In additions to the motherboard ,processor,memory ,sound card ,PC card slot and drive bay, a mobile computers system unit  also  house devices , speakers  and display.The system  unit also houses devices such as the keyboard ,pointing devices ,speakers and display .The system unit for a typical notebook computer often has keyboard/mouse,IrDA,serial,parallel,video , and USB ports.Tablet PCs usually include severals slots and ports .PDAs often have a IrDA port or are Bluetooth  enabled so users can communicate wirelessly.

A bus is an electrical  channel along which bits transfer within the circuitry of a computer , allowing devices btoh inside and attached to the system unit to communicate.The size of a bus, called the bus width determines the number of bits that the computer can transmit at one time .The larger the bus data .

A port is the point at which a peripheral  attaches to a system unit so it can send data to or receive information from the computers.A serial port ,which transmits data 1 bit a time , usually connects devices that do not requires fast data transmission such as mouse ,keyboard,or modem .A parellel port  which transfers more than 1 bit a time , often connects s printer to the system unit.A USB port can connect up to 127 different peripherals together with a single  connector type.Five specials purpose ports are FireWire ,MIDI ,SCSI,IrDA, Bluetooth.A FireWire port can be connect port  can connect multiple types of devices that requires faster data transmission speeds.A MIDI port connects the system unit to a musical instrument .A SCSI port attaches the system unit to SCSI peripherals such as disk drives.An IrDA port and Bluetooth  technology allow wireless devices  to transmit signals to a computer via infrared light waves or radio waves.

An expansion slot is a socket on the motherboard that can hold an adapter card.An adapter card is a circuit  board that enhances functions of a component of the system unit and /or provides  a connections  to a peripheral such as a modem disk drives,printer ,scannner ,or keyboard .Severals types of adapter  cards exist .A sound  cards enhances  the sound generating capabilities  of a  personal computers.A video card  converts  computers  output  into a video signal that display  an image on the screen .A modem card enables  computers to communicate .A network card allows a computer to access a network.

The system unit contains volatile and nonvolatile  memory .Volatile memory loses its  contents  when the computers  power is turned off .Nonvolatile  memory does not lose its contents  when the computers power is turned off.RAM is the most common  type of volatile  memory .ROM flash memory and CMOS are examples of the nonvolatile  memory.RAM consist to memory chips that can be read from and written  to by the processor  and others devices.ROM refers to memory chips  storing permanent  data  and instructions that usually cannot be modified .FLASH memory can be erased electronically and reprogrammed. CMOS technology uses battery power to retain information even when the power to the computer is turned off.

A PC video camera is a digital  video camera that enables users to capture video and still images and then send to capture video and still  images  and then send or broadcast  the images  over  the  internet .A web cam is a video camera that display its output on a web page .A video conference  is a meeting  between  geographically separated people  who use a network or the internet to transmit audio and video data.

A scanners  is a light sensing  input devices  that reads printed text and graphics  and translates the results into a  form  the computer can process .A flatbed scanner  works in a manner  similar to a copy machine  except  it creates a file of the document.Optical Character  recognition (OCR) reads characters from ordinary documents  optical mark recognitions(OMR) reads handrawn marks such as small circles .A bar code scanner is an optical  reader that uses laser beams to read a bar code ,or identifications code .MICR (magnetic -ink-character recognition) reads  text printed  with magnetized ink.

A terminals  consist of a keyboard .a monitor a video card , and memory . A dumb terminal has no processing power and relies on a host computer for proccessing .POS terminals  ATMs and smart display are special- purpose terminals .A POS ( point of scale) terminal records  purchases ,processes credit  crds or debit cards, and updates delivery at a consumer's locations .POS terminals  have card reader ,which reads personals data from the magnetic strip on a credit or debit card.An automated teller machine (ATM) is a self-service banking  machine.

A biometric devices translates personals characteristics into digital code that is compared with a digital code.A face recognitions system captures a live face shape image .A hand geometry system measures tha shapes and  size of a hand . A voice verifications system compares live speech with a stored  voice pattern .An iris recognitions  system  reads  patterns in the  iris of the eye.

Voice recognitions is ideal for visually impaired users. Keyboard with larger keys or an on-screen keyboard displayed on a user's screen also are available. A small trackball or a head-mounted pointer helps users with limited hand movement to control the pointer.