The CPU

1
The CPU

• Central Processing Unit

2
The Computer System

• Remember our computer system?

CPU
Input Unit
Output Unit
3
CPU

• Now we take a closer look inside the CPU

CPU
Arithmetic and Logic (ALU)
Control
4
Executing Instructions

• All instructions in a computer are either control
  instructions or arithmetic/logic instructions
• So each instruction is either handled by the ALU
  or the control unit

5
ALU

• Most computer operations are executed in the
  arithmetic and logic unit or ALU
• The ALU is responsible for performing
  mathematical as well as logical operations
• Mathematical operations include addition,
  subtraction, division, multiplication, modulo
• Logical operations include evaluating conditions

6
Control Unit

• The control unit is the nerve centre of the
  computer
• It handles instructions dealing with control
  signals
• You can think of the control unit as a manager
  that controls information flow in a computer

7
Computer Performance

• The most important measure of a computer is how
  quickly it can execute programs
• The processor is controlled by a clock where one
  basic instruction is completed in one clock cycle
  
• So if a clock cycle is shorter, then the computer
  takes less time to execute a basic instruction

8

• Which means we would have a FASTER computer!

9
Computer Performance

• Let P be the length of one clock cycle
• The measure of a computer is the clock rate, R
  1/ P
• Thus, when P is small, R is large

10
Computer Performance

• R is measured in cycles per second or Hertz (Hz)
• Computers today have clock rates in the billions
• The term million is denoted by the prefix Mega
  (M) and billion by Giga (G)

11
Computer Performance

• So a 500 MHz processor operates at 500 million
  cycles per second
• A 2 GHz processor operates at 2 billion cycles
  per second
• This is the number of basic instructions the
  processor can go through in 1 second

12
Practice

• How long does it take a computer to execute a
  basic instruction if the processor clock rate is
  3.2 GHz?
• If a processor can execute a basic instruction in
  0.8 ns (nanoseconds), what is the clock rate of
  the computer?