• Bit Combinations
  • A Computer Specification
  • Storing Information
  • Storage Capacity
  • The Central Processing Unit
  • The Central Processing Unit (CPU)
  • Network Connections
    		•

    Chapter 1: Computer Systems




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    Bit Combinations

    • 1 bit
    • 0
    • 1
    • 2 bits
    • 00
    • 01
    • 10
    • 11
    • 3 bits
    • 000
    • 001
    • 010
    • 011
    • 100
    • 101
    • 110
    • 111
    • 4 bits
    • 0000
    • 0001
    • 0010
    • 0011
    • 0100
    • 0101
    • 0110
    • 0111
    • 1000
    • 1001
    • 1010
    • 1011
    • 1100
    • 1101
    • 1110
    • 1111
    • Each additional bit doubles the number of possible combinations

    Bit Combinations

    • Each combination can represent a particular item
    • There are 2N combinations of N bits
    • Therefore, N bits are needed to represent 2N unique items
    • 21 = 2 items
    • 22 = 4 items
    • 23 = 8 items
    • 24 = 16 items
    • 25 = 32 items
    • 1 bit ?
    • 2 bits ?
    • 3 bits ?
    • 4 bits ?
    • 5 bits ?
    • How many
    • items can be
    • represented by

    A Computer Specification

    • Consider the following specification for a personal computer:
      • 600 MHz Pentium III Processor
      • 256 MB RAM
      • 16 GB Hard Disk
      • 24x speed CD ROM Drive
      • 17” Multimedia Video Display with 1280 x 1024 resolution
      • 56 KB Modem
    • What does it all mean?

    Memory

    • Main memory is divided into many memory locations (or cells)
    • 9278
    • 9279
    • 9280
    • 9281
    • 9282
    • 9283
    • 9284
    • 9285
    • 9286
    • Each memory cell has a numeric address, which uniquely identifies it

    Storing Information

    • 9278
    • 9279
    • 9280
    • 9281
    • 9282
    • 9283
    • 9284
    • 9285
    • 9286
    • Large values are
    • stored in consecutive
    • memory locations
    • 10011010
    • Each memory cell stores a set number of bits (usually 8 bits, or one byte)

    Storage Capacity

    • Every memory device has a storage capacity, indicating the number of bytes it can hold
    • Capacities are expressed in various units:
    • KB 210 = 1024
    • MB 220 (over 1 million)
    • GB 230 (over 1 billion)
    • TB 240 (over 1 trillion)
    • Unit Symbol Number of Bytes
    • kilobyte
    • megabyte
    • gigabyte
    • terabyte

    Memory

    • Main memory is volatile - stored information is lost if the electric power is removed
    • Secondary memory devices are nonvolatile
    • Main memory and disks are direct access devices - information can be reached directly
    • The terms direct access and random access are often used interchangeably
    • A magnetic tape is a sequential access device since its data is arranged in a linear order - you must get by the intervening data in order to access other information

    RAM vs. ROM

    • RAM - Random Access Memory (direct access)
    • ROM - Read-Only Memory
    • The terms RAM and main memory are basically interchangeable
    • ROM could be a set of memory chips, or a separate device, such as a CD ROM
    • Both RAM and ROM are random (direct) access devices!
    • RAM should probably be called Read-Write Memory

    The Central Processing Unit

    • A CPU is also called a microprocessor
    • It continuously follows the fetch-decode-execute cycle:
    • fetch
    • Retrieve an instruction from main memory
    • decode
    • Determine what the
    • instruction is
    • execute
    • Carry out the
    • instruction

    The Central Processing Unit (CPU)

    • The CPU contains:
    • Arithmetic / Logic Unit
    • Registers
    • Control Unit
    • Small storage areas
    • Performs calculations and decisions
    • Coordinates processing steps

    The Central Processing Unit

    • The speed of a CPU is controlled by the system clock
    • The system clock generates an electronic pulse at regular intervals
    • The pulses coordinate the activities of the CPU
    • The speed is measured in megahertz (MHz)

    Monitor

    • The size of a monitor (17") is measured diagonally, like a television screen
    • Most monitors these days have multimedia capabilities: text, graphics, video, etc.
    • A monitor has a certain maximum resolution , indicating the number of picture elements, called pixels, that it can display (such as 1280 by 1024)
    • High resolution (more pixels) produces sharper pictures

    Modem

    • Data transfer devices allow information to be sent and received between computers
    • Many computers include a modem, which allows information to be moved across a telephone line
    • A data transfer device has a maximum data transfer rate
    • A modem, for instance, may have a data transfer rate of 56,000 bits per second (bps)

    Networks

    • A network is two or more computers that are connected so that data and resources can be shared
    • Most computers are connected to some kind of network
    • Each computer has its own network address, which uniquely identifies it among the others
    • A file server is a network computer dedicated to storing programs and data that are shared among network users

    Network Connections

    • Each computer in a network could be directly connected to each other computer in the network
    • These are called point-to-point connections
    • This technique is not feasible for
    • more than a few close machines
    • Adding a computer requires
    • a new communication line
    • for each computer already
    • in the network

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