Is a case that contains electronic components of the computer that are used to process data.

​The System Unit

         ​The system unit is a case that contains electronic components of the computer used to process data. System units are available in a variety of shapes and sizes. The case of the system unit is made of metal or plastic and protects the internal electronic components from damage. All computers and mobile devices have a system unit. 
​          On desktop personal computers, the electronic components and most storage devices are part of the
system unit. Other devices, such as the keyboard, mouse, microphone, monitor, printer, USB flash drive, scanner, Web cam, and speakers, normally occupy space outside the system unit. An all-in-one desktop personal computer is an exception, which houses the monitor and the system unit in the same case. On most notebook computers, including netbooks, the keyboard and pointing device often occupy the area on the top of the system unit, and the display attaches to the system unit by hinges. The location of the system unit on a Tablet PC varies, depending on the design of the Tablet PC. With the slate Tablet PC, which typically does not include a keyboard, the system unit is behind the display.
   The processor interprets and carries out the basic instructions that operate a computer. Memory typically holds data waiting to be processed and instructions waiting to be executed. The electronic components and circuitry of the system unit, such as the processor and memory, usually are part of or are connected to a circuit board called the motherboard. Many motherboards also integrate sound, video, and networking capabilities. Adapter cards are circuit boards that provide connections and functions not built into the motherboard or expand on the capability of features integrated into the motherboard. For example, a sound card and a video card are two types of adapter cards found in some desktop personal computers today. Devices outside the system unit often attach to ports on the system unit by a connector on a cable. These devices may include a keyboard, mouse, microphone, monitor, printer, scanner, USB flash drive, card reader/writer, Web cam, and speakers. A drive bay holds one or more disk drives. The power supply allows electricity to travel through a power cord from a wall outlet into a computer.

The Motherboard

The motherboard, sometimes called a system board, is the main circuit board of the system unit. Many electronic components attach to the motherboard; others are built into it. Memory chips are installed on memory cards (modules) that fit in a slot on the motherboard. A computer chip is a small piece of semi conducting material, usually silicon, on which integrated circuits are etched. An integrated circuit contains many microscopic pathways capable of carrying electrical current. Each integrated circuit can contain millions of elements such as resistors, capacitors, and transistors. Specific types of processor, memory, and other chips are discussed later in the chapter.

​Processor

     The processor, also called the central processing unit (CPU), interprets and carries out the basic instructions that operate a computer. The processor significantly impacts overall computing power and manages most of a computer’s operations. On a personal computer, all functions of the processor usually are on a single chip. Some computer and chip manufacturers use the term microprocessor to refer to a personal computer processor chip. Most processor chip manufacturers now offer multi-core processors. A processor core contains the circuitry necessary to execute instructions. The operating system views each processor core as a separate processor. A multi-core processor is a chip with two or more separate processor cores. Two common multi-core processors are dual-core and quad-core. A dual-core processor is a chip that contains two separate processor cores. Similarly, a quad-core processor is a chip with four separate processor cores.

The Control Unit

​The control unit is the component of the processor that directs and coordinates most of the operations in the computer. The control unit has a role much like a traffic cop: it interprets each instruction issued by a program and then initiates the appropriate action to carry out the instruction. ​Types of internal components that the control unit directs include the arithmetic/logic unit and buses.

​​The Arithmetic Logic Unit

The arithmetic logic unit (ALU), another component of the processor, performs arithmetic, comparison, and other operations. Arithmetic operations include basic calculations such as addition, subtraction, multiplication, and division. Comparison operations involve comparing one data item with another to determine whether the first item is greater than, equal to, or less than the other item. Depending on the result of the comparison, different actions may occur.

Machine Cycle

For every instruction, a processor repeats a set of four basic operations, which comprise a machine cycle: fetching, decoding, executing, and, if necessary, storing. Fetching is the process of obtaining a program instruction or data item from memory. The term decoding refers to the process of translating the instruction into signals the computer can execute. Executing is the process of carrying out the commands. Storing, in this context, means writing the result to memory (not to a storage medium).

The System Clock

The processor relies on a small quartz crystal circuit called the system clock to control the timing of all computer operations. Just as your heart beats at a regular rate to keep your body functioning, the system clock generates regular electronic pulses, or ticks, that set the operating pace of components of the system unit. The pace of the system clock, called the clock speed, is measured by the number of ticks per second. Current personal computer processors have clock speeds in the gigahertz range. Giga is a prefix that stands for billion, and a hertz is one cycle per second. Thus, one gigahertz (GHz) equals one billion ticks of the system clock per second. A computer that operates at 3 GHz has 3 billion (giga) clock cycles in one second (hertz). The faster the clock speed, the more instructions the processor can execute per second. The speed of the system clock is just one factor that influences a computer’s performance.

Data Representation

Most computers are digital. They recognize only two discrete states: on and off. The two digits, 0 and 1, easily can represent these two states. The digit 0 represents the electronic state of off (absence of an electronic charge). The digit 1 represents the electronic state of on (presence of an electronic charge).​

The computer uses a binary system because it recognizes only two states. The binary system is a number system that has just two unique digits, 0 and 1, called bits. A bit (short for binary digit) is the smallest unit
of data the computer can process. By itself, a bit is not very informative. When 8 bits are grouped together as a unit, they form a byte. A byte provides enough different combinations of 0s and 1s to represent 256 individual characters. These characters include numbers, uppercase and lowercase letters of the alphabet, punctuation marks, and others, such as the letters of the Greek alphabet.

​The combinations of 0s and 1s that represent characters are defined by patterns called a coding scheme. Coding schemes make it possible for humans to interact with a digital computer that processes only bits. When you press a key on a keyboard, a chip in the keyboard converts the key’s electronic signal into a scan code that is sent to the system unit. Then, the system unit converts the scan code into a binary form the computer can process and is stored in memory. Every character is converted to its corresponding byte. The computer then processes the data as bytes, which actually is a series of on/off electrical states. When processing is finished, software converts the byte into a human-recognizable number, letter of the alphabet, or special character that is displayed on a screen or is printed. All of these conversions take place so quickly that you do not realize they are occurring. Standards, such as those defined by ASCII, also make it possible for components in computers to communicate successfully with each other.

Memory

Memory consists of electronic components that store instructions waiting to be executed by the processor, data needed by those instructions, and the results of processing the data. Memory usually consists of one or more chips on the motherboard or some other circuit board in the computer. Memory stores three basic categories of items:

• the operating system and other system software that control or maintain the computer and its devices
• application programs that carry out a specific task such as word processing
• the data being processed by the application programs and resulting information.

Bytes and Addressable Memory

A byte is the basic storage unit in memory. When application program instructions and data are transferred to memory from storage devices, the instructions and data exist as bytes. Each byte resides temporarily in a location in memory that has an address.

​Memory Sizes

Manufacturers state the size of memory and storage devices in terms of the number of bytes the chip or device has available for storage.

•  A kilobyte (KB or K) is equal to exactly 1,024 bytes.
• A megabyte (MB) is equal to approximately 1 million bytes.
• A gigabyte (GB) equals approximately 1 billion bytes.

• A terabyte (TB) is equal to approximately 1 trillion bytes.

Types of Memory

The system unit contains two types of memory, the volatile and nonvolatile. When the computer’s power is turned off, volatile memory loses its contents while nonvolatile memory does not lose its contents when power is removed from the computer. Volatile memory is temporary and nonvolatile memory is permanent. 

RAM

RAM is the most common type of volatile memory which means it loses its contents when the power is removed from the computer. Random Access Memory, also called main memory, consists of memory chips that can be read from and written to by the processor and other devices. When you turn on power to a computer, certain operating system files (such as the files that determine how the desktop appears) load into RAM from a storage device such as a hard disk. These files remain in RAM as long as the computer has continuous power. As additional programs and data are requested, they also load into RAM from storage. The processor interprets and executes a program’s instructions while the program is in RAM. During this time, the contents of RAM may change. RAM can hold multiple programs simultaneously, provided the computer has enough RAM to accommodate all the programs.  

Cache

Most of today’s computers improve processing times with cache (pronounced cash). Two types of cache are memory cache and disk cache. Memory cache helps speed the processes of the computer because it stores frequently used instructions and data. Most personal computers today have at least two types of memory cache: L1 cache and L2 cache.

•  L1 cache is built directly in the processor chip. L1 cache usually has a very small capacity, ranging from 8 KB to 128 KB.
• L2 cache is slightly slower than L1 cache but has a much larger capacity, ranging from 64 KB to 16 MB. 

ROM

Read-only memory refers to memory chips storing permanent data and instructions. ROM is nonvolatile, which means its contents are not lost when power is removed from the computer. Manufacturers of ROM chips often record data, instructions, or information on the chips when they manufacture the chips. These ROM chips, called firmware, contain permanently written data, instructions, or information.

Flash Memory

Flash memory is a type of nonvolatile memory that can be erased electronically and rewritten. Most computers use flash memory to hold their startup instructions because it allows the computer easily to update its contents. ​Flash memory chips also store data and programs on many mobile computers and devices, such as smart phones, portable media players, PDAs, printers, digital cameras, automotive devices, digital voice recorders, and pagers.

CMOS

Complementary Metal-Oxide Semiconductor technology because it provides high speeds and consumes little power. CMOS technology uses battery power to retain information even when the power to the computer is off. For example battery-backed CMOS memory chips can keep the calendar, date, and time current even when the computer is off. The flash memory chips that store a computer’s startup information often use CMOS technology.

Memory Access Times

Access time is the amount of time it takes the processor to read data, instructions, and information from memory. A computer’s access time directly affects how fast the computer processes data. Accessing data in memory can be more than 200,000 times faster than accessing data on a hard disk because of the mechanical motion of the hard disk. Some use fractions of a second, which for memory occurs in nano seconds. A nanosecond is one billionth of a second. A nanosecond is extremely fast.

Expansion Slots and Adapter Cards

         An expansion slot is a socket on the motherboard that can hold an adapter card. An adapter card, sometimes called an expansion card, is a circuit board that enhances functions of a component of the system unit and/or provides connections to peripherals. A peripheral is a device that connects to the system unit and is controlled by the processor in the computer.  
​        Other motherboards may require adapter cards to provide capabilities such as sound and video. A sound card enhances the sound-generating capabilities of a personal computer by allowing sound to be input through a microphone and output through external speakers or headphones. A video card, also called a graphics card, converts computer output into a video signal that travels through a cable to the monitor, which displays an image on the screen.

Removable Flash Memory

 Removable flash memory includes these devices: memory cards, USB flash drives, and PC Cards/Express Card modules.
​
• A memory card is a removable flash memory device, no bigger than 1.5" in height or width, that you insert and remove from a slot in a personal computer, game console, mobile device or card reader/writer. 
• A USB flash drive is a flash memory storage device that plugs in a USB port on a computer or mobile device.
• PC Card slot or an Express Card slot, which is a special type of expansion slot that holds a PC Card or an Express Card module. A PC Card is a thin, credit card-sized removable flash memory device that primarily is used today to enable traditional notebook computers and Tablet PCs to access the Internet wirelessly. An Express Card module, which can be used as a removable flash memory device, is about one-half the size of a PC Card and adds memory, communications, multi media, and security capabilities to computers.

Ports and Connectors

A port is the point at which a peripheral attaches to or communicates with a system unit so that the peripheral can send data to or receive information from the computer. An external device, such as a keyboard, monitor, printer, mouse, and microphone, often attaches by a cable to a port on the system unit. 

USB Ports

A USB port, short for universal serial bus port, can connect up to 127 different peripherals together with a single connector. Devices that connect to a USB port include the following: mouse, printer, digital camera, scanner, speakers, portable media player, optical disc drive, smart phone, PDA, game console, and removable hard disk. Personal computers typically have six to eight USB ports on the front and/or back of the system unit. ​USB hub is a device that plugs in a USB port on the system unit and contains multiple USB ports in which you plug cables from USB devices. 

FireWire Ports

FireWire port is similar to a USB port in that it can connect multiple types of devices that require faster data transmission speeds, such as digital video cameras, digital VCRs, color printers, scanners, digital cameras, and DVD drives, to a single connector. A FireWire port allows you to connect up to 63 devices together. 

Bluetooth Port

Bluetooth technology uses radio waves to transmit data between two devices. Bluetooth devices have to be within about 33 feet of each other. Many computers, peripherals, smart phones, PDAs, cars, and other consumer electronics are Bluetooth-enabled, which means they contain a small chip that allows them to communicate with other Bluetooth-enabled computers and devices.

SCSI Port

A special high-speed parallel port, allows you to attach SCSI peripherals such as disk drives and printers. SAS (serial-attached SCSI) is a newer type of SCSI that transmits at much faster speeds than parallel SCSI. 

MIDI Port

A special type of port that connects the system unit to a musical instrument, such as an electronic keyboard, is called a MIDI port. Short for Musical Instrument Digital Interface, MIDI (pronounced MID-dee) is the electronic music industry’s standard that defines how devices, such as sound cards and synthesizers, represent sounds electronically

Port Replicators and Docking Stations

A port replicator is an external device that provides connections to peripherals through ports built into the device. The mobile user accesses peripherals by connecting the port replicator to a USB port or a special port on the mobile computer.  A docking station, which is an external device that attaches to a mobile computer or device, contains a power connection and provides connections to peripherals; it usually also includes slots for memory cards, optical disc drives, and other devices .

Power Supply

​The power supply is the component of the system unit that converts the wall outlet AC power into DC power. Built into the power supply is a fan that keeps the power supply cool. Some have variable speed fans that change speed or stop running, depending on temperature in the system unit. Some external peripherals such as a cable modem, speakers, or a printer have an AC adapter, which is an external power supply. One end of the AC adapter plugs in the wall outlet and the other end attaches to the peripheral. The AC adapter converts the AC power into DC power that the peripheral requires. ​

Bays

A bay is an opening inside the system unit in which you can install additional equipment. It is actually  different from a slot on the motherboard, which is used for the installation of adapter cards. A drive bay is a rectangular opening that typically holds disk drives. An external bay allows a user to access openings in the bay from outside the system unit. A good example are the optical disc drives installed in external bays.  An internal bay is concealed entirely within the system unit. Hard disk drives are installed in internal bays.

Buses

​Each channel, called a bus, allows the various devices both inside and attached to the system unit to communicate with each other. Buses are used to transfer bits from input devices to memory, from memory to the processor, from the processor to memory, and from memory to output or storage devices. Buses consist of two parts: a data bus and an address bus. The data bus is used to transfer actual data and the address bus is used to transfer information about where the data should reside in memory.  The bus width, which is the size of the bus,  determines the number of bits that the computer can transmit at one time. The larger the number of bits handled by the bus, the faster the computer transfers data. Today's personal computers uses a 64-bit bus. Every bus also has a clock speed. Just like the processor, manufacturers state the clock speed for a bus in hertz. Remember that one megahertz (MHz) is equal to one million ticks per second. The higher the bus clock speed, the faster the transmission of data, which results in programs running faster.
A computer has these basic types of buses: a system bus, possibly a backside bus, and an expansion bus. The system bus, also called the front side bus (FSB), is part of the motherboard and connects the processor to main memory.  A backside bus (BSB) connects the processor to cache. An expansion bus allows the processor to communicate with peripherals. 

What contains the electronic components of the computer?

Is a boxlike case that houses the electronic components a computer uses to process data?

What computer component is used for data processing?

What is a box