The Components of the System Unit

Chapter 4: The Components of the System Unit


  1. Describe the components in the system unit
  2. Explain how the CPU uses the four steps of a machine cycle to process data
  3. Define a bit and describe how a series of bits represents data
  4. Differentiate between the various types of memory
  5. Describe the types of expansion slots and cards in the system unit
  6. Explain the difference between a serial, a parallel, and a USB port
  7. Describe how buses contribute to a computer’s processing speed
  8. Identify components in a notebook computer
  9. Identify components in a handheld computer

Chapter 4 presented the components in the system unit, described how memory stores data, instructions, and information, and discussed the sequence of operations that occur when a computer executes an instruction. The chapter included a comparison of various microprocessors on the market today.

1 | Describe The Components In The System Unit

The system unit, sometimes called the chassis, is a box-like case housing the electronic components of a computer that are used to process data. System unit components include the processor, memory module, cards, ports, and connectors. Many of the system unit’s components reside on a circuit board called the motherboard. The motherboard contains many different types of chips, or small pieces of semiconducting material, on which one or more integrated circuits (IC) are etched. An integrated circuit is a microscopic pathway capable of carrying electronic current. Each IC can contain millions of transistors, which act as switches for electronic signals.

2 | Explain How The CPU Uses The Four Steps Of A Machine Cycle To Process Data

The central processing unit (CPU), also called a processor, significantly impacts overall computing power and manages most of a computer’s operations. The CPU contains the control unit and the arithmetic/logic unit. The control unit directs and coordinates most of the operations in the computer. For every instruction, the control unit repeats a set of four basic operations called the machine cycle: (1) fetching the instruction or data item from memory, (2) decoding the instruction into commands the computer understands, (3) executing the commands, and, if necessary, (4) storing, or writing the result to memory. The arithmetic/logic unit (ALU) performs the execution part of the machine cycle. Specifically, the ALU carries out three operations:

  • Arithmetic operations – performing calculations, which include addition, subtraction, multiplication, and division
  • Comparison operations – comparing data items to determine if the first item is greater than, equal to, or less than the other item
  • Logical operations – working with conditions and logical operators such as AND, OR, and NOT
  • Compare and contrast various personal computer processors on the market today

A personal computer’s CPU usually is contained on a single chip, which some call a microprocessor. Intel is a leading manufacturer of personal computer processors. Most high-performance PCs use a processor from Intel called the Pentium® processor. A second Intel brand, called the Celeron™, is designed for less expensive PCs. Two more brands, called the Xeon™ and Itanium™ processors, are ideal for workstations and low-end servers. Intel-compatible processors have the same internal design as Intel processors and perform the same functions, but are made by other companies and often are less expensive. An alternative design to the Intel-style processor, the Motorola processor, is found in Apple Macintosh and Power Macintosh systems. A new type of processor designed for lower-costing personal computers and Internet appliances, called an integrated CPU, combines functions of a processor, memory, and a video card on a single chip. Today’s processors are equipped with MMX™ technology, a built-in set of instructions that manipulates and processes multimedia data more efficiently. Intel’s SSE instructions and AMD’s 3DNow!™ are two other technologies that improve a processor’s performance of multimedia, the Web, and 3-D graphics. To optimize and extend battery life for notebook computers, Intel® mobile processors use SpeedStep™ technology and AMD processors use PowerNow!™ technology.

3| Define A Bit And Describe How A Series Of Bits Represents Data

Most computers are digital, meaning they understand only two discrete states: on and off. These states are represented using two digits, 0 (off) and 1 (on). Each on or off value is called a bit (short for binary digit), the smallest unit of data a computer can handle. Eight bits grouped together as a unit form a byte. A byte provides enough different combinations of 0s and 1s to represent 256 individual characters including numbers, letters of the alphabet, punctuation marks, and other characters.

The combinations of 0s and 1s used to represent data are defined by patterns called coding schemes. Popular coding schemes are ASCII, EBCDIC, and Unicode. Coding schemes make it possible for humans to interact with a digital computer that recognizes only bits. Every character you type on a keyboard is converted into a corresponding byte, a series of on/off electrical states the computer can process.

4 | Differentiate Between The Various Types Of Memory

Memory is a temporary storage place for data, instructions, and information. Memory stores the operating system, application programs, and the data processed by application programs. A byte is the basic storage unit in memory. Memory size is measured by the number of bytes available for use. A kilobyte (KB or K) of memory is approximately one thousand bytes, a megabyte (MB) is approximately one million bytes, and a gigabyte (GB) is approximately one billion bytes. The system unit contains several types of memory.

RAM (random access memory) consists of memory chips that the processor can read from and write to. Most RAM is volatile memory, meaning that its contents are lost when the computer’s power is turned off. Two basic types of RAM chips are dynamic RAM and static RAM. Dynamic RAM (DRAM) must be re-energized constantly or it loses its contents. Static RAM (SRAM) is faster and more reliable than DRAM and has to be re-energized less often, but it is much more expensive.

Memory cache, also called a cache store or RAM cache, improves processing time by storing frequently used instructions and data. ROM (read-only memory) refers to memory chips that only can be read and used; that is, they cannot be modified. ROM is nonvolatile memory (NVM), meaning that its contents are not lost when the computer’s power is turned off. A variation of the ROM chip, called programmable read-only memory (PROM), is a blank chip on which you can place items permanently.

Flash memory, also known as flash ROM or flash RAM, is nonvolatile memory that can be erased electronically and reprogrammed. Complementary metal-oxide semiconductor (CMOS) memory, another type of memory chip, stores configuration information about the computer and uses battery power to retain information when the power to the computer is off.

5 | Describe The Types Of Expansion Slots And Cards In The System Unit

An expansion slot is an opening, or socket, where you can insert a circuit board into the motherboard. These circuit boards – called cards, expansion cards, boards, expansion boards, adapters, adapter cards, interface cards, add-ins, or add-ons — add new devices or capabilities to the computer. Four types of expansion cards found in most computers are a video card, a sound card, a network interface card, and a modem card.

A video card converts computer output into a video signal that is sent through a cable to the monitor, which displays an image. A sound card enhances the sound-generating capabilities of a personal computer by allowing sound to be input through a microphone and output through speakers.

A network interface card (NIC) is a communications device that allows the computer to communicate via a network. A modem card is a communications device that enables computers to communicate via telephone lines or other means. Many of today’s computers support Plug and Play, a capability with which the computer automatically can configure expansion boards and other devices as you install them.

Notebook and other portable computers have a special type of expansion slot used for installing a PC Card, which is a thin credit card-sized device that adds memory, disk drives, sound, fax/modem, and communications capabilities to a mobile computer.

6 | Explain The Difference Between A Serial, A Parallel, And A USB Port

A cable often attaches external devices to the system unit. A port is the interface, or point of attachment, to the system unit. Ports have different types of connectors, which are used to join a cable to a device. Male connectors have one or more exposed pins, while female connectors have matching holes to accept the pins. Most computers have three types of ports: serial, parallel, and USB. A serial port is a type of interface that connects a device to the system unit by transmitting data only one bit at a time. Serial ports usually connect devices that do not require fast data transmission rates, such as a mouse, keyboard, or modem. A parallel port is an interface that connects devices by transferring more than one bit at a time. Many printers connect to the system unit using a parallel port. A universal serial bus (USB) port can connect up to 127 different peripheral devices with a single connector type, greatly simplifying the process of attaching devices to a personal computer.

7 | Describe How Buses Contribute To A Computer’s Processing Speed

Bits are transferred internally within the circuitry of the computer along electrical channels. Each channel, called a bus, allows various devices inside and attached to the system unit to communicate with each other. The bus width, or size of the bus, determines the number of bits that can be transferred at one time. The larger the bus width, the fewer number of transfer steps required and the faster the transfer of data. In most computers word size (the number of bits the CPU can process at a given time) is the same as the bus width. Every bus also has a clock speed. The higher the bus clock speed, the faster the transmission of data, which results in applications running faster. A computer has two basic types of buses. A system bus connects the CPU to main memory. An expansion bus allows the CPU to communicate with peripheral devices.

8 | Identify Components In A Notebook Computer

Users with mobile computing needs often have a mobile computer, such as a notebook computer and/or handheld computer. A notebook computer, also called a laptop computer, can run either using batteries or using a standard power supply. In addition to the motherboard, processor, memory, sound card, PC Card slot, and drive bay, the system unit for a notebook computer also houses other devices, such as the keyboard, pointing device, speakers, and display.

9 | Identify Components In A Handheld Computer

Handheld computers run strictly on battery. Similar to desktop and notebook computers, handheld computers have a system unit that contains electronic components that process data. A handheld computer’s system unit also contains a display and may house speakers and some form of keyboard and/or pointing device. Handheld computers often have an IrDA port so you can communicate wirelessly with other computers. Many also include a serial port.

Expand Your Knowledge

  1. The system unit
  2. The CPU
  3. Processor comparison
  4. Data representation
  5. Memory
  6. Expansion slots and expansion cards
  7. Ports
  8. Buses
  9. Notebook computers
  10. Handheld computers

Here you will find additional information that will expand and enhance your knowledge beyond that contained in your textbook. Compare this information to what may be provided in a traditional classroom by your instructor or peers.

1 | The System Unit

The motherboard in the system unit contains different types of chips. Manufacturers package chips so the chips can be attached to circuit boards, such as the motherboard. Types of chip packages include:

  • Dual inline package (DIP), which consists of two parallel rows of downward-pointing thin metal feet (pins)
  • Pin grid array (PGA) package, which holds a larger number of pins because the pins are mounted on the surface of the package
  • Flip chip-PGA (FC-PGA) package, which places chips on the opposite side (flip side) of the pins
  • Single edge contact (SEC) cartridge, which connects to the motherboard on one of its edges

2 | The CPU

The power of personal computer processor chips (the chips that contain the CPU) has grown at an astounding rate. As chips become older and more widely used, price cuts usually are introduced.

Several factors affect CPU processing speed. CPUs in most of today’s personal computers use pipelining, a technique that increases processing speed by beginning execution of a second machine cycle instruction before the first instruction is completed. CPUs also use high-speed storage locations, called registers, to hold data and instructions temporarily. The control unit relies on a small chip called the system clock to synchronize all computer operations. The speed at which a processor executes instructions is called clock speed, or clock rate, and is measured in megahertz (MHz). The system clock is a major factor affecting processor speed. A higher clock speed means the CPU can process more instructions per second.

3 | Processor Comparison

Although once frequently used, the term “microprocessor” is much less common today.
Sometimes you can upgrade your processor to increase the computer’s performance. There are three forms of upgrades:

  • With a chip for chip upgrade, the existing processor chip is replaced with a new one
  • With a piggyback upgrade, the new processor is stacked on top of the old one
  • With a daughterboard upgrade, the new processor is on a small circuit board (the daughterboard) that plugs into the motherboard

The past three years have seen a steady drop in the cost of computers. PC prices plunged as a result of lower prices for processors, memory chips, and hard drives. Consumers also are showing increased interest in new less powerful, but less expensive, personal computers that work perfectly well for the most popular uses – word processing, Internet access, and spreadsheet applications. The surge in low-priced computer sales has had an impact on Intel, the world’s largest processor manufacturer. By focusing on making inexpensive processor chips, rivals AMD and Cyrix are making inroads into Intel’s dominance. Intel’s response, the Celeron™, has proven popular, but the lower-priced chip offers a smaller profit margin.

4 | Data Representation

Just as the decimal system (10 digits) is suited to human anatomy (10 fingers), the binary system (2 digits) is perfect to represent the on-off states (2 states) of a computer. Basic coding standards make it possible for components within computers to communicate, allow manufacturers to be confident that the components they produce will operate correctly in a computer, and enable consumers to purchase components that are compatible with their systems. In the ASCII-8 and EBCDIC codes, the first four characters represent the zone, and the last four characters represent the digits 1 through 8. ASCII, originally a seven-bit code, was expanded to eight bits in an effort to provide for symbols used in other nations. Unicode, a 2-byte (16-bit) code, can represent 216, or 65,536, characters. The system employs the codes used by ASCII and also includes other alphabets (such as Cyrillic and Hebrew), special characters (including religious symbols), and some of the “word writing” symbols used by various Asian countries.

5 | Memory

Because computers use the binary number system, the actual values for the units in which memory and storage are measured are based on powers of 2. For example, one kilobyte = 210 = 1,024.

RAM’s volatility, and its ability to be changed, are its most distinguishing characteristics. When RAM is purchased it comes in banks of nine chips – eight are needed to represent a byte and the ninth is needed for parity. RAM chips usually are packaged on small circuit boards called single inline memory modules (SIMMs) or dual inline memory modules (DIMMs) that are inserted into the motherboard. During the past 20 years, the price of RAM has dropped an average of 20 percent each year, but its capacity has more than doubled every two years.

Similar to flash ROM, another variation of ROM, called EEPROM (electrically erasable programmable read-only memory), can be erased electrically and rewritten. Because of its nonvolatile nature, EEPROM is used in electronic cash registers to store item prices.

The amount of time it takes the processor to read data from memory, called access time, directly affects how fast the computer can process data. Memory access time is measured in terms of nanoseconds, or billionths of a second.

6 | Expansion Slots And Expansion Cards

Plug and Play was a much-touted feature of the Windows 95 and Windows 98 operating systems.

A PC Card slot, usually located on the side of a notebook computer, allows a PC Card to be changed without having to open the system unit. There are three types of PC Cards:

  • Type I cards add memory capabilities to the computer
  • Type II cards contain communications devices
  • Type III cards house devices such as hard disks

7 | Ports

Port connectors are devised to be foolproof – each is designed so it can fit only one type of socket in only one correct position. Serial ports always are male on the system case. Cables connected to parallel ports often are employed over shorter distances.
Special-purpose ports include:

  • 1394 port – a port that can connect multiple devices requiring faster data transmission speeds such as digital cameras and DVD drives
  • MIDI (musical instrumental digital interface) port – a special type of serial port designed to connect the system unit to a musical instrument
  • SCSI (small computer system interface) port – a high-speed parallel port used to attach peripheral devices such as disk drives and printers
  • IrDA port – a port that allows wireless devices to transmit signals to a computer via infrared light waves

8 | Buses

A highway analogy can help clarify how bus width affects the speed of data transfer. Data moves like cars – the more lanes (greater the bus width) the faster the traffic (data) flow. Ideally, buses used to transfer data should be large enough to use the processing power of registers. Sometimes, however, manufacturers reduce bus size to cut costs.

Word size, which indicates the number of bits processed in each machine cycle, has been compared to the amount of coffee produced with each turn of a coffee grinder’s handle. Theoretically, if word size doubles then processor throughput also could double.

The types of expansion buses on a motherboard determine the types of cards you can add to a computer. Types of expansion buses include:

  • An ISA (Industry Standard Architecture) bus, the most common and slowest expansion bus, connects to devices such as a mouse, modem card, sound card, and low-speed network card
  • A local bus is a high-speed expansion bus used to connect higher speed devices such as hard disks
  • An Accelerated Graphics Port (AGP) is a bus designed by Intel to improve the speed with which 3-D graphics and video transmit
  • The universal serial bus (USB) and 1394 bus are buses that eliminate the need to install expansion cards into expansion slots
  • A PC Card bus is the expansion bus for a PC Card

9 | Notebook Computers

A typical notebook computer often has a keyboard/mouse, IrDA, serial, parallel, video, and USB ports. The keyboard/mouse port allows users who are uncomfortable with a notebook computer’s smaller keyboard and less-traditional pointing devices (often a touch pad or pointing stick) to connect a full-sized keyboard or a mouse to the computer.

10 | Handheld Computers

One of the most popular handheld computers is the Palm Pilot from 3Com. First introduced in 1996, a recent version, Palm IIIc, offers a color screen and an expandable, full-sized keyboard. Another pioneering handheld computer is Visor from Handspring. Visor runs the Palm operating system and features an expansion slot that can accommodate add-ons such as digital cameras and music players. Visor’s greatest innovation, however, may be the slot in the back, which can accommodate modules with various functions including a pager, an MP3 music player, videogame cartridges, and a module that converts the Visor into a cell telephone.


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