Understanding the Central Processing Unit (CPU) in Computer Hardware and OS, Lecture notes of Computer Science

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CHAPTER- 3

COMPUTER HARDWARE & OS

Introduction to Central Processing

Unit(CPU)

Introduction to CPU

• CPU imprinted on silicon chip called microprocessor.
• Contain tens of millions of tiny transistors.
• CPU is the brain and heart of the computer.
• Key components:
  • CPU.
  • Registers.
  • System clock.

Microprocessor Speeds

• Measure of system clock speed :
  • How many electronic pulses the clock produces per second
  • Usually expressed in gigahertz (GHz)
    • Billions of machine cycles per second
    • Some old PCs measured in megahertz (MHz)
• Comparison of clock speed only meaningful between identical microprocessors.
• CPU cycle time – Clock rate or speed.

Current Technology Capabilities and

Limitations

• Moore’s Law
  • Rate of increase in transistor density on microchips doubles every 18-24 months with no increase in unit cost.
• Rock’s Law
  • Cost of fabrication facilities for chip generation doubles every four years.
• Increased packing density.
• Electrical resistance.

Components of the CPU

• Control Unit (CU)
  • Moves data and instructions between main memory and registers.
• Arithmetic Logic Unit (ALU)
  • Performs computation and comparison operations.
• Set of registers (R)
  • Storage locations that hold inputs and outputs for the ALU.

CPU Registers

• Primary roles
  • Hold data for currently executing program that is needed quickly or frequently (general-purpose registers).
  • Store information about currently executing program and about status of CPU (special-purpose registers).

General-Purpose Registers

• Hold intermediate results and frequently needed

data items.

• Used only by currently executing program.
• Implemented within the CPU; contents can be

read or written quickly.

• Increasing their number usually decreases

program execution time to a point.

Word Size

• Number of bits a CPU can process simultaneously.
• Increasing it usually increases CPU efficiency, up to a point.
• Other computer components should match or exceed it for optimal performance.
• Implications for system bus design and physical implementation of memory.

The Physical CPU

• Electrical device implemented as silicon-based

microprocessor.

• Contains millions of switches, which perform

basic processing functions.

• Physical implementation of switches and circuits.

Switches and Gates

• Basic building blocks of computer processing circuits.
• Electronic switches
  • Control electrical current flow in a circuit.
  • Implemented as transistors.
• Gates
  • An interconnection of switches.
  • A circuit that can perform a processing function on an individual binary electrical signal, or bit.

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