MEMORY

A hard disk drive contains rigid disk shaped platters, usually constructed of aluminum or glass which cannot be bent like floppy disc hence termed as hard disk. Since the platters cannot be removed they are sometimes called fixed disk drives.

The physical constructions of the hard disks contains of spinning disks with the heads that move over.

The physical construction of a hard disks called platters with heads, they are that move over the disk stacked on top of each other and spin at same speed, each with two sides tracks and sectors which the drive stores data. The heads read and write data in tracks which are divided up into segment called sectors. Most drives have at least 2-3 platters, resulting four or six sides, some driver have up to 11 or more platters. The identically positioned tracks on each side of every platter together make up a cylinder. A hard disk drive normally has one head per platter with all heads mounted on common rack. The hard disk spins the disk as 3600, 5400, and up to 7200 or even more RPM (rotation per minute).

Basic Hard drive components:

* Disk platters
* Read/ write head
* Spindle motor
* Head acutator mechanism
* Logic board
* Cable and connectors
* Configuration items

The platters, spindle motor, heads and head actuator mechanism are contained in a sealed chambers called the Head Disk Assembly (HDA), usually treated as a single components and is rarely opened. Other parts external to HDA such as the logic board, cover, and other configuration items can be disassembled from the drive.

TYPES OF HDD


Desktop Hard Disk Types: 3.5', IDE and Sata Interfaces

Laptop Hard Disk Types:2.5', 1.8', IDE, Sata, SCSI

Server Hard Disk types:3.5' Sata, SCSI

Hard Disk Interface(s)
There are a few ways in which a hard disk can connect/interface with:
• Advanced Technology Attachment (Also known as IDE, ATAPI and Parallel ATA)
• Serial ATA
SCSI(aka Scuzzy)
There are variants of each interface, and this article will not do justice to the different types of ATA, SATA and SCSI interfaces. Thus, it will only highlight the more common interfaces as used by the home user.
ATA (IDE, ATAPI, PATA)
ATA is a common interface used in many personal computers before the emergence of SATA. It is the least expensive of the interfaces.
Disadvantages
Older ATA adapters will limit transfer rates according to the slower attached device (debatable)
• Only ONE device on the ATA cable is able to read/write at one time
• Limited standard for cable length (up to 18inches/46cm)
Advantages
• Low costs
• Large capacity
SATA
SATA is basically an advancement of ATA.
Disadvantages
• Slower transfer rates compared to SCSI
• Not supported in older systems without the use of additional components
Advantages
• Low costs
• Large capacity
• Faster transfer rates compared to ATA (difference is marginal at times though)
• Smaller cables for better heat dissipation

SCSI
SCSI is commonly used in servers, and more in industrial applications than home uses.
Disadvantages
• Costs
• Not widely supported
• Many, many different kinds of SCSI interfaces
• SCSI drives have a higher RPM, creating more noise and heat
Advantages
• Faster
• Wide range of applications
• Better scalability and flexibility in Arrays (RAID)
• Backward compatible with older SCSI devices
• Better for storing and moving large amounts of data
• Tailor made for 24/7 operations
• Reliability

RAM

Random access memory (RAM) is the best known form of computer memory. RAM is considered "random access" because you can access any memory cell directly if you know the row and column that intersect at that cell.

The opposite of RAM is serial access memory (SAM). SAM stores data as a series of memory cells that can only be accessed sequentially (like a cassette tape). If the data is not in the current location, each memory cell is checked until the needed data is found. SAM works very well for memory buffers, where the data is normally stored in the order in which it will be used (a good example is the texture buffer memory on a video card). RAM data, on the other hand, can be accessed in any order.

Similar to a microprocessor, a memory chip is an integrated circuit (IC) made of millions of transistors and capacitors. In the most common form of computer memory, dynamic random access memory (DRAM), a transistor and a capacitor are paired to create a memory cell, which represents a single bit of data. The capacitor holds the bit of information -- a 0 or a 1 (see How Bits and Bytes Work for information on bits). The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.

­A capacitor is like a small bucket­ that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor's bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge.

Ram is simply Random Access Memory. RAM is a part of main memory. RAM is called main memory because we can randomly and quickly access any location in memory. It is used to store instructions waiting to be obeyed, instructions currently being obeyed, data awaiting processing, data currently being processed, and data awaiting output.

RAM are of two types:

1. DRAM
2. SRAM

DRAM stands for Dynamic RAM. It is the type of memory in a modern computer. It is dense, very small, and it is very inexpensive, which makes it affordable for large amount of memory. The memory cells in a DRAM chip are tiny capacitors that retain a charge to indicate a bit.

The problem with DRAM is it is dynamic, and because of the design it must be constantly refreshed or the electrical

charges in the individual memory capacitors will drain and the data will be lost.

SRAM stands for Static RAM. It does not need the periodic refresh rate like DRAM. It is also much faster than DRAM and is fully able to keep pace with modern processor. However SRAM are both physically larger and store less bits overall and are more expensive to produce.

KEYBOARD

One of the main input devices used on a computer, a PC's keyboard looks very similar to the keyboards of electric typewriters, with some additional keys. Below is a graphic of the Saitek Gamers' keyboard with indicators pointing to each of the major portions of the keyboard.
Keyboard Types
QWERTY Keyboard
A standard computer keyboard is called a QWERTY keyboard because of the layout of its typing area. This keyboard is named after the first six leftmost letters on the top alphabetic line of the keyboard. A QWERTY keyboard might limit your typing speed. The QWERTY keyboard, which was named after the first six letters of its layout, is the standard design for both typewriters and computer keyboards. The QWERTY was originally designed to decrease the pace of text entry and to prevent key jamming in early mechanical typewriters. Modern computers to not suffer from key jamming, and many feel that it is time to consider alternative keyboard layouts.
Because the fluency of text entry is determined by the speed and accuracy of the user, it is important to examine these measures for alternative keyboard options. One option presently available is the Chubon keyboard, which was designed to improve the speed and efficiency of single digit entry (Chubon & Hester, 1988). Consequently, a comparison of the QWERTY keyboard and the Chubon keyboard can provide valuable information for occupational therapists who prescribe alternative keyboards for persons with disabilities, including those with low endurance and decreased strength.
Dvorak Keyboard
A keyboard with an alternative layout was designed to improve typing speed. Called the Dvorak keyboard, this type of keyboard places the most frequently typed letters in the middle of the typing area.

Here's a brief history of this alternative to QWERTY. In the 1930s, two efficiency experts named August Dvorak and William Dealey conducted a study of workplace efficiency in the office. One of their first discoveries was the strange QWERTY keyboard layout, which seemed to have been designed with at best no consideration for efficient typing. They began work on an alternative keyboard layout, and after many years published a new design called the American Simplified Keyboard or ASK. This design eventually became called the Dvorak Simplified Keyboard or DSK, named after Dr. Dvorak, who spent much of his life working on it, and then trying to promote it. (No, it's not named after PC industry pundit John C. Dvorak, but I read somewhere that they might be related. Perhaps if Mr. Dvorak reads this he could let me know. :^) )

The goals behind the Dvorak design are relatively common sense ones: move the most commonly-used letters to the home row where they are easy to reach, and exile infrequently-used keys to the outer reaches of the layout. Dvorak refined the design over many years until he came up with a design that he felt was ideal.

Now the interesting thing is that quite a lot of controversy swirls around the Dvorak design. Proponents of this layout consider it vastly superior to the QWERTY arrangement, and many have spent years in frustration attempting to overcome the inertia of the QWERTY design--including Dvorak himself, who lived a rather quixotic life trying to sell his improved design to an industry that just wasn't interested. At the same time there are skeptics that say the advantages of the Dvorak keyboard have been overstated. Some even claim the studies showing the superiority of the Dvorak design were conducted incompetently, or even dishonestly! I don't really know the truth behind all the claims and counter-claims, so I am not going to touch this controversy with a ten-foot pole. But I did want to mention that Dvorak is not universally acknowledged to be superior to QWERTY.

Those who like the Dvorak design make many claims regarding its superiority, and from a common-sense standpoint, they do make sense. The Dvorak design is supposed to allow faster typing with more accuracy, since 70% of keypresses are on the home row of the keyboard, compared to 31% with QWERTY. The layout is organized with the vowels on one side of the keyboard so that more alternation between right and left hand is used in typing common words. The reduced motion of the fingers is claimed to allow for greater comfort on the part of the typist, and some go so far as to say that using a Dvorak design will alleviate the symptoms of repetitive stress injuries associated with keyboards. I would have to see some valid medical evidence to be convinced of that, but again, it makes some intuitive sense to me.

COMPUTER MONITOR

Monitor

Also called a video display terminal (VDT) a monitor is a video display screen and the hard shell that holds it. In its most common usage, monitor refers only to devices that contain no electronic equipment other than what is essentially needed to display and adjust the characteristics of an image.

Types of Monitor

CRT Monitor

This one is heavy and thick and has a normal tube. Because of this normal tube you could get a glare on the tube from light sources within the room. That could be irritating so that’s why they developed the flat CRT screen which has a flat tube.

Flat

This one is also heavy and thick but this one has a flat tube. Flat CRT screens are designed to reduce the glare and distortion created by conventional CRT screens. The flat tube increases image clarity while reducing glare from light sources within the room.

LCD

LCD (Liquid Crystal Display) screens are most commonly used for laptop screens but are becoming increasingly popular and more affordable for desktop users. These screens use a TFT (thin film transmitter) to produce a more secure picture with a relatively wide angle of view. TFT provides the best resolution of all of the flat panel technologies. These are the most expensive screens on the market. Also these are available in several sizes

There are currently three types of monitors: CRT, LCD, and Plasma.

How can you tell what type of monitor your computer has? CRT and Plasma monitors

are used only on desktops. Laptops always use LCD monitors. Desktops can use

CRT, LCD, or Plasma monitors. CRT monitors are the largest. They are usually a

cube in shape and take up the most space. Also, the screen is usually curved on a

CRT monitor; if you run your finger across it it’ll go up and down. However, the

screen is not always curved on a CRT monitor. LCD monitors are a lot smaller.

Modern LCD monitors are usually less than in inch in thickness. That’s why

they’re called “flat-screened.” Plasma monitors are also flat-screened, however,

they cost tons more money then LCD monitors do, since they can display the

clearest images. Plasma monitors are very new and expensive, and very few people

have them – most people have CRT or LCD monitors.

CRT monitors are the oldest types of monitors, they have been around for decades,

but they are still made today. CRT stands for “Catho-RayTube”. The screen on a

CRT monitor is literately a screen, just like a window screen. It is divided into

millions of tiny squares. In fact, you can even see each square if you look

closely, especially on older monitors that only support low resolutions. These

tiny squares are called “pixels.” To display an image, each pixel changes color.

The monitor isn’t really displaying the image as a whole; it’s displaying

thousands or millions of tiny dots of color, that together, look likes the image.

It’s sort of like print dots on a piece of paper printed form an inkjet printer.

To display a video or animation, the colors of the pixels rapidly change. In CRT

monitors, a device called the “cathoraytube,” located inside the monitor, fires

different color light, sort of like a laser, into each pixel of the monitor. The

light stays there in that pixel for maybe a thousandth of a second, so the

cathoraytube has to keep firing more light into each and every pixel extremely

fast. The cathoraytube goes one pixel at a time, usually working its way left to

right across each row then going down to the next row until it reaches the bottom

of the screen. Then it starts over again with the top left pixel on the screen.

It might sound like it would take a long time to fire a light beam into each and

every pixel on the screen; however, the cathoraytube is extremely fast. So fast,

in fact, that it fires a light beam into every pixel on the screen 60-120 times

every second! The number of times per second that the cathoraytube fires a beam

of light into every pixel on the screen is called the monitors “refresh rate.”

LCD monitors have no “cathoraytube”, but they still have a refresh rate. The

refresh rate for LCD monitors is how many times each pixel on the screen changes

per second. Most LCD monitors refresh at a rate of 100 Hz, that’s 100 times a

second. For more info on LCD monitors, scroll down two paragraphs.

With CRT monitors, especially older CRT monitors, if the same image is fired by

the cathoraytube over and over again over a period of several hours, the image can

get “burned” into the pixels, so the pixels only show that image and cannot show

any other images. This can be a major problem. That’s why screen-savers were

invented! Screen-savers literately save the screen from freezing from

“cathoraytube burn.” However, if you have a LCD or Plasma monitor, or if your

monitor is set to automatically turn of after so many minutes without use, then

you don’t need a screen-saver. You can still have one, but it is not necessary.

LCD stands for “Liquid-Crystal Display” or “Liquid Crystal Diode.” Sometimes LCD

displays are also called LED displays. LED stands for “Light Emitting Diode.”

Like CRT monitors, LCD monitors are divided into millions of tiny squares called

“pixels.” In LCD monitors, each pixel has a tiny LCD (a Liquid Crystal Diode) in

it. LCDs, also known as LEDs (although there are some slight differences between

LCDs and LEDs) are basically tiny light bulbs that can light up as any color. In

this way, each pixel can change color to display an image, similar to the way CRT

monitors display images, except that the pixels change colors from the LCDs in

them changing colors, not because a cathoraytube fires a different color into the

pixel. LCD monitors have no cathoraytube.

All monitors, no matter what type of monitor it is, gets the information on what

image to display from a video card (also known as a “graphics card”), located in

one of the expansion slots of the motherboard

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