Chapter 1: Introduction - I

Chapter Goals

• To understand the activity of programming
• To learn about the architecture of computers
• To learn about machine languages and higher-level programming languages

What is a Computer?

• Computers can handle repetitive chores without becoming bored or exhausted.
• A computer is programmable to handle different tasks:
  • Balancing checkbooks
  • Processing words
  • Playing games
• Computer actions are composed of huge numbers of extremely primitive operations
• The computer gives the illusion of smooth interaction because it executes these operations a great speed.
• Examples:
  • Put a red dot at this screen position
  • Get a number from the location in memory
  • Send a letter A to the printer
  • Add up these two numbers
  • If this value is negative, continue the program at that instruction.
• Computers are flexible: they can handle a wide range of tasks.

What is Programming?

• A program tells the computer the sequence of steps needed to fulfill a task.
• A programmer designs and implements these programs.
• Most computer users are not computer programmers.
• Programming is an essential skill for a computer scientist.
• Programming is not the only skill required to be a successful computer scientist.
• Early programming experiences are mundane and will not rival sophisticated software that you are familiar with.
• Once certain skills are developed, even simple programs can be thrilling.

The Anatomy of a Computer

• CPU (Central Processing Unit):
  • Plastic, metal and mostly silicon.
  • Composed of several million transistors.
  • Enormously complicated wiring.
  • Performs program control, arithmetic, and data movement.
  • Locates and executes program instructions.
• Memory:
  • RAM (Random Access Memory): read-write memory.
  • ROM (Read Only Memory): contains certain programs that must always be present.
  • Secondary storage (e.g. a hard drive) provides persistent storage.
• The bus is a set of electrical lines that connect the CPU, RAM, and other devices.

• The motherboard holds the CPU, memory, and bus, as well as card slots which connect peripherals to the computer.
• Peripherals allow the user and computer to interact:
  • Printer
  • Mouse
  • Keyboard
  • Modem
• Some computers are connected to one another through networks.

Translating Human-Readable Programs to Machine Code

• Machine instructions:
  • Extremely primitive:
    1. Move the contens of memory location 40000 into register eax.
    2. Subtract the value 100 from register eax.
    3. If the result is positive, continue with the instruction that is stored in memory location 11280.
  • Encoded as numbers:
    

    161 40000 45 100 127 11280

  • Tedious and error prone to look up numeric codes and enter them manually.
  • Each processor has its own set of machine instructions.

• Assembler
  • Assigns short names to commands:

    mov 40000, %eax
    sub 100, %eax
    jg 11280

  • Makes reading easier for humans.
  • Translated into machine instructions by another program.
  • Still processor dependent.

• Higher-level languages:
  • Independent of the underlying hardware.
  • Easiest for humans to read and write.
    

    if (int_rate > 100) message_box("Interest rate error");

  • Translated by compilers into machine instructions.