Performance measures of computer

Computer education point

PERFORMANCE MEASURES: In this section, we consider the important issue of assessing the performance of a computer. In particular, we focus our discussion on a number of performance measures that are used to assess computers. Let us admit at the outset that there are various facets to the performance of a computer. For example, a user of a computer measures its performance based on the time taken to execute a given job (program). On the other hand, a laboratory engineer measures the performance of his system by the total amount of work done in a given time. While the user considers the program execution time a measure for performance, the laboratory engineer considers the throughput a more important measure for performance. A metric for assessing the performance of a computer helps comparing alternative designs.
Performance analysis should help answering questions such as how fast can a program be executed using a given computer? In order to answer such a question,
we need to determine the time taken by a computer to execute a given job. We define the clock cycle time as the time between two consecutive rising (trailing) edges of a periodic clock signal (Fig. 1.1). Clock cycles allow counting unit computations, because the storage of computation results is synchronized with rising (trailing) clock edges. The time required to execute a job by a computer is often expressed in terms of clock cycles.
We denote the number of CPU clock cycles for executing a job to be the cycle count (CC), the cycle time by CT, and the clock frequency by f ¼ 1/CT. The time taken by the CPU to execute a job can be expressed as

CPU time ¼ CC CT ¼ CC=f
It may be easier to count the number of instructions executed in a given program as compared to counting the number of CPU clock cycles needed for executing that

program. Therefore, the average number of clock cycles per instruction (CPI) has been used as an alternate performance measure. The following equation shows how to compute the CPI.

It is known that the instruction set of a given machine consists of a number of instruction categories: ALU (simple assignment and arithmetic and logic instructions), load, store, branch, and so on. In the case that the CPI for each instruction category is known, the overall CPI can be computed as

where Ii is the number of times an instruction of type i is executed in the program and CPIi is the average number of clock cycles needed to execute such instruction.
Example Consider computing the overall CPI for a machine A for which the following performance measures were recorded when executing a set of benchmark programs. Assume that the clock rate of the CPU is 200 MHz.

Assuming the execution of 100 instructions, the overall CPI can be computed as

It should be noted that the CPI reflects the organization and the instruction set architecture of the processor while the instruction count reflects the instruction set architecture and compiler technology used. This shows the degree of interdependence between the two performance parameters. Therefore, it is imperative that both the CPI and the instruction count are considered in assessing the merits of a given computer or equivalently in comparing the performance of two machines.
A different performance measure that has been given a lot of attention in recent years is MIPS (million instructions-per-second (the rate of instruction execution per unit time)), which is defined as

Addressing mode

Autodecrement Mode Similar to the autoincrement, the autodecrement mode uses a register to hold the address of the operand. However, in this case the content of the autodecrement register is first decremented and the new content is used as the effective address of the operand. In order to reflect the fact that the content of the autodecrement register is decremented before accessing the operand, a (2) is included before the indirection parentheses. Consider, for example, the instruction LOAD (Rauto), Ri. This instruction decrements the content of the register Rauto and then uses the new content as the effective address of the operand that is to be loaded into register Ri. Figure 2.11 illustrates the autodecrement addressing mode.
The seven addressing modes presented above are summarized in Table 2.2. In each case, the table shows the name of the addressing mode, its definition, and a generic example illustrating the use of such mode.
In presenting the different addressing modes we have used the load instruction
for illustration. However, it should be understood that there are other types of instructions in a given machine. In the following section we elaborate on the different types of instructions that typically constitute the instruction set of a given
machine.

Computer structure

The computer interacts in some fashion with its external environment. In general, all of its linkages to the external environment can be classified as peripheral devices or communication lines.We will have something to say about both types of linkages.

But of greater concern in this book ismthe internal structure of the computer itself, which is shown in Figure 1.4.There are four main structural components:

• Central processing unit (CPU): Controls the operation of the computer and performs its data processing functions; often simply referred to as processor.
• Main memory: Stores data.
• I/O: Moves data between the computer and its external environment.
• System interconnection: Some mechanism that provides for communication among CPU, main memory, and I/O. A common example of system

interconnection is by means of a system bus, consisting of a number of conducting
wires to which all the other components attach. There may be one or more of each of the aforementioned components.Traditionally, there has been just a single processor. In recent years, there has been increasing use of multiple processors in a single computer. Some design issues relating to multiple processors crop up and are discussed as the text proceeds; Part Five focuses on such computers.
Each of these components will be examined in some detail in Part Two. However, for our purposes, the most interesting and in some ways the most complex component is the CPU. Its major structural components are as follows:

• Control unit: Controls the operation of the CPU and hence the computer
• Arithmetic and logic unit (ALU): Performs the computer’s data processing functions
• Registers: Provides storage internal to the CPU
• CPU interconnection: Some mechanism that provides for communication among the control unit,ALU, and registers

Each of these components will be examined in some detail in Part Three, where we will see that complexity is added by the use of parallel and pipelined organizational techniques. Finally, there are several approaches to the implementation of the control unit; one common approach is a microprogrammed implementation. In essence, a microprogrammed control unit operates by executing microinstructions that define the functionality of the control unit.With this approach, the structure of the control unit can be depicted, as in Figure 1.4.This structure will be examined in Part Four.

Vonneumann model of computer architecture

VONNEUMANN MODEL The von Neumann model of computer architecture wasfirst described in 1946 in the famous paper by Burks, Goldstein, and vonNeumann (1946). A number of very early computers or computerlike devices hadbeen built, starting with the work of Charles Babbage, but the simple structureof a stored‑program computer was first described in thislandmark paper. The authors pointed out that instructions and data consist ofbits with no distinguishing characteristics. Thus a common memory can be usedto store both instructions and data. The differentiationbetween these two is made by the accessing mechanism and context; the programcounter accesses instructions while the effective address register accessesdata. If by some chance, such as a programming error, instructions and data areexchanged in memory, the performance of the program is indeterminate. Beforevon Neumann posited the single address space architecture, a number ofcomputers were built that had disjoint instruction and data memories. One ofthese machines was built by Howard Aiken at Harvard University, leading to thisdesign style being called a Harvard architecture. A variation onthe von Neumann architecture that is widely used for implementing calculatorstoday is called a tagged architecture. With these machines, each data type inmemory has an associated tag that describes the data type: instruction,floating-point value (engineering notation), integer, etc. When the calculatoris commanded to add a floating‑point number to aninteger, the tags are compared; the integer is converted tofloating point, the addition is performed, and the result is displayed infloating point. You can try this yourself with your scientific calculator. Memory Thecomputer will have memory that can hold both data and also the programprocessing that data. In modern computers this memory is RAM. Image result for von neumann architecture images ControlUnit Thecontrol unit will manage the process of moving data and program into and out ofmemory and also deal with carrying out (executing) program instructions - oneat a time. This includes the idea of a 'register' to hold intermediate values.In the illustration above, the 'accumulator' is one such register. Image result for von neumann architecture images Input- Output Thisarchitecture allows for the idea that a person needs to interact with themachine. Whatever values that are passed to and forth are stored once again insome internal registers. Image result for von neumann architecture images ArithmeticLogic Unit Thispart of the architecture is solely involved with carrying out calculations uponthe data. All the usual Add, Multiply, Divide and Subtract calculations will beavailable but also data comparisons such as 'Greater Than', 'Less Than', 'EqualTo' will be available. Bus Noticethe arrows between components? This implies that information should flowbetween various parts of the computer. In a modern computer built to the VonNeumann architecture, information passes back and forth along a 'bus'. Thereare buses to identify locations in memory - an 'address bus' Image result for von neumann architecture images

Boot process

The last step in the PC start-up is reading the operating system. The start-up program is instructed to find the Master boot sector. The boot sector is the very first sector on either hard disk (C) or floppy drive A.

By default, the PC will look for a boot sector in floppy drive A. That is why the PC "drops dead" if there is a different diskette in A drive. If there is no diskette in A drive, the start-up program will search for the boot sector on hard drive C. When the boot sector is found, a small program segment (boot-strap) is read from there. The boot-strap then takes over control of the PC. The start-up program has done its job. Now DOS, Windows, or another operating system takes control.

Here is an illustration of the start-up process:

Bits and digital data

Each 0 or 1 is called a bit. Bit is an abbreviation of the expression BInary digiT. It is called binary, since it is derived from the binary number system

The binary number system

The binary number system is made up of digits, just like our common decimal system (10 digit system). But, while the decimal system uses digits 0 through 9, the binary system only uses digits 0 and 1.

If you are interested in understanding the binary number system, then here is a brief course. Try if you can follow the system. See how numbers are constructed in the binary system, using only 0's and 1's:

 

Digital data

We have seen that the PC appears capable of handling data, if it can receive them as 0's and 1's. This data format is called digital. If we can translate our daily data from their analog format to digital format, they will appear as chains of 0's and 1's, then the PC can handle them.

So, we must be able to digitize our data. Pour text, sounds, and pictures into a funnel, from where they emerge as 0's and 1's:

The most basic data processing is word processing. Let us use that as an example. When we do word processing, we work at a keyboard similar to a typewriter. There are 101 keys, where we find the entire alphabet A, B, C, etc. We also find the digits from 0 to 9 and all the other characters we need:,.-;():_?!"#*%&etc.

All these characters must be digitized. They must be expressed in 0's and 1's. Bits are organized in groups of 8. A group of 8 bits is called a byte.

8 bits = 1 byte, that is the system. Then, what can we do with bytes? First, let us see how many different bytes we can construct. A byte is an 8 digit number. We link 0's and 1's in a pattern. How many different ones can we make? Here is one: 01110101, and here is another: 10010101.

We can calculate that you can make 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 different patterns, since each of the 8 bits can have 2 values.

 2⁸ (two in the power of eight) is 256. Then there are 256 different bytes. Now we assign a byte to each letter and other characters. And since we have 256 patterns to choose from, there is plenty of room for all. Here you see some examples of the "translation:"

When you write the word "summer", you write 6 letters. If the computer has to process that word, it will be digitized to 6 bytes. In other words, the word summer occupies 6 bytes in the PC RAM, when you type it, and 6 bytes on the hard disk, if you save it

.ASCII means American Standard Code for Information Interchange. It is an industry standard, which assigns letters, numbers, and other characters within the 256 slots available in the 8 bit code.

The ASCII table is divided in 3 sections:

•  Non printable system codes between 0 and 31.
• "Lower ASCII" between 32 and 127. This part of the table originates from older, American ADP systems, which work d on 7 bit character tables. Foreign letters, like Ø and Ü were not available then.
• "Higher ASCII" between 128 and 255. This part is programmable, in that you can exchange characters, based on which language you want to write in. Foreign letters are placed in this part.

Let us imagine a stream of bits sent from the keyboard to the computer. When you type, streams of 8 bits are sent to the computer. Let us look at a series of bits:

001100010011001000110011

Bits are combined into bytes (each 8 bits). These 24 bits are interpreted as three bytes. Let us read them as bytes:

00110001,
00110010, and 00110011.

When we convert these byte binary numbers to decimal numbers, you will see that they read as 49, 50, and 51 in decimal numbers. To interpret these numbers, we have to look at the ASCII table. You will find that you have typed the numbers 1, 2, and 3.

Storage hardware

STORAGE HARDWARE
A limitation of any CPU is the size of its primary storage, also called main memory. The retention of programs and data reduces the amount of primary storage available for processing.
The secondary storage or auxiliary storage, is permanent storage, which is separate from the CPU. This storage is non-volatile. The contents still exist after the power is turned off.

BENEFITS OF SECONDARY STORAGE

Economy
It is less expensive to store data on magnetic tape or disk, the principle means of secondary storage, than in filling cabinets. Optical storage media are also relatively low in cost. In expensive storage space permits the storage of vast amount of data. With increased accuracy in filling and retrieving if.

Reliability
Data in secondary storage is basically safe, since secondary storage is physically reliable. Also, the data in it is stored in such a way that data tampering is difficult. 

Convenience
With the help of a computer, authorized people can locate and access data quickly

MAGNETIC STORAGE
The three most common storage devices floppy disk drives, hard disk drives and tape drives all uses similar techniques for reading and writing data, because all use the same medium (the material on which the data are stored)

EXAMPLE OF MAGNETIC STORAGE
i)   Floppy Disk
ii)  Hard Disk
iii) Magnetic Tape

OPTICAL STORAGE
Optical disk coated with plastic that can store digital data as tiny pits etched in the surface; is read with a laser that scans the surface

EXAMPLE OF OPTICAL STORAGE
i)    CD-ROM Disk
ii)   CD- R Disk iii)  CD-RW Disk
iv)  DVD ROM Disk

Floppy Disks
A floppy disk also called a disk or diskette is a removable storage disk used for storing data. It is called a floppy disk because of the round film inside the disk’s plastic shell is flexible (floppy). You can use a floppy to store, move data easily from one PC to another. You can make copy of any information from hard disk on to a floppy.

TYPES OF FLOPPY DISK AND THEIR PARTS
During the early 1990’s floppies were available in three sizes (identified by diameter) with varying storage capacities. They were.

i)     8   Inches
ii)  5.25 Inches
iii)  3.5 Inches

Today the 3 ½ - inch diskette completely replaced 5 ¼ and 8 - inch diskette. 3 ½ - inch diskette comes in three capacities double, high and extra high density. The higher the density the more closely the iron oxide particles are packed.

The  3 ½- inch Floppy Disks
They incased in a hard plastic jackets which makes it secure. Though it does not feel “floppy” it is still called  floppy disk. The 3 ½-inch floppy disk has a few holes and notches

HUB
The HUB is the metallic disc having a small hole that allows the disk inside the protective jacket to be centered in the disk drive for proper rotation.

Data Access
The data access area has a metal plate or shutter that protects the read/write opening. When the disk is inserted in the disk drive, the shutter moves aside. This protective slide means there is n need to keep the disk in a paper envelop when unused. <br>

Write /Protect Notch
The hole is lower right corner on the back is write/protect notch. When the slide covers the hole, it can be read from and written to. When slide does not cover the hole, the contents are write-protected. They cannot
be erased, altered.

High Density Disk Indicator
Some disks have 4 holes in the opposite corner from the write\protect notch. This indicated that the disk is a high density disk MB rather than 720 KB.

Stress Relief Notches
The two small notches besides the data access area are stress relief notches. They allow you to bend the disk without damaging it.

Sectors
Each track on a disk is divided into ple-wedged-shaped sections known as sectors. These sectors are used by the computer system for storage reference purposes. Disks are identified as being either soft sectored or
hard sectored
Soft sectored disks are marked magnetically by the user computer system during formatting that is, the sectors are not already defined on the disks.
Hard sectored disk are fixed by the disk manufacturer and always have same number and size of sectors. Hard sectored disk cannot be used in disk drive designed for soft sectored disks, nor the reverse.

Sides
The early disks were either single sided or double sided. A single sided disk stores data on one side. A double sided disk stores data on both sides. The standard 3 ½ inch floppy disks are double sided, holding twice the
data. To use a double sided disk, you must have a computer with a double sided floppy disk drive. These drives are equipped with read\write head for both the top and the bottom surface of the disk.

Densities
Floppy disks may be double density, high density or extra high density. Density here means recording density, the numbers of bits per inch (Bpi) of data that can be written onto the surface of a disk. The more tracks, the
more sectors the more sides, and the higher density that the disks has, the more data it can hold a standard 3 ½ inch disks for IBM micro computer has 512 per track. At 512 bytes per sectors, 18 sectors per track, 80
tracks per disk side and two sided per disk, this equals 1474560 bytes or 1.44 megabytes (1.44 MB). Mathematically, the capacity of disk is calculated as .

1 track = 18 sectors
80 tracks = (18x80) 1440 sectors
Double sided, 80 tracks = 1440x2 = 2880 sectors But 1 sector = 512 bytes
Double sided, 80 tracks = 2880x 512 = 1474560
But 1024 bytes = 1 KB :.
Double sided, 80 tracks = 1474560 / 1024   = 1.44 MB

Care of Diskettes
a)  Keep floppies in their protective covers anytime you are not using them. b)  Avoid bending them.
c)  Insert them into disk drives carefully.
d) Do not touch the area of the diskette that is covered by metallic shutter.
e) Do not expose floppies to sunlight, high temperature or strong magnetic field.

HARD DISK
Hard disk is also called a fixed disk, Hard disk is the basic storage device for all computers. Since a hard disk stores so much data , it is sometimes called a mass storage device along with the tape. A hard disk contains
one or several rigid platters on which data may be recorded. There may be as many as 12 platters in a unit,
all connected to a central spindle or shaft. Unlike floppy disks, where the disk and drive are separate the hard disk drive or hard drive is the whole unit. It includes the hard drive, is the whole unit. It includes the hard
disk, the motor that spins the platters, and a set of read/write heads (one for each platter. You generally cannot remove the hard disk from its drive the two terms are used interchangeable to mean the whole unit,
both disk and drive.

A faster technology is that of fixed hard disks. These devices have fixed access arms with separate read \ write mechanisms for each track of the disks. The disks, rather than the head, whirls to bring the correct
sector to the arm; the read \ write mechanism for the correct track then retrieves or output the data, depending on the application. Other disk devices combine the technologies of both moving and fixed head
access to produce a high capacity rapid access device.

The breakdown for a disk that is sold as 50 GB disk: :.  12 Platter x 2 Side = 22 Platters
12024 cylinders x 22 heads (side) = 264528.
264528 tracks x 369 sectors / tracks = 97610832 sectors.  :. But 1 sector = 512 bytes
97610832 sectors x 512 bytes / sectors = 49976745984 bytes

MAGNETIC TAPE
With hard disks capable of storing large amounts of data, the loss of data due to a failure of the hard disk itself could be disastrous. It becomes critical to keep a backup copy of data on another secondary storage device.
Micro computer systems are generally strictly disk based and use diskettes backups. However since a diskette can hold only a megabyte or so, about three thousand diskettes would be needed to back up a 4.3 Giga byte hard disk. This approach is expensive, awkward and slow. The amount of data on a tape is expressed in terms of density, which is the numbers of characters per inch (epi) or bytes per inch (bpi) that can be stored on the tape. 

Advantages of Magnetic Disks
a) Less expensive
b) Ideal for sequential processing <l
c) Generally used for backup.

Disadvantages of Magnetic Disks
a) Update requires new tape
b) Slow access to records
c) Not suitable for on line interactive processing. '
d) Records must be retrieved in sequence

Advantages of Magnetic Tape
a) Fast access to records
b) Direct updating can be preferred

Disadvantages of Magnetic Tape
a) More expensive
b) Read\write heads can crash and destroy files
c) Separate back up procedure required.

OPTICAL STORAGE DEVICES
The need forever greater storage capacities drive hardware manufacturers to improve the available technologies and to develop alternative storage media. Today, optical storage is the main alternative to magnetic storage.

Optical storage techniques make use of the laser beams to write and read data which can consists of text, graphics, audio clips, or video and images at densities many times finer than those of a typical magnetic disk.
A single optical disk of the type called CD ROM can hold up to about 700 megabytes of data.

CD-ROM
CD ROM  which stands for compact disk read only memory  it’s an optical disk format that is used to hold pre recorded text, graphics, sounds and videos. Data on disk is imprinted by the disk manufacturers and cannot be altered or erased by the user. Read only mean that data can be retrieved but not but not altered or erased by the users. This type of optical disks is used to store huge volume of data that rarely change. For example dictionaries, encyclopedias, medical legal or other professional references libraries, music and video all required tremendous amount of data that you would not normally want to alter. In addition to these uses, many software companies distribute their products on CD-ROM.

CD-R DISK <br>

The tremendous capacities of CDs have compelled hardware manufacturers to work hard to develop recordable optical devices. The first ventures into developing a recordable optical technology resulted in the
CD-R (Compact Disk-Recordable) disk. A CD-R disk can be written, or recoded onto and then cannot be erased or altered; it can be read many times by using and CR ROM drive. This allows consumers to make their own
CD disks, though it is slow process (recording a full disk takes 20-60 minutes).

CD-RW DISKS
CD-RW (Compact Disk Re- writeable) disks. Are both readable and writable; their contents can be erased or altered by the user. These in effect resemble magnetic disks, although they store huge amount of data. The
CD RW drives and software are required. 

DVD-ROM
The acronym DVD ROM (for digital versatile disk read only memory) refers to a relatively new high capacity CD storage format that was initially developed to store the full contents of the standard three hour movie.
The acronym DVD origionally stored for digital video disk. Proponents of the technology later felt that the word video was too limiting. The disks can also store text, graphics and audio data so the longer name is
often dropped.

Memory

Main Memory or RAM
RAM or Random access memory described earlier as main memory or primary storage is used to store programs and data that the CPU is presently in processing. The reason it is called “random access” memory is that data can be retrieved at random from anywhere in a RAM chips equal amounts of time. RAM is volatile memory

Volatile Memory
It means that data stored in RAM is lost when computer is switched off that is, it is temporary storage.<br>
RAM chips are often mounted on a small circuit board, such as SIMM or DIMM which is plugged into the Motherboard.

SIMM
Single in line memory module has Ram Chips only on one side. The SIMM was used with computers using a 486, early Intel Pentium, and compatible processors. SIMM is only 32-bits wide, they must be installed two at
a time when used with any 64-bit processor.

DIMM
Dual inline memory module has RAM chips on both sides. Dual In-line Memory Module, DIMM is a circuit board that holds memory chips. DIMMs have a 64-bit path because of the Pentium Processor requirements.
Because of the new bit path, DIMMs can be installed one at a time

DRAM
(Dynamic Random Access Memory) chips used for most main memory. DRAM stores its information in a cell containing a capacitor and transistor; because of this design, these cells must be refreshed with new
electricity every few milliseconds allowing the memory to keep its charge and hold the data as long as need

SRAM
(Static Random Access Memory) chips, used for some specialized purpose within main memory. Static RAM, SRAM is computer memory that requires a constant power flow in order to hold information. Power
consumption varies widely based on how frequently the memory is accessed. It is commonly only used in cache and video card memory.

ROM
Read only memory is memory from which data may retrieve, but the contents cannot be change by the user. They store instructions in permanent, or non-volatile, form. Non-volatile storage does not lose its contents when the power is turned off. Variation of ROM Chip

PROM
Programmable Read Only Memory. It is also a non-volatile storage.

EPROM
Erasable Programmable Read Only Memory. PROM cannot be changed or altered and EPROM when in use information can only be read and remain in the chip until it is erased.    

EEPROM
Electrically Erasable Programmable Read Only Memory. EEPROM chip can be erased, either within a computer or extrenally, by electric power. the process usally requires more voltage than the common +5 volts used in login circuits. 

other forms of memory
Moving data between R M and the CPU’s registers is one of the most time consuming operation a CPU must  perform, simply because RAM is much slower than the CPU. A partial problem to this solution is to include a cache memory on the micro processor chip or elsewhere on motherboard. Cache (pronounced “Cash”) is  similar to RAM except that it is extremely fast compared to normal memory and is used in a different way.

VIRTUAL MEMORY
It is a condition in which part of a program is stored on disk and is brought into memory only as needed. This features which requires, special software, uses the disk as an extension of RAM. The virtual memory program
puts as many piece of the program into RAM as possible.

VIDEO MEMORY
V(RAM) chips are used to store display images for the monitor. The amount of video memory determines how fast images appear and how many colures are available. Video memory chips are particularly desirable if you are running programs that display a lot of graphics.

FLASH MEMORY
Flash memory is non-volatile memory. That is, it retains data even when the power is turned off flesh memory can be used not only to simulate main memory but also supplement or replace hard disk drives for permanent
storage.

Pointing device

Pointing Device
The kind of direct entry input device known as pointing device.

MOUSE
A mouse is a small, hand-operated device that is connected by cable to the computer and that moves the cursor on the display screens. As you move the move  the mouse around the desktop, the cursor moves across the screen. When the cursor reaches your desired location, you may push a button on the mouse to signal a command to the computer.

>TRACK BALL
A track ball is pointer device resembling a mouse but is stationary and is guided by the fingers intead of rolled on the desktop. Some computers have built-in track balls. Track balls may also appear on the side of computer keyboard. These devices are useful in situations where there is no room to move a mouse, as in the airplanes.

JOYSTICK
A joystick usually used to play games on computer. It is a device that controls the movement of certain object on the screen. The buttons on the joystick allow the users to perform different actions on the nature of the program.

TOUCH SCREEN
A Touch screen is a special type of computer display screen that allows commands to be entered by the users touching screen. Labeled boxes on the screen display choices available to the users.

LIGHT PEN
The light pen consists of a light-sensitive pen like device that enters commands into a special display screen when the users touch the screen with pen. Light pens are frequently used by graphic designers and drafting engineers.

DIGITISING TABLET
A digitizing tablet, or digitizer, consists of a flat drawing surface and a pointing tool, which creates images that are converted to computers usable form. The tablet which is covered by a grid of tiny wires, is connected by cable to the computers

PEN BASED COMPUTER
Pen based computers allow users to write on the screen of the computer, which than interprets the hand-writing and displays the result on the screen in printed form. Users can edit what they have entered and give commands by circling words, checking boxes, and using symbols developed by the manufacturer.

File

Understanding File Types in Windows

Successive versions of windows have reduced the amount of information displayed to the user. This is microsoft's attempt at making your PC easier to use, but for many it just makes things more confusing. You can reveal some of the most important hidden things and may well find it helpful.

Just a bit of history first so you can understand what we are about to reveal. Files used to be named according to the convention 8.3, that is 8 letters and a dot and 3 letters. Maybe

- myletter.doc

- somecalcs.xls

- myhold.jpg , etc.

The first (up to) 8 are picked by the user to describe the file contents. The 3 are used by the operating system (eg, windows) to note what type of file it is. Some common file types are;

.doc --Microsoft Word File

.xls --Microsoft Excel File

.exe --Executable File - one that starts up a software application, for instance

.txt --A plain text file, can be read by Word, Notepad, Wordpad and many others

.bmp --A picture file created with paint

.jpg --A compressed picture file

.wav --A sound file

.mp3 --A compressed sound file , etc.

This was quite restrictive, with only 8 characters available to describe the file contents, and in windows 98 onwards you can use 255 characters to name your file.

Hardware

Hardware refers to the physical components of a data processing system. Input, storage, processing and control devices are hardware such as Hard disk, Printer, Keyboard etc. Without any hardware your computer would not exist and software would have nothing to run on. Computer software is something that executes within the hardware.

The hardware of a computer is infrequently changed, in comparison with software and data. Hardware professionals deal with the manufacturing and maintenance of computers.

Many manufacturers mass produce computer systems and sell them either through direct marketing or retail chains.

There are also a number of vendors that can custom assemble computer systems to the end-user's specifications. It is also possible to purchase the individual parts and component of a computer and build it.

Some of the items to consider when purchasing a computer include:

• The motherboard 
• Processor 
• RAM 
• Storage 
• Adapter cards
• Power options