Storage

Storage is similar to memory in that both are used to retain information. Storagedevices use magnetic or optical means to retain data where memory uses an electronic circuit. 

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

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

Input Devices

Input devices let you enter information into a PC. 

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

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

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

When you press a key on the keyboard the same thing happens. An interrupt handler (software routine) says 'Hey CPU, stop what you're doing, go over to the keyboard port, read the value of the key the operator 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 goes back 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 consuming routine to service the interrupt. 

The difference is that you probably get interrupted infrequently compared to a PC, unless you have a really busy job, like a switchboard operator, where it could be a few times every minute. A CPU can be interrupted hundreds, even thousands 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 status about 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 called video memory. This data is continuously updated and sent to the display at the right rate and in the right order, so that the screen displays images we can read or interpret. 

Printer

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

Speakers

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 as music, voice and status information (beeps), such as those made during start up. 

Input/Output (I/O) 

There are some ports on the PC that are used to move data out of as well as back 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. 

Software

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

Operating Systems 

The operating system (OS) is essential code that a PC must have. It is stored on your hard drive. It provides the basic operating platform that brings the PC to a point where it can interpret the keyboard and mouse input and display data and images on the video display. Other programs can't run on a PC until the OS is loaded and 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 Windows Desktop appears. All you have to do is point and click on a desktop icon or the Start Button to select an application 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 the OS. It can loosely be considered to be a part of the OS. When you first turn on your PC it is under the control of BIOS code which is stored in a special memory chip on the mother board. 

The BIOS stores configuration information about all the hardware in the PC. The type of hard drive, floppy drive, video display, CD etc. is kept here so that the 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 with it. In older BIOS, if hardware is changed without updating the BIOS the PC may not complete it's boot up cycle, and will just sit there until the new device parameters are loaded into it. 

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

Application Programs 

These are workhorses of code. They are the ones you are probably most familiar with. 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 of the thousands of Application Programs that are available to you.

What happens when your PC is powered on?

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

Power-On Self Test (POST) is a series of tests to make certain that necessary is present and properly 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 completed the CPU searches Storage devices for special files that indicate the beginning of the OS and then loads 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.