept in a register • Can also hold the address of a memory location • Registers are used to process instructions and data during the Fetch Execute Cycle • Two main types of register: • Instruction Register (IR) • Commands to be performed • Data Registers • Data upon which operations will be performed Computer Systems 1 (2004 - 2005)

7. CPU Registers • Common registers in the CPU: • Program Counter (PC) • Holds the memory address of the next instruction to be executed • Memory Buffer Register (MBR) • Briefly holds data and instructions that travel to and from memory • Sometimes called MDR (Memory Data Register) Computer Systems 1 (2004 - 2005)

8. CPU Registers • Common registers in the CPU: • Memory Address Register (MAR) • Holds the memory address locations of data and/or instructions to be read / written to memory • Current Instruction Register (CIR) • Holds the instruction which is to be executed • General Purpose Registers • ‘Working areas’ for data processed by the ALU Computer Systems 1 (2004 - 2005)

9. Buses • A collection of wires which connects together the internal components of the computer • Allows transfer of data • Main types of bus: • Data bus • Carries actual data bits (information) • Address bus • Transfers locations where data should be sent • Control bus • Carries status information Computer Systems 1 (2004 - 2005)

10. CPU INPUT OUTPUT DATA BUS ADDRESS BUS CONTROL BUS MEMORY Buses • How buses fit into the computer system: Computer Systems 1 (2004 - 2005)

11. CPU Buses • Parallel connections between low-level components of the computer • Size is measured by the number of parallel connections on the bus • E.g.- 32-bit wide bus = 32 individual wires • These bursts are called words • A word is a set measure of bits (in this case we have a 32-bit word) • Signals on buses follow strict timing sequences • Some buses are bi-directional • Allowing two way flow of information Computer Systems 1 (2004 - 2005)

12. Computer Architecture • The Von Neumann Model • 1903 - 1957 • Mathematician • Quantum physicist • Worked on ENIAC • Electronic Numerical Integrator and Computer • Major development in computer technology • Responsible for developing the Fetch Execute Cycle, and his namesake - • ‘Von Neumann Model’ • The original computer geek?! Computer Systems 1 (2004 - 2005)

13. Von Neumann Model • Logically defines a complete computer system • Centralised control of all processes of the computer system • Defines main parts of the machine: • Memory • Storage for instructions and data • Processing unit • ALU functions • Control unit • Interpreting instructions • Issuing commands • Input and Output • For entering and retrieving data Computer Systems 1 (2004 - 2005)

14. Von Neumann Model • Logical Structure of the computer system • Routes of data transfer during processing Computer Systems 1 (2004 - 2005)

15. Von Neumann Model • Problem? • Von-Neumann Bottleneck • All instructions must be retrieved from memory before they are processed • Memory (RAM) runs at slower speeds than the processor is capable of • The difference between the speed of the RAM and the speed of the processor is the ‘bottleneck’ • This is being remedied by faster RAM technologies • Such as SDRAM Computer Systems 1 (2004 - 2005)

16. CS1: Week 6 • What you know now: • CPU Architecture • Bit size • ALU • Control Unit • Registers • Many different types • Buses • Address • Control • Data • Von Neumann Model • Problems! Computer Systems 1 (2004 - 2005)