Which of the following devices is the device that connects all of the physical components of a computer together?

Basic Definitions

Transistor: A tiny electrically operated switch that can alternate between "on" and "off".

Chip (Microchip): A tiny piece of silicon that contains millions of transistors and other electronic components.

• The System Unit
  • Motherboard (aka System Board)
  • CPU (Central Processing Unit)
  • Memory, such as RAM, ROM, Cache, and Flash
  • Ports
  • Expansion Slots
• Secondary Storage, such as hard disk drives, CD-ROMs, etc.
• Input / Output Devices
• Communications Devices

The System Unit

• The Motherboard is the main circuit board for the computer, containing both soldered, nonremovable components along with sockets or slots for components that can be removed. The motherboard holds the CPU, RAM and ROM chips, etc.
• The Central Processing Unit (CPU) is the "brain" of the computer. It executes instructions (from software) and tells other components what to do.
  • The Intel Pentium is a popular processor for IBM-PCs.
  • The PowerPC is a popular processor for Macintoshes.
  • There are 2 parts of the CPU: The ALU and Control Unit.
  • The Arithmetic Logic Unit (ALU) performs arithmetic operations (such as addition and subtraction) and logical operations (such as comparing two values).
  • The Control Unit deciphers and carries out instructions.
  • Different CPUs have different types of instructions, so software made for one type of CPU will not run on other kinds.
  • The word size denotes how many bits of data a CPU can process at once. 32 bits is the standard word size for CPU's used in personal computers today. The higher the word size, the faster a CPU can execute instructions.
• The System Clock is an "electrical pulse generator" that sends out a pulse of electricity at regular intervals. The electronic components of the computer need these electric pulses in order to perform work. The more pulses sent out by the system clock, the faster the computer. The first personal computers had clock speeds of 8 MHz (8 million pulses per second); today's PC's have clock speeds greater than 3.2 GHz (3.2 billion pulses per second).
• Bus Lines are "electrical data roadways" (i.e. wires) through which bits of information are transmitted between the CPU and other components. The bus size denotes how many bits can be transmitted at once. In general, this should be the same as the CPU word size.
• Memory Chips
  • Random Access Memory (RAM), also known as Main Memory or Primary Storage, is used to hold instructions and data while they are being used. RAM is volatile, meaning its contents are lost when the power goes off. RAM is more than 1000x faster than the fastest secondary storage (see below).
  • Read Only Memory (ROM) chips are non-volatile memory that generally contains instructions for "booting" the computer (i.e. loading the operating system when the computer starts up).
  • CMOS chips are powered by a battery and contain so-called "flexible information" such as the type of hard drive your computer is using and the current date and time.
  • Flash chips do not require electricity or a battery yet are non-volatile. They are used in computers, cell phones, digital cameras, etc.
  • Cache memory is special high-speed memory that temporarily stores instructions and data the CPU is likely to use frequently. This speeds up processing. Level 2 or external caches generally range in size from 64 Kilobytes to 2 Megabytes.
• Expansion Slots are sockets on the motherboard that you can plug expansion cards into. To plug a card into a slot, you must open the system unit. A card contain a socket on its end that sticks out from the system unit so a cable can be plugged into it. Common types of cards are graphics, sound, and network cards.
• Ports are sockets that are on the outside othe system unit, meaning you can easily plug a cable into a port without opening the system unit.
  • Serial ports transmit one bit of data at a time.
  • Parallel ports transmit 8 bits of data at a time.
  • Universal Serial Bus (USB) ports are much faster than serial or parallel ports and allow multiple devices to be connected to the same port.

Secondary Storage

• Devices that "permanently" hold data and information (i.e. programs).
• Non-volatile memory; when the power goes off, contents are still saved (unless there is an error).
• Used to store instructions and data while they are not being used.
• A floppy disk is a removable (i.e. portable) platter made of mylar plastic that is magnetized. Bits of information are stored in concentric rings called tracks on either side of the platter. The current floppy disk standard is a 3 1/2" platter in a hard plastic case that holds 1.44 Megabytes of information. A Zip disk, on the other hand, can hold up to 250 Megabytes.
• A hard disk is similar to a floppy disk but uses metal platters to store information. Hard disks are not only much faster than floppy disks but can hold huge amounts of data (hundreds of gigabytes).
• Both floppy and hard drives use a read/write head, which is basically a magnet, to read/write information from/to tracks on a platter. In a hard drive, the read/write head and platter(s) are enclosed together in an air-tight package, making hard drives less susceptible to damage. The read/write head hovers above the platter but should not touch it. If touched, the platter can be damaged, resulting in the loss of some or all the data on the platter. This is known as a head crash.
• Magnetic tape is used mostly for backups. These are very slow because you have to fast forward or rewind to the right spot. However, they are very reliable.
• Optical discs use optical technology (i.e. lasers) instead of magnetic technology to store information.
  • CD-ROM stands for Compact Disc - Read Only Memory.
  • CD-R stands for Compact Disc - Recordable and can be written to only once.
    (Also known as CD-WORM: Compact Disc - Write Once, Read Many.)
  • CD-RW stands for Compact Disc - Re-writeable (or Read/Write).
  • DVD-ROM stands for Digital Versatile Disc - Read Only Memory.
  • DVD-R stands for Digital Versatile Disc - Recordable and can be written to only once.
    (Also known as DVD-WORM: Digital Versatile Disc - Write Once, Read Many.)
  • DVD-RW stands for Digital Versatile Disc - Re-writeable (or Read/Write).
  • CD's can hold approximately 650 Megabytes of data while DVD's can hold up to 17 Gigabytes.

Input / Output Devices

• Input devices translate data into a form the computer can understand.
  • The keyboard is the most common input device, but this type of data entry is very slow and error-prone.
  • Direct input devices are much faster and less error-prone.
    • Pointing devices such as the mouse, trackball, and touchpad allow you to manipulate a cursor on the screen.
    • Scanning devices read data directly. For example, OMR (Optical Mark Recognition) devices (such as a scantron machine) can sense marks on paper. Even more advanced are OCR (Optical Characgter Recognition) devices, which attempt to read letters. Bar Code Readers are often used in grocery stores to scan items.
• Output devices translate information into a form humans can understand.
  • The Monitor (or Display Screen) is the most common type of output device. It produces softcopy (i.e. temporary) output on a screen.
  • The Printer is the most second most common type of output device. It produces hardcopy (i.e. "permanent") output on paper.
    • A Laser Printer uses a photoelectric drum and powdered ink, similar to a copying machine, to produce output.
    • An Inkjet Printer produces output by spraying droplets of liquid ink onto the paper from small nozzles. It is the most common type of printer in use today and is generally very inexpensive.

Communications Devices

• These allow your computer to send/receive data to/from other computers.
• A modem sends information over a phone line. Modems are slow and susceptible to problems such as phone line static.
• A network card sends information over a network cable. These can be used to hook up a computer to a local area network (LAN) or to an Internet Service Provider via a cable modem or DSL (for Internet access).

Measurements in Computer Science

• Bit (Binary Digit): Can only have a value of either 0 or 1.
• Byte: 8 bits (also known as a Character).
  • ASCII: A code that assigns characters, such as 'a', 'b', etc. unique 8-bit values. This allows information created on one computer to be understood by other computers.
  • There are other codes besides ASCII, but ASCII is the most common, used in virtually all personal computers and most larger computers as well.
• Kilobyte: 1024 bytes (or 2^10 bytes).
• Megabyte: 1024 * 1024 bytes (or 2^20 bytes). Roughly one million bytes.
• Gigabyte: 1024 * 1024 * 1024 bytes (or 2^30 bytes). Roughly one billion bytes.
• Terabyte: 1024 * 1024 * 1024 * 1024 bytes (or 2^40 bytes). Roughly one trillion bytes.

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