Storage is similar to memory in that both are used to retain information. Storagedevices usemagnetic or optical means to retain data where memory uses an electroniccircuit. 

They are non-volatile, which means they do not lose the information theycontain when they are powered off. They have greater capacity but are muchslower than memory devices. 

Floppy Disks, Hard Disks and Tapes all store information magnetically. CDs andDVDs retain information optically. Storage is used to retain importantinformation like programs (software) and data files. 

Input Devices

Input devices let you enter informationinto a PC. 

The keyboard, mouse and touch-screen display areexamples of input devices.

Input devices get information into the computer by interrupting the CPU fromit's main program, in much the same way that you are interrupted in your dailylife. 

If you are home preparing dinner or watching TV andthe door bell rings you have to stop what you are doing, go to the door, seewho it is, see what they want, take any appropriate action, say good bye andthen go back and pick up where you left off, right? 

When you press a key on the keyboard the same thing happens. An interrupthandler (software routine) says 'Hey CPU, stop whatyou're doing, go over to the keyboard port, read the value of the key theoperator just pushed and store this value in memory location X.' 

The CPU stops, reads the key, stores the key value in location X, and then goesback to doing what it was doing before the interrupt.This is a simple example.Often an interrupt will cause a PC to run a very complex, time consumingroutine to service the interrupt. 

The difference is that you probably get interrupted infrequently compared to aPC, unless you have a really busy job, like a switchboard operator, where itcould be a few times every minute. A CPU can be interrupted hundreds, eventhousands of times per second depending on the environment it is running in. 

Output Devices

Output devices are used to provide usable output for us, as well as statusabout the operation of the PC. 

The video display, printer and speaker are examples of output devices. 

Video Display 

The video display uses data that the CPU puts into a section of memory calledvideo memory. This data is continuously updated and sent to the display at theright rate and in the right order, so that the screen displays images we canread or interpret. 


Most of you have probably used a printer. The PC sends data that you selectsuch as a file or picture to the printer, usually through either a USB orparallel port on the PC. 


Most PCs come standard with external speakers that plug into the sound card.These speakers play back any audio source that your PC can send to them such asmusic, voice and status information (beeps), such as those made during startup. 

Input/Output (I/O) 

There are some ports on the PC that are used to move data out of as well asback into the PC. They are called I/O ports. 

  • Serial Port - The serial port is an I/O port. It connects to external devices that need to communicate with the PC. Scanners and digital cameras are examples of devices that can connect to the serial port. It is called the serial port because only one bit of information passes across it during each transfer. 
  • Parallel Port - The parallel port is also an I/O port that is almost always used for output. More often than not a printer connected to it. It's called a parallel port because it moves a byte (8 bits) of data in parallel during each data transfer. 
  • Universal Serial Bus - The Universal Serial Bus (USB) is a newer higher speed serial I/O port that will eventually replace the serial and parallel ports. It has been appearing in PCs for a relatively short time. Many printers and other external devices are already equipped with USB ports. The USB port on a PC can connect directly to any device with a USB port. It can also connect to a USB Controller which in turn can connect to many devices that have USB ports. 


The words software, code and programs are often usedinterchangeably. They all refers to instruction sequences that run on the hardware of a computer. They can be dividedup into two general areas: operating systems and applicationprograms. 

Operating Systems 

The operating system (OS) is essential code that a PC must have. It is stored on your harddrive. It provides the basic operating platform that brings the PC to a pointwhere it can interpret the keyboard and mouse input and display data and imageson the video display. Other programs can't run on a PC until the OS is loadedand running. DOS, Windows 95 and Windows 98 are examples of operating systems. 

When an OS like Windows 98 has been loaded and is ready to go, the WindowsDesktop appears. All you have to do is point and click on a desktop icon or the Start Button to select anapplication program you want to use and away you go! 

BIOS (Basic Input Output System) 

This is essential code that initializes the hardware in your PC and loads theOS. It can loosely be considered to be a part of the OS. When you first turn onyour PC it is under the control of BIOS code which is stored in a specialmemory chip on the mother board. 

The BIOS stores configuration information about all the hardware in the PC. Thetype of hard drive, floppy drive, video display, CD etc. is kept here so thatthe PC knows how to communicate with these devices. Newer BIOS code has 'auto'modes that can figure out what hardware is attached and how to communicate withit. In older BIOS, if hardware is changed without updating the BIOS the PC maynot complete it's boot up cycle, and will just sit there until the new deviceparameters are loaded into it. 

After BIOS copies the OS code from your hard drive into memory, the copy of theOS in memory takes over and finished the initialization process. When it iscomplete the PC is ready to use application programs. 

Application Programs 

These are workhorses of code. They are the ones you are probably most familiarwith. They run on top of the OS and allow you to use your PC for work or play. 

Word, Excel, Frame Maker, Corel Draw and PC games are a just a few examples ofthe thousands of Application Programs that are available to you.

What happens when your PC is powered on?

When your PC is powered on asequence called the bootstrap is run. It occurs in two steps, Power-On SelfTest and Operating System Load. 

Power-On Self Test (POST) is a series of tests to make certain that necessary is present andproperly operating. 
  1. First the Central Processing Unit (CPU) is reset. 
  2. The CPU checks itself and the POST program stored in the BIOS memory. 
  3. Then the CPU begins using the code located in BIOS to check components in the PC. 
  4. The DRAM is tested by writing and reading back every byte to insure proper operation. 
  5. The Keyboard is checked for attachment and to see if any key has been pressed. 
  6. Pushing certain keys during boot up tells the CPU you want to modify the BIOS code. You would want to do this if a new piece of hardware has been added and you need to set up BIOS to talk to it. 
  7. The Floppy and Hard Drives are checked to make sure they are present. 
  8. The Mouse is also checked to make sure it is attached. 
  9. Finally, the test results are compared to the configuration data in the BIOS. 
  10. Any mismatches causes the POST to fail and an error message appears on the display.

Operating System Load - After the POST is successfully completedthe CPU searches Storage devices for special files that indicate the beginning of the OS and thenloads it into memory. 
  1. The sequence in which the storage devices are searched is part of the configuration set-up stored in BIOS. 
  2. Once the Operating System (OS) is located, the CPU copies what’s called the 'boot record' from the OS into DRAM. 
  3. Control is then passed to the 'boot record' in DRAM, which then continues loading the rest of the OS. 
  4. This process continues until the OS load is completed. 
  5. When the OS load is completed the Desktop appears in the display and waits for you, the user, to tell it what you want it to do.