Before we go anywhere let's have a look a how data is stored on a hard disk.

As i've said elsewhere the disk is formatted and devided into 'sectors'. and below we see a window showing a readout of just one of the sectors on an 80GB hard disk to the left we see the Binary, or Hexadecimal codes, to the right the programs attempt to decypher it. If you look carefully you can see there are 160,055,528 sectors on that drive. Each of the used sectors has a similar amount of data, the unused sectors are populated with the '00's you can see in this sector.

The information dosn't make much sense does it?, well examine the sector closer and you'll see that there are a lot of 'f's, see if you can work out which of the pair of numbers it coresponds to.

Not all the characters shown translate into Hexadecimal Hex codes, the others are machine codes and are translated as the CPU processes tham.

In case you didn't get it it's '66'

Here we see a hard disk with the cover off, you can see the platters (there are two of them) and one of the arms, with the head at the end, there are four arms, one for each of the faces of the two platters. All hard disks wether thay're EIDE, SCSI, or SATA are the same inside.

The disk need not be permanently mounted in the PC. Here we see a 'Caddy' commonly available, currently for EIDE but it won't be long before SATA versions are avialable. Basially it's a housing for a standard hard disk. The caddy is mounted in a drive bay and the drive unit is inserted. This is a good way of safeguarding a copy of your data, if it is removed and stored away from the machine.

Here we have another type of 'Caddy' this time completely external. Yet again it's a housing for a standard hard disk. You can either buy it 'Made up' with the drive, or as I did buy the case and put an old (in this case (?)) 60GB EIDE drive in it. The only real disadvantage is that you need an external power supply and a USB lead. One more thing, if you do take the route I did the drive must be set as master and formatted before fitting.

Both 5.25" and 2.5 external USB drives. the 5.25" needs an external power supply, the 2.5 dosn't.

EIDE

Stands for Extended Interface Device Electronics and was originally called IDE.

When you buy a new PC it currently comes with two hard drive controllers built in and probably a new SATA connector. Each EIDE controller can handle up to two devices, so as a result the computer can handle up to four EIDE devices, which may be two hard disks, or a hard disk and CDROM, on two chains, without buying any additional controller cards.
Each of those chains must have one of the devices set as a master and the other set as slave (Fig 1). The settings are usually set by positioning jumpers according to details either printed on the device or available on the manufacturers web site .

A hard disk showing the jumper settings

Cable Select (CSEL)

Most motherboards and drives are capable of supporting this feature. Drives configured in a multiple drive system are identified by CSEL's value (assuming a dual drive IDE cable):

If the CSEL jumper is closed, the drive address is 0. This will be the first drive on the IDE cable, the Primary Drive. If CSEL is open, then the drive address is 1. This will be the drive plugged into the second connector (the last one on the cable), the Secondary Drive.

EIDE is the best choice for their home systems. EIDE drives often cost less than a comparably sized SCSI drive. EIDE drives have a much lower price per megabyte than SCSI, so a user can obtain more space for the same cost.

EIDE drives waste a lot of processor (CPU) power. The CPU usage often hits 100% when accessing the hard drive. The high CPU utilization is because the CPU directly controls everything that the EIDE hard drive does.

On newer Pentium systems, EIDE controllers can be configured to utilize DMA bus-mastering, which uses very little CPU time and still transfers data at a very reasonable rate. The motherboard manufacture can supply users with the proper drivers for running EIDE devices using DMA bus-mastering. Since Windows 95 Mocrosoft's OS's have supported bus-mastering chipsets. If you have an older version of Windows 95, it is recommended that you consult your motherboard manufacture for the correct drivers.

EIDE drives are a very good choice for most users who need inexpensive storage space, or users who plan to use only a single drive in their system. If you need the most bang for your hard earned buck, you should use EIDE devices. If you do not have a SCSI controller, you are probably already using EIDE devices. Adding SCSI storage space would be expensive because a SCSI controller would also need to be purchased.

EIDE (Extended Interface Device Electronics) is by far the most common of the two, with late 486 and Pentium MOBO's having two EIDE controllers embedded in the motherboard. An IDE controller is only able to control two devices.

EIDE is the best choice for their home systems. EIDE drives always cost less than a comparably sized SCSI drive which needs a lot more onboard electronics. EIDE drives have a much lower price per megabyte, so a user can obtain more space for the same cost.

EIDE drives waste a lot of processor (CPU) power. The CPU usage often hits 100% when accessing the hard drive. The high CPU utilization is because the CPU directly controls everything that the EIDE hard drive does.

S.M.A.R.T.

- top-Monitoring Analysis and Reporting Technology

What is S.M.A.R.T.?

S.M.A.R.T. Is a system that enables the PC to predict the future failure of devices such as disk drives. Armed with a failure prediction, the user or system manager can back up key data, replace a suspect device prior to data loss, or avoid undesired downtime. S.M.A.R.T. Is a key component of improving data integrity and data availability of the PC.

Installing an IDE(EIDE) Hard Disk

Installing an IDE(EIDE) hard disk in a PC is a three stage process.

(1) The installation of the hardware

Basically consisting of opening the PC case with an appropriate screwdriver and securing the drive in an available bay with the screws provided and connecting the leads. You may however need to purchase an IDE ribbon cable with two drive plugs on. You may also need to buy an adapter tray to allow the drive to be mounted in a 5.25" bay.

Fig 29 5.25" bay adaptors

You must be aware that the drive's jumpers should be set so that it is either `Master' or `Slave on the IDE line.

(2) Setting the drive's parameters in the PC's BIOS.

The BIOS stores details of the IDE hard disks fitted to the PC and modern BIOS's have the ability to `autodetect' the drives parameters. Pressing the <DEL> key when the PC is powering up will start the BIOS's setup program, one option of which will be `Autodetect hard disks', this option should be selected for all four drives. Other devices such as CDROMS and ZIP drives will also be detected.

(3) A new drive will usually not have been formatted. If this is to be the Primary Master drive and you are installing the OS from a CDROM the formatting will be done during the setup process. If the drive is to be either the Primary slave, or a drive on the Secondary chain it will be recognised and assigned a drive letter when the machine is restarted.

Assuming the drive is a disk (HDD) attempting to open it from `My Computer' will probably return an error message "This Drive is not formatted, Do you wish to Format it now?." Your response should be yes. If you do not get this message `click, on the drive and a `pop up' dialogue box will appear, one of the options of which will be Format, Do so! and the drive will be ready for use.

SCSI

The acronym means "Small Computer Systems Interface." Simply put, it is a way to connect multiple devices to your computer. It is not a disk-drive only bus, unlike EIDE.

One of the major advantages of SCSI is that it can control multiple devices. With any SCSI controller, you may connect up to seven devices. These may be a tape backup unit, and a CDROM, external hard disks, or a scanner.

With a SCSI controller, the problems would be solved. Two interrupts would be left after installing everything except for the CDROM, tape, and disk drives. The user would then install a SCSI controller and use ONE interrupt and ONE DMA channel. The user would then have one interrupt free to use.

SCSI hard drives manage their own bad block maps. An EIDE drive has X number of sectors. If a sector fails, the operating system must mark that sector bad and will not to use it again. However, the OS only marks sectors bad after a read error has occurred on that sector. In other words, the sectors usually are marked bad after your data was stored on it and after the data that was stored there is gone.

SCSI devices function very differently. Almost every SCSI drive has what is known as a "hot-fix" area. A SCSI drive will have X active data sectors, plus Y sectors that are reserved to correct defects. If a SCSI drive detects an error that can be fixed using the EDC (Error Detection Code), it can correct and relocate the original data to a reserved sector. When the OS asks to access the damaged sector after it is relocated, the drive goes to the reserved and corrected sector instead. Some SCSI drives will relocate data in this manner automatically, and some will do this only when given special commands. This hardware approach is a much more elegant and reliable solution than the disk error correction provided with a software approach such as Norton Disk Doctor.

When a SCSI drive is SCSI formatted, any bad sectors are marked unusable by the drive ittop , and all the used "hot-fix" blocks are released.

One important feature of SCSI devices is a concept called disconnection. This allows a SCSI

device to use the SCSI bus only during the transfer of information, and not during the execution of a command. For example, let's say that the user has a CDROM and a hard drive attached to the same controller. Let's also say that the person is searching the CDROM for the word "teacup", and simultaneously playing a video file from the hard drive.

SCSI can connect more than just disk drives. Common uses for SCSI include any high data throughput devices, such as scanners, colour film recorders, very high speed printers, fast tape drives, and removable media devices such as CDROMS and Zip drives.

SCSI drives are typically of higher quality than their EIDE counterparts. Additionally, the latest technology is always available for the SCSI format first. This is because SCSI devices are used exclusively in high performance servers. Companies that buy servers are typically willing to pay a price premium for extra performance. Because of this, hard drive manufacturers usually incorporate new performance enhancing technologies into their SCSI devices first.

EIDE devices accept the commands in the order they are given. Seek to sector 10. Read it. Seek backwards to sector 5. Read it. Seek forwards to sector 7. Read it. Seek back to sector 1. Read it. Etc.

Command Queuing and Reordering

Earlier interfaces like SCSI-1 or IDE/ATA will allow only one command to be issued at a time to a device. Meaning that when a command was sent to a device, any other commands must wait for the first one to be actioned,

From SCSI-2 devices can re-order the commands so that the hard drives seek along a smaller summed distance. If the commands can be reordered so that they fall into a linear order, overall access time will decrease, increasing data throughput..

Here we see the way that both EIDE and SCSI would access three files. The three files are not stored in a continuous sequence and an explanation of why this occurs, follows. The EIDE drive will make three passes of the head, each picking up one file. You should remember that with EIDE devices, the CPU controls the head movements. The SCSI drive will pick up a part of each file as the head passes over it, sort the files into sequence and then pass them to the CPU, The SCSI controller manouvers the head, releasing the CPU to perform other tasks.

Fig 30 EIDE and SCSI drives accessing files

It is wrong to assume that files are stored as a continuous length on a hard disk, this may be the case when the disk was new but, as files are removed or updated they may be split into several segments and distributed about the disk. The record of where these parts of a file are kept is stored in the File Allocation Table (FAT) and it is this which most boot sector viruses attempt to destroy.

Fig 31 illustrates the way that each of a number of files, each represented by a different colour, may be distributed about a disk. The Grey areas are used by other files and the White areas are free space. As you can see there are small pockets of unused and because of their size, unusable space scattered around the disk, `Defragging' the disk will locate all of this spare space in on place, making it available to be used.

So SCSI drives `Reordering' allows these files to be retrieved faster.

Fig 31 `Fragmented' files

SCSI interface cards are available in two types, with or without a BIOS chip. Those without the BIOS cannot be used to boot the PC from, that is to say the PC must boot from an EIDE hard disk before you can access any SCSI device. Those cards with the BIOS may be used with the boot drive. If you do install a bootable SCSI card you will need to disable the embedded EIDE controllers on the MOBO, this is done within the motherboard's BIOS.

The following varieties of SCSI are currently implemented:

SCSI-1: Uses an 8-bit bus, and supports data rates of 4 MBps ·
SCSI-2: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector, and supports multiple devices. This is what most people mean when they refer to plain SCSI.

· Wide SCSI: Uses a wider cable (168 cable lines to 68 pins) to support 16-bit transfers.
· Fast SCSI: Uses an 8-bit bus, but doubles the clock rate to support data rates of 10 MBps.
· Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps.
· Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps.
· SCSI-3: Uses a 16-bit bus and supports data rates of 40 MBps. Also called Ultra Wide SCSI.
· Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MBps.
· Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps.

Installing A SCSI Device

A SCSI card with both SCSI 1 and SCSI 2 ports, one on the back to connect ecternal devices

The SCSI interface requires either a card to be added to the MOBO, or in the case of some high end MOBO's it may have the interface built in. With all but very old SCSI cards the system will detect the presence of the card and either install the drivers when the OS is installed, or if the card is added later prompt you to provide the drivers on disc, or CDROM.

The procedure when installing a SCSI device will depend upon wether it is a hard disc or not, as SCSI is not a hard disc only interface like EIDE, If the device is, for example a Scanner the drivers will need to be loaded from disc, or CDROM.

Each SCSI device will have to be configured with its own unique number rangeing from 0-8. This is usually done with jumpers, or if for example a scanner a small, often rotary switch on the device.

If the device is a hard disc, then, as with the EIDE installation the new drive will usually not have been formatted. If this is to be the Primary Master drive and you are installing the OS from a CDROM the formatting will be done during the setup process. If the drive is to be either the Primary slave, or a drive on the Secondary chain it will be recognised and assigned a drive letter when the machine is restarted.

Assuming the drive is a disk (HDD) attempting to open it from `My Computer' will probably return an error message "This Drive is not formatted, Do you wish to Format it now?." Your response should be yes. If you do not get this message `click, on the drive and a `pop up' dialogue box will appear, one of the options of which will be Format, Do so! and the drive will be ready for use.

Here we see a SCSI device, in this case a 35mm Slide scanner. Each SCSI device will need an ID number assigned, the chart abd dip wsitches will allow you to set the desired number. It should be remembered that the ends of the SCSI chain will need a 'terminator' this will either be as a switch on the last device, or as a plugin module,

A floppy drive chain

Floppy Drives

Most PC's Come with a single floppy drive normally designated A:, however the PC will support a second floppy drive should you wish to install one. A normal two floppy drive installation is shown here. The ribbon cable has a number of lines reversed just prior to the end connection. The A: drive should be connected to this plug, with the second drive or other device connected using the other plug. There are no jumpers on floppy drives that designate the drives identification (A: or B:) the reversed cables do this. The illustration also shows connectors for the now redundant 5.25" floppy drives. This cable will be plugged into the MOBO

A floppy drive chain

Floppy drives have been an integral part of the PC since its beginning in the early 1980's, originally as a 5.25" disk with a capacity of 360k. We are now all familiar with the 1.44Mb floppy disk, and although there is a 2.88Mb version available, it never seemed to find favour.

There is a feeling that the days of the floppy disk are numbered.

USB

Stands for Universal Serial Bus. It is an external bus and supports data transfer rates of 12 Mbps. A single USB port can be used to connect up to 127 peripheral devices, with each MOBO supporting two USB roots. USB allows the connection of devices such as mice, modems, scanners, printers and keyboards. USB also provides Plug-and-Play installation and hot plugging.

Hot plugging is the ability to add and remove devices to a computer while the computer is running then have the OS automatically recognize the change and install any drivers needed to make the device function. Both USB and Firewire support hot plugging. This is also a feature of PCMCIA. Hot plugging is sometimes referred to as hot swapping.

USB was first implemented in 1996, as a few MOBO manufacturers included USB support in their boards. It seems likely that USB will replace serial and parallel ports.

Firewire

Is another external bus standard that supports data transfer rates of up to 400 Mbps (400 million bits per second). Products supporting the 1394 standard go under different names, depending on the company. The technology which originally developed for the Apple Macintosh, It uses the trademarked name FireWire, but is known by other names, such as i.link and Lynx. A single 1394 port can be used to connect up to 63 external devices.

In addition to its high speed, 1394 also supports isochronous data, delivering data at a guaranteed rate. This is ideal for devices that need to transfer high levels of data in real-time, such as video devices.

Like USB, Firewire supports both Plug-and-Play and hot plugging, and also provides power to peripheral devices.

RAID

Stands for Redundant Array of Inexpensive Disks, a category of disk drives that uses two or more drives in combination to provide fault tolerance and performance. RAID disk drives are commonly used in servers but really are not necessary for domestic users where other forms of data security are available.

There are number of different RAID levels, some of them fairly esoteric, but in it's simplest form (RAID 0) information written to one disk is also written to it's 'mirror' disk. If one disk fails the other is seemlessly active.

© Allen. C. Roffey primerpc.com 12:13 15/01/2006