A+ Hardware by nuhman10


									                                   COMPUTER FUNDAMENTALS
                Computer is an electronic machine. Which take input in the form of the data and
instructions through input unit. The input goes to its processing unit. Where data process
according to instruction and give output to output unit.

                              INPUT                PROCESSING                    OUTPUT
                               UNIT                   UNIT                        UNIT

                         i.    Key Board
                        ii.    Mouse

                         i.    Monitor
                        ii.    Printer

                         i.    Mother board
                        ii.    Processor
                       iii.    Memories
                       iv.     Casing

     Facts and figures are called data.

          The command which process the data called instruction.

                         i.    Text
                        ii.    Pictures
                       iii.    Numbers
                       iv.     Audio
                        v.     Video

         Physical components of computer system are called hardware.

         The programs like (windows, Ms Word, Ms Excel) called software.

                                  i.   OPERATING SYSTEM:
                                       That software’s through which user can operate the
                                       computer system like (Dos, Windows)
                                 ii.   APPLICTOIN SOFTWARE’S:
                                       Those software’s in which user can create or run any
                                       application called application software’s like (Ms Word,
                                       Ms Excel, paint, internet etc)

             Memories are use to store data. There are two types of memories.

                                   i.   INTERNAL MEMORIES
                                  ii.   EXTERNAL MEMORIES

                                  INTERNAL MEMORIES

                       RAM                                          ROM

                            EXTERNAL MEMORIES

                                                     Drives of Computer System

       Ram stand for (Random Access Memory)

                i.   Currently executed data store in ram.
               ii.   It is a temporary memory.
              iii.   It is a volatile memory.
              iv.    Random Access means to store and read without any sequence which
                     make it faster.
               v.    Physically ram is a module of ICs.

       Rom stand for (read only memory)

                i.   It is use to store personal data of computer system.
               ii.   It is permanent memory
              iii.   It is a nonvolatile memory
              iv.    Physically it is a IC placed on mother board

                            All the drives of computer system called external memories.

                             1. Floppy Drive
                             2. Hard Disk Drive
                             3. Cd Rom Drive

                     Use to store data and data transportation.

               Use to store data and software.

                      Use to store data, data transportation and installation of software.

                                           1. Floppy Drive ( A,B )
                                           2. Hard Disk Drive (C,D,E,F)
                                           3. Cd Rom Drive (G)

                              Basic unit of memory is called Bit (binary digit)

                              8bits            =       1 byte
                              1024 bytes       =      1KB
                              1024 KB          =      1 MB
                              1024 MB          =      1 GB
                              1024 GB          =      1 TB

        File contains data. Data may be text, pictures, numbers, audio and video.
File having two parts.

                             (i)      File name      (ii) File extension


                                              File name represents its name
                                              File extension represents its type


        1. SYSTEM FILES:
              System files run the operating system. The extension of system files is (sys)
              For Example:
              Ms Dos run through (MSDOS. SYS and IO.SYS files)
              Program files run the application software on operating system. The extension of
              the program file is (EXE or COM)
              For Example:
              Ms Word runs through (winword.exe)
              Paint runs through (Ms Paint.exe)
              Command prompt runs through (Command .com)

        3. USER DATA FILES:
              These files are created by the user in any application software. The extension of
              these files changed program to program.
              For Example:
              Ms Word______ doc
              Excel ______ xls
              Paint     ______ Bmp

    Folder is a portion which contains files & sub folders. The folder having only its name.

               In (CUI) user can operate the computer system through commands.

                    In (GUI) user can operate the computer system through graphics.

1       Windows XP Screen layout Open close the programs
        Minimize, Maximize
        Short cut keys
2       Introduction to My Computer Icon
        Checking the properties of different drives
        Create Folders & files
        Checking the size and extension of files
        Copy Files and Folders from one location to other location
        Search the files & folders


              Dos run through the following files.

                   i. Command. Com
                  ii. Io.sys
                 iii. Ms Dos.sys

Dos interface is called DOS prompt

In Dos user operate the computer through Dos commands. There are two categories of
                 i.   DOS INTERNAL COMMANDS
                ii. DOS EXTERNAL COMMANDS

     Internal Dos commands load through command.com file in computer memory.

                a)   DIR                (display the list of files and directory)
                b)   CLS                (clear the last screen)
                c)   CD                 (change directory)
                d)   MD                 (make directory)
                e)   RD                 (remove directory)
                f)   DATE               (display and change the current date)
                g)   TIME               (display and change the current time)
                h)   VOL                (display the volume label of the drive)
                i)   VER                (display the version of the O/S)
                j)   COPY               (copy files from one location to other location)

    External Dos commands run through their executable files.

                a)   Label              (display and change the volume label of drive )
                                        Run through Label.EXE file

                b)   Diskcopy           (Make a copy of Floppy disk to Floppy Disk )
                                        Run through diskcopy.EXE file

                c)   XCOPY              (Copy Directories, sub directories and files from one
                                        location to other location)
                                        Run through xcopy.exe file
                d)   SYS                (Copy DOS bootable files from one drive to other
                                        Run through sys.com file

                e)   Format             (Formatting of HDD or Floppy Disk)
                                        Run through format.com file

                f)   DATE               (display and change the current date)
                g)   TIME               (display and change the current time)
                h)   VOL                (display the volume label of the drive)
                i)   VER                (display the version of the O/S)

                Motherboard is main circuit of computer system. All the internal and external
hardware like microprocessor, RAM, ROM, Drives, Key Board, Mouse, Add in card, Printer
etc.are directly attached on mother board.


Following components found on a typical motherboard:

          Expansion slots
          Memory slots
          Processor slots or sockets
          Power connectors
          On-board disk drive connectors
          Keyboard connector
          Peripheral port connectors
          BIOS chip
          CMOS battery
          Jumpers and DIP switches

      Expansion slots are use to install add-in card like display card, sound card, module,
network etc. As their name suggests, they are used to install various devices in the
computer to expand its capabilities.


        ISA SLOT (Industry Standard Architecture): -
        These slots are used for add-in cards like display card, sound card, modem, network card
etc. These expansion slots are available on 486,P-1, P-2 & P-3 mother boards.
        PCI SLOT (Peripheral Component Interface):
        These slots are used for add-in cards like display card, sound card, modem, network card,
TV card. . These expansion slots are available on 486,P-1, P-2, P-3 & P-4 mother boards.

       AGP SLOT (Accelerated Graphics Ports): -
                                    This slot is used only for display card. A mother board
having only on AGP port available on P-2, P-3, & P-4 mother boards.


    Communication ports provide interface for external devices like Mouse, Printer, External
MODAM, and any other external hardware.


       SERIAL PORT:-
            There are two types of serial ports available on mother boards
            1. COM Ports
            2. USB (Universal Serial Bus) Ports

            COM 1-COM 2
                 Both ports having 10 pins on motherboard and 9 pines on cable port.
                 Mouse & External MODAM are connected with these ports.

            USB Ports
                  USB ports are used for USB Devices like printer, scanner, keyboard,
                  mouse, Web camera etc
                  A mother board having one or more USB ports and single USB port can
                  support 127 devices at once through USB hub.

          LPT 1
                This port having 26 pins on mother board and 25 holes on cable port
                Printer & scanner are connected with these ports.
                This port is Labeled with Printer, LPT, and Prn.


Processor socket is use to install microprocessor chip on motherboard.

          1- Socket 3 (For 486 processor)
          2- Socket 7 (For P-1 processor)
          3- Slot 1 (for P-2 & P-3 processor)
          4-Socket 370 (For P-3 & celeron Processor)
          5-Socket 478 ( For P-4 Processors)


              IDE (Integrated Drive Electronics)
              HDD Data cable connectors are use to attached the HDD or CDROM Drive on

            A mother board having two IDE controllers
            1- Primary (IDE)               2- Secondary (IDE)
               IDE1                           IDE2
     A single IDE controller can support two drives (HDD or CDROM drive)
     So a mother board can support Four IDE drives. These are:

        1.    Primary Master
        2.    Primary Slave
        3.    Secondary Master
        4.    Secondary Slave

Power Connector:

              Power connectors are used for powers cables on mother board


   1.   AT Connector                        (used on 486 and P-1 mother boars
   2.   ATX P-3 Connector                   (used on P-1, P-2, P-3 mother boards)
   3.   ATX P-4 Connectors                  (used on P-4 mother boards)
   4.   ATX P-4 (24 Pins) Connectors        (used on P-4 mother boards)

Keyboard Connectors

The most important input device for a PC is the keyboard. All PC motherboards
contain a connector (as shown in Figure) that allows a keyboard to be connected
directly to the motherboard through the case. There are two main types of
keyboard connectors: AT and PS/2. The AT connector is round, about 1/2" in
diameter, and has 5 sockets in the DIN-5 configuration.

The second style, the PS/2 connector (as shown in Figure), is smaller and is more
ubiquitous than the AT connector. Most new PCs that you can purchase today
contain a PS/2 keyboard connector on the motherboard.


The most important chip on the motherboard is the basic input/output system
(BIOS) chip. This chip is a special memory chip that contains the BIOS software
that tells the processor how to interact with the rest of the hardware in the

CMOS Battery

Your PC has to keep certain settings when it’s turned off and its power cord
unplugged. Some of these settings include:

          Date
          Time
          Hard drive configuration
          Memory

Your PC keeps these settings in a special memory chip called the Complimentary
Metal-Oxide Semiconductor (CMOS) chip. Actually, CMOS (usually pronounced
see-moss) is a type of memory chip.
To keep its settings, the memory has to have power constantly. To prevent CMOS
from losing its information (and it’s rather important that it doesn’t, actually),

motherboard manufacturers include a small battery, called the CMOS battery, to
power the CMOS memory. The battery makes comes in different shapes and sizes,
but they all perform the same function.


The role of the CPU, or central processing unit, is to control and direct all the
activities of the computer using both external and internal buses. It is a processor
chip consisting of an array of millions of transistors.

CPU Manufacturers

With the computer industry being as profitable as it is, there are many companies
making CPUs for PCs today. The market leader in the manufacture of chips is Intel
Corporation. Intel’s competition includes Motorola, Advanced Micro Devices
(AMD), Cyrix, and IBM. When it first started making CPUs for the IBM PC, Intel
shared its designs with other manufacturers, but with the introduction of the 80386
model in 1985, Intel ceased licensing its designs to other manufacturers.

Processor Performance Issues

Don’t forget, there’s more to a chip than what meets the eye. There are several
factors that affect the performance of a processor. Among them are availability of a
math coprocessor, clock speed, internal cache memory, and supporting circuitry.

Math Coprocessor

The math coprocessor is used to improve the processor’s number-crunching speed.
It does not, however, increase the speed of simple additions and subtractions. What
it does is increase the speed of calculations that involve floating decimal point
operations (such as calculations for algebra and statistics). Since the introduction
of the 486, the math coprocessor has been built into the processor. CPU models
that preceded the 486 can add a math coprocessor as an option. (There is a special
slot for it next to the CPU.)

Clock Speed

The clock speed is the frequency with which a processor executes instructions.
This frequency is measured in millions of cycles per second, or megahertz (MHz).
There is actually a “clock” of sorts within the CPU. This clock signal is generated
by a quartz crystal, which vibrates as electricity passes through it, thereby
generating a steady pulse to every component synchronized with the signal. A
system cycle is generated by this pulse (called a clock “tick”), which sends a signal
through the processor telling it to perform another operation. To transfer data to
and from memory, an 8086 computer needed four cycles plus “wait states.” Wait
states allow the processor to wait for the slower speed RAM that was used in 8086-
based computers. Generally speaking, the higher the MHz value, the faster the PC
will be.

Cache memory

Cache memory is a storage area for frequently used data and instructions. It
requires a small amount of physical RAM that can keep up with the processor. It
uses this RAM for storage. The processor contains an internal cache controller that
integrates the cache with the CPU. The controller stores frequently accessed RAM

locations to provide faster execution of data and instructions. This type of cache is
known as a Level 1 Cache. It is also possible to have a cache external to the CPU,
called a Level 2 Cache. This type of cache performs the same functions as a Level
1 Cache and can speed up the perceived performance. Basically, a larger cache
leads to the perception of a faster CPU.

The Bus

Finally, the processor’s ability to communicate with the rest of the system’s
components relies on the supporting circuitry. The system board’s underlying
circuitry is called the bus. Although this is not the bus used to get to the mall or the
football game, the idea is similar: the computer’s bus moves information into and
out of the processor and other devices. A bus allows all devices to communicate
with each other. The bus consists of several components, including the external
bus, the data bus, and the address bus.

The External Bus (System Bus)

The external bus is also referred to as the system bus or expansion bus. The
expansion bus is a bus system that allows the processor to talk to another device. It
is known as an external bus system because it is outside of the processor. The
devices are connected through expansion cards and slots. (An expansion card is a
removable circuit board that expands the capability of the computer.) We will
cover bus types in more detail in Chapter 5, “PC Bus Architectures.”

The Data Bus

The data bus is used to send and receive data. The larger the bus width, the more
data that can be transmitted (and, therefore, the faster the bus). The data bus and
address bus are independent of each other, but for better performance larger data
buses require larger address buses. The data bus width indicates how much data the
chip can move through at one time, and the size of the address bus indicates how
much memory a chip can handle. Data in a computer is transferred digitally. A
single wire carries 5 volts to indicate a 1 data bit or carries zero volts to indicate a
0 data bit. Remember, computers use the binary system to transmit information.
The greater number of wires allows more bits to be transmitted at the same time.
For example, a 16-bit data bus width has 16 wires to transmit data, and a 32-bit
data chip can transmit twice the amount of data as a 16-bit chip. A good
comparison would be to the highway system. A single lane for traffic allows only
one car through at a time whereas two lanes allow twice the amount of traffic to
pass through at one time. Be careful not to assume that data flows at the same
speed within the processor as it does outside of the processor. Some Intel
processors have an internal data bus that is greater than the external data bus. The
8088 and 803 86SX are good examples of this. They are designed with an internal
bus that has twice the width of the external bus. This provides for backward
compatibility with other processors that do not support a wider bus.

The Address Bus

The address bus also contains a set of wires to carry information in and out of the
processor, but the information the address bus sends is addressing information used
to describe memory locations. This location is used for data being sent or retrieved.
The address bus carries a single bit of information, representing a digit in the
address, along each wire. The size of the address bus corresponds to the number of
address locations. The larger the address bus, the more memory address locations
that can be supported. The more memory address locations a processor can
address, the more RAM a processor can use. As an analogy, we can compare the
address bus to the address of a house or its house number. If the house numbers for
a street were limited to two digits, then the street could have only 100 addresses
(00 to 99). For an address bus, which communicates in binary language, a limit of
two digits would give four addresses (00, 01, 10, and 11). Thus, the larger the
address bus, the more combinations of 0 and 1 that would be permitted to pass
through at one time. Take a look back to Table 2.3. A 286 processor has a 24-bit
address bus width. Using binary theory, this translates to a little over 16 million
locations, which means it allows access to as much as 16MB of RAM. Using
similar calculations, a 386DX with a 32-bit address bus will allow access up to
4GB of RAM. For more information on bus types, see Chapter 5, “PC Bus

Bus Speed

Motherboards are designed to be more or less universal. They have a processor
socket or slot that can support many different processor types and speeds. The
speed at which a processor can run is set via jumper, as is the speed at which the
expansion bus transfers data to and from the processor. The speed of the bus and
the speed of the processor are directly related. When installing a processor into a
motherboard, you must set both the processor speed and bus speed with a jumper.
Typically, the bus speed is set to 66MHz, 100MHz, or 133MHz plus a multiplier.
For example, when you have a 450 MHz processor, you would set the processor
speed jumper to 450MHz, the expansion bus speed to 100MHz, and the multiplier

speeds without a multiplier.


BIOS stands for Basic Input/Output System. The BIOS communicates between the
computer and devices. The BIOS is usually stored in ROM. When the operating
system needs to access a piece of hardware, it would now ask the BIOS, rather than
just taking control of the hardware. There are three major companies that
manufacture ROM BIOS software:

Phoenix Technologies

Phoenix Technologies was the pioneer who developed BIOS in the first place.
Their BIOS software supports user-defined hard drive types and 1.44MB floppy
disk drives.

American Megatrends International (AMI)

AMI BIOS has been very popular. AMI not only added a more extensive
diagnostic program, but, more than that, AMI produced its own system board. This
provided guaranteed reliabilitybetween BIOS and computer.

Award Software

Award Software is the next largest manufacturer of BIOS software. Award took its
production one step further by allowing other companies that purchase their BIOS
to make modifications as needed to the software. In 1998, Award and Phoenix
Technologies merged. The new company is Phoenix Technologies. The new
company will provide technical support for older products from both companies as
well as produce new BIOS software.

Hard Drives
Hard Drives (HD, HDD)
In the early days of personal computing, files and programs were quite small by
today's standards. They required little storage space. Programs and files easily fit
on floppy disks. Even the OS (operating system) could be kept on a floppy to be
inserted and loaded into memory at start-up.
As the need for storage increased, tape drives were used. These were very slow and
storage was linear. This meant that you had to 'fast forward' and 'rewind' the tape
continuously. Technology progressed fairly rapidly and program and file sizes
increased dramatically. Hard drives were introduced in the early 80's, and the 5MB
of storage space they provided seemed to be more than anyone would ever use. Of
course, programs continued to become larger, more complex and diverse. People
now use computers for a wide variety of applications. Today's entry-level
computer has a hard drive with more than 13-20 GB of storage.
You'll often hear that the CPU and the motherboard are the brain and the backbone
of your computer, absolutely necessary for the proper function and performance of
you system. However, a hard drive failure will definitely bring your computer to a
screeching halt. You can lose all your programs, information, data, and your
operating system. All you'll see is a flashing cursor on your monitor and an error
message indicating a hard drive failure. Also, because your hard drive is a
mechanical device, it is more prone to failure.
At one time, the BIOS knew how the drive was sectored (by calculating the
information in setup) and would access data through a controller card in an
expansion slot on the motherboard. Two cables ran from the controller to the drive,
one for information on where to position the read/write heads and another to
transmit data.
Today's hard drives have the controller and a hard drive BIOS built right on the
drive. Not only does this setup control the read/write operations, but it performs
many other functions as well. One of which is to translate or interpret positioning
of data to the system BIOS. The system BIOS no longer understands the physical
organization of the platters inside
Five Main Components
Platters - The aluminum alloy disks upon which data is stored. It's a magnetic
media, somewhat akin to the surface preparation on a cassette tape. However, the
surface of the platter is magnetically formatted (on both sides) into sectors and
tracks where digital information is written.
Spindle - the platters or disks spin on the spindle, which is run by a motor on the
drive. I guess you could say it's kind of like the axle on a wheel.
Read/Write Heads - The heads move across the platters to write data to, and read
data from the platters. There's a read/write head for each side of each platter.
Access is random, meaning that the heads can jump straight to the information they
want without having to fast-forward or rewind past unneeded information.
Head Actuator - Controls the read/write heads. The heads are at the end of an
actuator arm which is attached to the actuator.
Circuit board - Receives commands from the hard drive controller and
translates them in order to move the head actuator, which moves the read/write
head across the platters to the required position.

Low Level Formatting - Low-level formatting divides the surface of the disk into
tracks and sectors. This sets up a kind of a grid so that the controller knows how to
access each individual sector. For instance, it might store part of a file at 'head #2,
track 40, sector 16'. It's almost like a 3D game of battleships.
It used to be necessary to individually low-level format the older drives before they
could be used by your computer. This is no longer the case. On all the drives you'll
see (unless you're working on a very old computer), low-level formatting is done at
the factory. You may see a choice in your CMOS setup program called 'Low-Level
Format'. I would suggest against using it. It's a dinosaur from days gone by.
There are, however, 3rd party programs which claim to safely low-level format
IDE and EIDE hard drives. If you wish to try them, that's your choice. I'm certainly
not one to discourage experimentation. Just take into consideration the value of the
data you plan on storing on them afterwards, and the fact that the manufacturers
recommend against it.
Tracks - When a read/write head is stationary, the area that spins directly under
the head is called a track. There is the same number of tracks on both sides of each
platter in a hard drive. Each track forms a complete circle, unlike a vinyl record,
which has a single track that spirals to the center.
Cylinders - The read/write heads all move on the actuator arm together. So they're
all positioned over the same track, on each side of each disk, simultaneously. The
entire set of tracks moving under all the heads when they're stationary makes up a
cylinder. If the platters in a drive each have 968 tracks, then there are 968
cylinders. Data is written from the outside cylinder inwards, using up space on
each cylinder before the heads move to the next track, or next cylinder.
Sectors - Each track is divided up into 512 byte blocks called sectors. The data
written to your drive is stored in these sectors, a cluster at a time.
Clusters - A defined number of sectors make up a cluster. The number of sectors
in each cluster varies depending upon the size of the HD (Hard Drive) and how it's


• FireWire/IEEE 1394
• Fiber

Stands for small computer system interface and is a parallel interface standard used
for attaching peripheral devices to computers at the same time, such as hard drives,
printers, scanners, CD-ROM/RW drives, DVD drives and tape drives. SCSI
connector provides for faster data transmission rates up to 320 Megabytes per
second (320 MBps). SCSI provides higher data transfer rates than ATA.


There are three basic specifications:

SCSI-1 features an 8-bit parallel bus running at 3.5 MB/s in an asynchronous mode
and 5 MB/s in a synchronous mode. SCSI-1 supports 8 devices at the maximum
with the maximum cable length of 6 meters.

It features a 16-bit bus width doubling the maximum transfer rate to 10 MB/s.
SCSI-2 supports up to 16 devices with the maximum cable length of 3 meters.

SCSI-3 uses a 16-bit bus and the maximum transfer rate to 320 MBps. It is also
called Ultra Wide SCSI.Other specifications include: Ultra2 SCSI, Wide Ultra2
SCSI, Wide SCSI, Fast SCSI, Fast Wide SCSI, Ultra SCSI.

IDE (Integrated Drive Electronics); EIDE (Enhanced Integrated Drive
Electronics); ATA (Advanced Technology Attachment); PATA (Parallel advanced
technology attachment). It is an interface for mass storage devices, in which the
controller is integrated into the hard disk, CD-ROM drive, and the floppy
drive.The IDE interface uses advanced technology attachment interface for cable
lengths up to 2 feet. A single IDE ATA channel supports up to two drives, master
and the slave. At a time, IDE can only access one drive per channel.

IDE is less expensive than SCSI but offers less performance level. IDE interface
could support drives up to 540 MB

Serial ATA is an evolution of the parallel ATA interface. A single cable with
atleast four wires creates a point-to-point connection between devices, which
defines that serial ATA is a serial link. Serial ATA support data transfer rates
starting from 150 MBps. Serial ATA cables can extend up to one meter. This
interface supports all ATA and ATAPI devices. Serial ATA has thinner cables,
hence, allow for smaller chassis designs. These cables can extend up to one meter.

Serial ATA is not backward compatible i.e. it will not work with the same
connectors that IDE, SCSI or any other interface.

USB: Stands for Universal Serial Bus is an external serial bus standard to interface
devices. USB is an external bus that supports three data transfer rate:

• A low speed rate of 1.5 Mbps, mostly used for human interface devices, such as
mice, keyboards, and joysticks.
• A full speed rate of 12 Mbps supported by all USB Hubs.
• A high speed rate of 480 Mbps. The high speed devices are commonly referred to
as USB 2.0.

A single USB port is capable of connecting up to 127 peripheral devices, such as
printers, scanners, digital cameras, mice, keyboards, joysticks, modems, speakers,
telephone, video phones etc.

Connecting a USB device to the computer requires the plugging of the USB device
cable connector to the USB connector at the back of the computer. A typical USB
socket at the back of the computer is rectangular. A USB standard uses two
different connectors, "A" and "B". "A" connectors head upstream toward the
computer and B connectors head downstream and connect to individual devices.

The main features of USB include:

+ Ease of Use.
+ The computer acts as a host.
+ Up to 127 devices can be connected to the host with maximum cable length of 5
meters for individual USB and 30 meters for USB hubs.
+ USB devices are hot swappable, i.e. they can be plugged and unplugged at any
+ USB supports Plug-and-Play installation.
+ High data transfer rate of 480 megabits per second with USB 2.0.


RAM is our working memory storage. All the data, which the PC uses and works
with during operation, are stored here. Data are stored on drives, typically the hard
drive. However, for the CPU to work with those data, they must be read into the
working memory storage, which is made up of RAM chips. To examine RAM, we
need to look at the following:
       RAM types (FPM, EDO, ECC, and SD RAM)
       RAM modules (SIMM and DIMM) in different versions
       RAM and the system bus
RAM types
The traditional RAM type is DRAM (dynamic RAM). The other type is SRAM
(static RAM).
SRAM continues to remember its content, while DRAM must be refreshed every
few milli seconds. DRAM consists of micro capacitors, while SRAM consists of
off/on switches.
Therefore, SRAM can respond much faster than DRAM. SRAM can be made with
a rise time
as short as 4 ns. It is used in different versions in L2 cache RAM (for example pipe
line Burst
Cache SRAM).
DRAM is by far the cheapest to build. Newer and faster DRAM types are
continuously. Currently, there are at least four types:
     FPM (Fast Page Mode)
     ECC (Error Correcting Code)
     EDO (Extended Data Output)
     SDRAM (Synchron Data RAM)
A brief explanation of DRAM types:

FPM was the traditional RAM for PC's, before the EDO was introduced. It is
    mounted in SIMM modules of 2, 4, 8, 16, or 32 MB. Typically, it is found in
    60 ns or 70 ns versions. 60 ns is the fastest and the one to use. You cannot
    mix different speeds on the same Pentium system board.
EDO is an improvement of FPM RAM. Data are read faster. By switching from
    FPM to EDO, one can expect a performance improvement of 2 to 5 percent.
    EDO RAM are usually sold in 60 ns versions. A 50 ns version is available at
    higher cost.
ECC RAM is a special error correcting RAM type. It is especially used in servers.
SDRAM is the newest RAM type for PC's. It comes only in 64 bit modules
           (long 168 pin DIMM's). SDRAM has a rise time of only 8-12 ns. The
           performance improvement over EDO RAM is only 5 percent running
           at 66 MHZ, but at 100 MHZ it will prove a lot better.
RAMBUS (RDRAM) is a future RAM type. Intel and others have great
                        expectations from this type.

RAM speeds
RAM speed is measured in ns (nano seconds). The fewer ns, the faster is the RAM.
Years ago, RAM came in 120, 100 and 80 ns. Today, we are talking about 60 ns
and faster It becomes complicated to describe the relationship between RAM speed

and the ability of the system bus to utilize fast RAM. I will gloss over that. But
here is a table which illustrates RAM
speed, relative to clock speed:
Clock speed          Time per clock tick
20 MHZ               50 ns
25 MHZ               40 ns
33 MHZ               30 ns
50 MHZ               20 ns
66 MHZ               15 ns
100 MHZ              10 ns


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