Chapter 4: The Components of 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 themotherboard. 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.
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 usePowerNow!™ technology.
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.
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.
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.
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.
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.
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.
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.
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