Hard disk drive

The hard disk drive (HDD) was invented by IBM in 1950s. Initially the HDD is known as Winchester  Disk. As we know very well that a hard disk is non volatile storage device, which stores the data even when the power is switched off. Although the hard disk drive is much faster than the floppy disk drive or cartridge drive, but its principle is same as that of the floppy disk drive or cartridge drive.

In a hard disk drive the data is stored on a magnetic media or surface layered onto hard disk platters. These platters are coated with magnetic particles. These magnetic particles are iron oxide particles. These platters are coated on both sides.

A hard disk drive consist of 3 to 10 magnetic disks called platters. These platters are mounted on a motor, which spin the platter at a very high speed. The rotation speed of a disk platters is up to 3600 rpm, which is 10 to 20 times faster than a floppy disk drive. The distance between the two adjacent platter is ½ inch. All the disks platters move simultaneously in same direction with the same speed. The entire arrangement is placed in a cover. The entire unit means platters or heads are called hard disk assembly (HAD).

The storage capacity range from 10 MB to several gigabytes.

Data is stored on to the disk in the form of files. A file is simply a named collection of bytes. When a program running on a computer request  a file, the hard disk retrieves its bytes and sends them to the CPU one at a time. There are the two ways  to measure the performance of a hard disk:

1. Data rate: The data rate is the number of bytes per second that the drive can deliver to the CPU. It is also known as transmission rate. the rate lie between 5 to 40 MBPS.
2. Seek time: It is the amount of time between when a CPU requests a file and when the first byte of the file is sent to the CPU. 10 to 20 millisecond is common.

Tracks and sectors

All hard disk drives write data on to the platters in concentric rings, called tracks. The spacing of the tracks and number of tracks on each platter surface depend upon the capacity of the drive. Today’s hard disk drives contain 1000 to 2000 tracks on each platter surface.

Each track on each platter surface is further divided into the pie shaped wedges, called sectors. Each sector stores 512 bytes of data. The number of sectors on a track can vary depending upon the density of the disk. Each sector is numbered, starting from 0. Each sector is divided in to two parts called

Address area and data area. The address area is used to identify a sector whereas the data area stores 512 bytes of data. The address area also contain the CRC information.

Hard disk drive components

Some of the important component of HDD are

• Platters: One of the most important component of the hard disk drive is platter. A platter is made up of rigid material like aluminum or glass ceramic, having coated with magnetic material on both sides. The hard disk write data on this platter. The number of platters in a hard disk drive varies from three to ten.
• Read / write head: Like FDD, the HDD also contain a read/write head. The head is responsible for writing data to and reading it from the magnetic material applied to the platter surfaces. For each platter surface there is one read/write head means if there are 3 platters means 6 surfaces, that means there are six read/write head.

Fixed head are not used in today’s hard disk. The moving head is mounted on a actuator arm. None of the head move independently, when the arm moves all the heads move back and forth. Where as in case of fixed head, there is a read/write head for each and every track.  The head is made up of iron oxide core and it is wrapped with electromagnetic coils.

• Head slider : The read/write head assembly of hard disk drive rides on arm called the head slider.
• Head actuator : Head actuator arm are light weight, rigid, triangular pieces of a metal alloy which holds the  head slider and the head and it move the head back and forth across the platter.
• Spindle motor: The spindle motor rotates the platters. The spindle motor rotate the platters at a constant speed so that there is no impact of vibration or noise can carried out, which disturb the position of the platters in relation to the head. Moreover the spindle motor is responsible for maximum part of power consumption of the hard drive.
• Circuit board or logic circuit: It contain  the electronic circuit. This board is mounted below the chassis and contain all circuits that is responsible for the communication with the system, movement of head, spinning of platters and so on.
• Cables and connectors: A HDD also contains the 2 cables and connectors.

Power cable provide two types of power to the HDD ; +5 V for logic circuit and +12 V for rotating the drive motor.

Operation on Hard Disk Drive

There are two types of operations performed on the HDD.

1. Read operation          2.    Write operation

Read operation: For reading data from the HD, CPU generate a read request/command and passes it to the drive controller along with the address of required data location. When the HDD controller get this address and command the head start working and it come closer to surface while the disk is spinning. When the required location is reached, it is confirmed by CRC codes and then the head generates a voltage pulse when it passes over the location. The drive controller amplifies the pulses and decode them to reproduce the original data.

Write operation: The write operation of the hard drive is very much similar to the read operation. The system send the write command and sends it through system bus to drive controller. The address of the required data location is also transmitted with the write command. When the write command is received by the HDD, it start spinning. Then the actuator arm moves and locate the required track. Once the required track is found the head waits for required sector to come below it, when the sector is reached below it, the head moves and stays on the sector. It encode the original data in to the bits and generate the magnetic or voltage pulses to write it on the desired sector.

The SMART Technology

More over the HDD also possesses the feature  called SMART (Self Monitoring Analysis & Reporting Technology). By this technology the system measure the performance of the HDD and system facing problem using a technique called Predictive Failure Analysis.

Key features of HDD:

1. Performance: The performance of a system is largely depends upon the performance of a HDD. How better the HDD work? How fast it load the PC Boot up program and other program and how’s its performance in case of the multitasking system? Such as graphics work, editing sound and video or working with databases.
2. Storage Capacity: Better be the performance of the system if the storage capacity is large or big. Because the large size disk store better features than the low space devices.
3. Software support: the hard disk should be fast and such that it provide the needy space to the large software and load them efficiently.
4. Reliability: A HDD must be reliable in case of delivering of the data, reading of data and writing of the data. It does not loss the data during the read and write operation.
5. Latency: It is the amount of time that it takes for the platter to spin, bringing the sector to the right position. The faster the platter spin, lower is the latency.
6. Access time: A drive’s access time represent the total delay between the beginning of a read and write operation and the time when the drive actually begins reading and writing.
7. Track switch time: Also known as Cylinder Switch Time or Track to Track seek time. This is the time taken by the actuator arm to move the head from one track to the next adjacent track.
8. Areal Density (AD): It is the measurement of the overall efficiency of a magnetic storage media. Greater the areal density, more data the drive can store in the same amount of the space. This is possible because manufacturer have increased both the number of bits stored in a single track and the number of tracks on each platter surface. The two factors are known as drive’s areal density. o To compute areal density multiply the bits per inch (BPI) to the track per inch (TPI). BPI is the number of bits stored in one inch length of a single track, and the TPI is number of track in one inch length from the centre of the platter to its outer edges. The areal density measures in bits per square inch (BPSI).

Some Advanced Technology

1. Cylinder Skewing: The primary purpose of the cylinder skewing is to minimize the movement of the head. When one track ends the heads move to next track and start reading it from the starting position of each track is same. But in case of cylinder skewing the starting position of each track is different from the adjacent one. By this feature the head immediately read the next track when the previous tracks ends.
2. Zoned bit recording or Sector sparing: In the starting days, the each track contain the same number of sectors. It lead to the fact that sectors on the outer tracks had lower density and the sectors on the inner track had higher density. To overcome this problem Zoned Bit Recording (ZBR) method was introduced. Under this method the total tracks are divided in to the zones. The tracks in a particular zone contain the same number of sectors. So that there is a stability between to two types of densities.

Note : When a hard disk is manufactured, a certain amount of pressure is maintained in the hard disk so that it works properly. Such pressure lift the heads at a proper height from the platter surface. That is why unlike FDD, the head of the hard disk drive does not touch the surface. But if the pressure reduces, then it is not able to maintain the proper height between the head and the surface and the head crash. Head crash can be caused by electronic failure, a sudden power failure, wear and tear, poor manufactured disks etc.

In the most desktop and server drives, when powering down, the heads are moved to the landing zone, an area of the disk usually near its inner diameter, where no data stored. This area is known ad Contact Start and Stop (CSS) zone.    

Motherboard:

The motherboard is, without a doubt, the primary component of the entire system. Without the support circuitry and functions this device provides, even the CPU is unable to work. A motherboard also known as main board, logic board, or system board. Typically the motherboard contains slots, and all the controllers required to control standard peripheral devices, such a display screen, keyboard and disk drive etc.

In addition to the hardware, the motherboard also contains some software. The system ROM actually contains three small, but very critical programs. These are the POST, the BIOS and the setup program. Power on Self Test (POST) is the program that initializes and tests each components of the motherboard every time it starts up to make sure everything is ready to run. The Basic I/O Service (BIOS) is a collection of very small programs that enable the system to communicate with hardware devices during the boot process. Finally the setup program is used to configure the features found on the motherboard.

Important component of the motherboard:

1. Chipsets:

Chipset is made up of one or more electronic components that connects the motherboard components such as, the processor, expansion and internal memory. Basically the work of the chipset is to control the system and its capabilities. Chipset determines the speed of the motherboard. It is also responsible for increasing the speed of the communication between processor and the peripheral devices. Mostly used chipset in the computer motherboard is Northbridge chip and Southbridge chip. Northbridge chip is the name given to the system controller chip. This is the chip that directly connects the CPU’s front side bus (FSB) to the high speed components of the system, which would include RAM, the AGP port and all other high speed peripherals. The FSB (also known as the external data bus) of the CPU must be able to synchronize with all the other devices. Where as the Southbridge chip is slower than the Northbridge chip. Southbridge chip is the component of the chipset that is responsible for interconnecting the CPU’s FSB to the slower component of the system. Device managed by this chip include IDE port, USB ports, the ISA bus and other devices that are not supported by the super I\O chip. The information from the CPU has to go through the Northbridge chip  before reaching the Southbridge chip.

2. Microprocessor:

Microprocessor is one of the main components of the motherboard. It is a square shaped chip installed on the motherboard and performs the operations like subtraction, addition, multiplication and division on the data and sends out the results. It also performs logical and comparison operations on data. Moreover, the microprocessor controls the activities of various components of the computer and also responds to the requests from the peripheral devices e.g printer signals indicating that it has run out of paper.

3. Clock Chip:

Clock chip is a small chip on the motherboard used as a timekeeper for the components of a computer, which are designed to operate in perfect synchronization. The chip provide the timing signal in the form of electronic pulses that are used by the computer components to set up a working pace. The clock speed of PCs ranges from a low of 16MHz to a high of 500MHz. The clock speed is one way to measuring the speed of a computer. Another unit for measuring the speed of a computer is MIPS (Million Instructions per second). It gives the number of instructions executed per second.

4. Bus:

A bus is simply a circuit that connects one part of the motherboard to another. The speed of the bus is measured in megahertz (MHz). The communication between the microprocessor and the memory chips as well as the other chips found on the motherboard is accomplished through a bus. The bus comprising of 8 wires for carrying data is called the data bus. When data is sent from one unit to another, the address (location in memory at which the data is stored) is also send with the data. The bus has a set of 20 wires for carrying the address. This type of bus is called the address bus. Bus speed usually refers to the speed of the Front Side Bus (FSB), which connects the CPU to the Northbridge. FSB speeds can range from 66 MHz to over 800 MHz.

5. Ports, sockets and memories:

Moreover the motherboard also contains some ports (like serial and parallel port) for connecting the peripheral devices to the computer motherboard. Motherboard also contains some sockets in which the main memory is placed.

Motherboard controller and system resources:

System resources are resources that are electrically used by hardware, allocated by firmware, and used by software for each device. In some ways, everything in a PC is a resource of system like RAM, processor speed, hard disk space, etc. But there are several special resources in the system that are shared by the various devices that use it. These are not physical "parts" of the system. Rather they are logical parts of the system that control how it works, and are referred to as the PC's system resources. System resources are important because they must be shared by the various devices in your PC. This includes not only the motherboard and other main components, but also expansion devices, plug-in cards and peripherals. The resources are primarily used for communication and information transfer between these devices.

Some of the system resources are given below:

Memory Address: Some devices require some space in the memory area for their own use. The devices that use a memory area generally use it for their own BIOS, which contain code to control the device and is invoked by direct calls or calls from the internal system BIOS.

Input/output Ports: Every device must have at least one I/O port to send data and commands to and from the device.

IRQ Line: Many devices use this resource type, but not all. Usually, if a device is designed to perform tasks independent of the CPU, it will need an IRQ line to alert the CPU when a task is done or attention is needed. A mouse and a network card are examples of devices that always use an IRQ.

DMA Channels: The DMA channels were originally designed to take advantages of faster transfers of data to and from memory using a DMA controller instead of the CPU. As CPU increased, however, DMA channel resources usage decreased, because the CPU was actually faster than DMA.

Memory Address:

There are a few devices that need to allocate memory addresses as a system resource for their own use. The most common add-in device to use a dedicated memory address space for its own BIOS is a SCSI host adapter. In addition, network cards that have the ability to boot the computer over the network typically also use a memory area for the boot BIOS. But fortunately there are very less conflicts in this area today. Most commonly used memory conflict in the case of memory address is that sometimes the two video adapter cards or other devices using the same memory address locations. The best method to overcome this problem is to draw a memory map which clearly show that which memory address location is assigned to which device.

I\O ports:

Every device in every PC uses at least one I\O port address, in most cases, a single device uses 4, 8, or 16 I\O port address. These port addresses are used to do the following:

1. Send commands to the device.

2. Get device information and status.

3. Set, check and clear device interrupts.

4. Send and receive data.

I\O port conflicts are usually only a problem with the older ISA cards, although they can also be a problem with devices that are integrated directly on the motherboard with their own connections to the ISA bus. Newer ISA cards that are PnP compatible can be automatically reconfigured by the operating system and therefore are not usually involved in conflicts.

I\O port conflicts are quite similar to memory address conflicts. In fact, the I\O address signals are actually carried on the same wires as the memory address bus, with one additional wire used by the CPU to tell the rest of the system that it is an I\O operation and not a memory operations. This is usually refers as IO/MEM wire.

IRQ Lines:

IRQ stands for Interrupt Request. An interrupt is an unavoidable task or command provided by the device of computer to the system processor. When the processor get such type of interrupt it stop the execution of the instructions of the current task and starts working on the interrupt. The purpose of the IRQ lines is to get the CPU’s attention when it is needed. Not all devices use interrupts, and some devices use interrupts only is certain cases. Interrupts are widely used during the modem communication. Interrupts are not provided by the CPU, it is provided by a special chip.

The reaction of the system when it got the interrupt is given below:

1. The IC chip on the motherboard sees the IRQ and passes it on to the CPU if there are no other interrupt in progress, or pending IRQs of higher priority.
2. When the CPU sees an IRQ, it stops the foreground task at the end of the current instruction and saves the location of the next instruction to be run for the foreground task and starts working on interrupt.
3. When the interrupt finishes the foreground task continues with the next instruction it had saved and all the internal registers back to the way they were before the interrupt.

The original PC had eight IRQ lines, of which six were usable. Starting with the AT computers and all PCs since, there are 16 IRQ lines of which 11 are usable. IRQ2 and IRQ9 are two names for the same wire, and can conflict with each other, no IRQs can conflict with other IRQs of a different number.

Interrupts are actually provided not by the CPU, but a separate IC chip that can prioritize eight different IRQ lines coming in and pass them on to the CPU, queuing them up if needed. On the original IBM PC, the 8259 IRQ controller chip handled this task. Because IRQ0 and IRQ1 were dedicated to motherboard functions, only six IRQs were actually available to other devices

When the AT computer came out, IBM added a second 8259 IRQ controller chip to the motherboard, which was cascaded through the first one which is shown below.

As you can see from the diagram that the cascading of the first 8259 IRQ controller chip and second 8259 IRQ controller chip is carried out through the IRQ2 line of first 8259 and IRQ9 line of second IRQ controller chip. Therefore the conflicts generate between the IRQ2 and IRQ9

Recordable Drives

Optical devices such as CD –ROMs and DVD –ROMs are good data storage solutions. They hold a lot of data. But the biggest drawback of optical devices is that they were read only devices. To write the data in the CD or DVD, the drive is able to create the pits in the CD or DVD. The drive which possesses such feature is called the Recordable Drives. CD – R (CD writer) and CD – RW (CD rewriter) are the example of good recordable drives. Some recordable drives enable you to write data to a disc only once where as other recordable drive enable you to write the data on the same disc many times. The CD – R drives write the data only once, they can’t remove the old data from the CD and make them as a reusable. But the CD – RW able to do that.

CD – R Drive

The first writable CD – ROM drives introduced in the market in 1990s were called the CD – R. In other words we can say that it is the first CD writable drive which is able to write the data on the blank CD by crating the pits on it. At the time of their original release, both the drive and the blank discs were extremely expensive. The CD – R drive itself contains most of the same components as a regular drive, except for the special laser. The drive can also function as a regular CD – ROM drive means you can read the data from the CD as well. CD – R drives generally uses two types of speed, one for reading discs and one for writing them. The writing speed of the drive is lower than the reading speed. Generally, speaking, CD – R drives are not as fast as standard CD – ROM drives.

Like a CD – ROM drive, CD – R drives are also available or work in both IDE and SCSI interfaces. Once the CD – R drives starts writing the data to the disc, it can not be stopped until the session is completed. If the stream of data traveling to the drive is interrupted, the disc generally is ruined. More over the CD – R drives work on different types of discs called the    CD – R discs.

CD – R disc

CD – R drives works by using a different type of disc and a different type of laser beam than CD – ROM drives. The disc is still made up of the same polycarbonate material which is used in the CD – ROM drive disc, but with a spiral pregroove instead of a data pattern. This groove is the path for the track that the drive will write onto the disc.

On top of the polycarbonate layer there is a layer of a photosensitive dye, which is reflective in nature and made up of metal (such as gold alloy), and a transparent layer of protective plastic. When you purchase a blank CD, it consist only a big land, which is reflective. And the process of writing data to it involves creating pits. For crating the pits, the drive uses a special laser that causes the reflective dye layer to react with the metallic layer, forming a composite (mixed form of two) material that is almost non reflective. These non reflective areas become the pits. Such process is known as the “burning” of the CD, because a chemical reaction is carried out between the two layers by using a laser beam.

CD – R software

In addition to the drive and the blank discs, to burn CDs on your computer, you need special software. This software provides an interface by virtue of which you can select the files that you want to write to the disc. Nero, CD Burner etc. are some of the example of such kind of CD – R software.

Single Session and Multi session discs

A session is a continuous written collection of data written to a CD – ROM. The CDs of movies, songs etc are available on the market are all single session disc in which the data in written only once. But when you burn or create your own CDs, you can conceivably write multiple sessions on a disc. This means that you can write 100 MB of data today, 72 MB tomorrow, and so on until the disc is full. This is a wonderful feature provide by the CD – R soft wares, but you must have a multisession drive in order to burn CDs in this way or create a multisession CD. In addition, you need a multisession CD – ROM drive to read them. If you use a single session CD – ROM drive to read a multisession CD, then you are able to read only or see the data written during the first session not written in the other sessions.

CD – RW drive

No doubt that the CD – Rs became a popular data storage media, but as the time spent, the people want to reuse discs by erasing the old data on them and writing new data. But CD – R are not able to do that. To overcome this problem the CD – RW (CD rewritable) drives are introduced which can do the same. Once again, the CD – RW differs from the CD – ROM and the CD – R primarily in the disc medium itself and the capabilities of the laser used in the drive. A CD – RW disc consists of the usual polycarbonate substrate, but it consist five additional layers. The recording layer in which the data is stored consists of a special chemical, sandwiched between two dielectric layers that draw heat away from the recording layer. On top of these layers comes a layer of reflective metallic material, followed by a protective coating.

To perform the different types of operations, the laser in a CD – RW drive can run at three power levels, which are as follows:

Read power: The lowest power generated by the laser. Used only to read the data already written to the disc. It does not alter the state of the recordable layer.

Erase power: The laser’s middle power. Used to return the pits in an already written disc to a crystalline (reflective) state.

Write power: The laser’s highest power. Used to create the pits by changing parts of the recoding layer to an amorphous (non reflective) state.

CD – RW drives are capable of writing to either CD – R or CD – RW discs, which is an excellent feature.

REPEATER :

A Repeater is the most basic device on a network. Repeater is device that operate at the physical layer of the OSI model. Repeater has two ports.

A Repeater generates any signal received in on one port and out the other port.

The basic puRepeaterose of Repeater is to extend the distance of something.

The primary puRepeaterose is simply to regenerate a signal received from input and correct the signal to its original state for output i.e repeater takes w weak signal from one segment, regenerates it and passes it to the next segment.

Repeaters are used to connect only same type of media, such as 10 base-2 Thin Ethernate to 10 base -2 thin ethernate, or Token ring twisted pair to Token ring twisted pair.

Repeaters are available in many types.

1- Single port Repeater.

2- Multiport Repeater.

3- Smart Repeater.

4- Optical Repeater.

Single port Repeater operates with two segments that is one segment send signsl to Repeater, it boost and pass to the next signal.

Multiport Repeater connect one segment to another cable segment. Multiport Repeater has one input port and multiple output ports.

Smart Repeater is hybrid device and are very similar to a bridge in functionality. Packet filtering is done by smart Repeater.

Optical Repeaters are Repeaters that operate on optical signal. Most Repeaters in use today are multiport repeaters that are called hubs.

Advantages of Repeater :

1- The least expensive way of expanding a network.

2- They can connect different types of media but they do not have ability ti connect different network architectures such as ethernet an token ring.

3- Passes all traffic in both directions.

Disadvantages of Repeater:

1- The main disadvantage of Repeater is that it passes along everything it detects on the line. If there is noise or distortion the Repeater passes it along with original signal.

2- Not support if network traffic is heavy.

3- Not support to connect different network architectures such as ethernet an token ring.

4- Repeater do not have ability to work beyond the physical layer, which means they do not examine any destination addresses and they do not filter or translate any data.

5- There is a limit to the numbers of Repeaters that can be used to extend a LAN's length and topology. that is ethernet LAN, support maximum five segments interconnected by four Repeaters.