Technical The Internal Components Disks - The Basics

Disks - The Basics

So that you have clear idea of exactly what a disk is, I am going to take the time to cover the basics. Every piece of software from applications, to documents created by them usually makes its home on your disks. Understanding how data is stored on disks, will help you better use what you have and give you a better understanding when it comes to purchasing upgrades.

Most users operate from Windows, which uses the disks under the supervision of DOS (Disk Operating System). So later I will delve into this perspective.

The disk storage that a computer uses is two things that work together. A recording / reading head technology and an access device. Other than optical disks, the recording is magnetic. This form of recording was first seen in sound, an analogue form of information. This was adapted for the digital recording that computers operate with.
The disk surface is treated as an array of bits that can be set to the equivalent of 0 or 1. The precise location of the bits isn't known until some markings for guidance are laid down on the surface. Hence the need to format a disk before it can be used.

After data is stored on a disk it needs to be accessed and is made possible by the access device. The two things that make it possible to get at anywhere on a disk surface is rotation and head movement. The disk spins round very quickly so that any part of it is under a given point. A floppy disk at 300 RPM and hard drives upwards of 5400 RPM. The head moves in a manner similar to a record player and can skip across the disk from outside to inside. Most hard drives have multiple heads and platters and also multiple heads on each platter to speed up access times and decrease delays. When you combine moving the head and rotating the disk into position under the head you can see any part of the disk very quickly and is why a computer disk is called random access storage, unlike a tape which needs to forward past all the unwanted data in a linear manner.

A disks surface is divided into tracks forming rings, starting from the outer edge of the disk. The number of tracks vary with the type of disk. Floppy disks generally have 80 tracks, while on hard disks it is common to have between say 500 to over 1000 tracks. However many there are they are numbered from zero (0), again, starting from the outer edge.

The disk surface is further broken down into sectors. Again the type of disk and its format determine the number. There is more sectors per track on the outer cylinders. However many there are, they remain a fixed size on the disk.

All reading and writing is done as complete sectors. Although data can be any size, the actual disk I/O is performed only in complete sectors. Like the tracks on the surface, the sectors are identified by numbers, starting from zero (o).

Like anything flat, a floppy disk has only two sides. However as hard drives generally contain more than one disk platter, they are often referred to as having more than two sides. Again, for identification purposes the sides of the disks are numbered starting from zero for the first side.

Another term used is cylinder which refers to the set of all tracks, one on each side, that lie at the same distance from the centre of the disk. If a drive has three platters then each cylinder consists of six tracks. The cylinders are also numbered started from zero (0).

If you combine all of these dimensions you arrive at the storage capacity of the disk. Multiply the number of tracks per side by the number of sectors per track, giving the total number of sectors per disk. Multiply this by the number of bytes per sector to get the raw capacity of the disk. Some of this is occupied by overhead, but the value is essentially the storage capacity of the disk.

Floppy Disks

The kind of floppy disk most of you will encounter is the 3.5". This type of disk is enclosed in a rigid protective case. They hold 1.44 MB of data in their most common format, but IBM created another rather unpopular 2.88 MB 3.5" floppy.

Inside, a floppy disk is a soft flexible plastic with a metal hub piece. A sliding cover on the outside protects the read/write opening.

A write protect notch is provided which you simply slide the tab, so that the window is either open or closed.

On a high capacity disk you will see a small hole in the corner, opposite the write protect tab, which signifies the disk is high density.

Hard Disks

There are many varieties of hard drives that differ in speed, the number of platters, cylinders, sectors and other features. Typically hard disks platters are made of an aluminium alloy, which are sealed inside the disk drive. All hard disk look the same from a users point of view.

Various restrictions have been imposed on hard drives in the past. Previous versions of DOS could not recognise hard disks greater than 32 MB. Large hard drives had to be broken into partitions not larger than 32 MB.

Hard disks are not removable media, however there are significant exceptions to this due to new features in the BIOS which allows booting from different drives and availability of removable drive caddies that facilitate this feature.

While the smaller hard disks which are now obsolete held a massive 5 MB newer systems typically hold over 50 GB. Don't be surprised if, when you read this, even these hard disks may be obsolete.

Disk Controller Types

Data is moved from the disk drive into memory and back again through ports. Controller ports are either plug in hardware boards, usually PCI these days or the disk interface is built into the motherboard. There are several disk controller standards namely IDE, E-IDE, SCSI and recently PCMCIA.

IDE (Integrated Drive Electronics) was a first attempt to resolve too many incompatible ways to interface a drive to a computer. There are three parts to the controller interface. The computer talks to the controller and vice versa, the controller manipulates the data and has to talk to the disk drive. Most of the intelligence for move the data back and forth used to be on the controller card itself, which meant that to add a new hard disk or replace an existing one, you had to ensure that the controller was completely compatible with any new hard drive. IDE controllers are on board the disk drive and hence the interface between the drive and computer is relatively simple. As long as the drive is compatible with the interface on the computer, it doesn't make a lot of difference how a manufacturer designs a hard drive.

IDE has several limitations. There is a maximum drive size of 528 MB and the controller only supports two hard disks.

E-IDE (Enhanced IDE and Mode 3) - Several manufacturers of IDE drives introduced the enhanced IDE interface in 1994. The E-IDE standard removes the 528 MB limit to an 8 GB limit. Also E-IDE controllers support four devices instead of two. Another difference is that E-IDE controllers also include standards for CD-ROM drives and tape backup drives as well. The data transfer rate of E-IDE drives changes to Mode 3 which can achieve data transfer rates several times that of IDE, which are Mode 2 specification.

These data transfer rates have since increased to Mode 4, UDMA, UDMA33, UDMA66 and recently UDMA100. Obviously these standards will continue to increase in line with common bus speeds found on modern motherboards.

To benefit from improved performance and capacity both the drive and controller must be of the highest mode. Installing a UDMA66 hard drive on a UDMA33 controller would mean the drive operating in UDMA33 mode and not the higher transfer rate it is capable of.

SCSI (Small Computer Systems Interface) and pronounced "skuzzy" has been widely available on the Macintosh and SUN platforms for some time. A SCSI disk interface is more like a bus than the dumb interface of IDE. A SCSI port can interface seven devices from a single interface card. In toady's desktop systems many users experience conflicts with interrupts and other bus signals, SCSI can help solve those problems. Additionally SCSI devices are intelligent and capable of communicating with each other, therefore offering performance in most circumstances. SCSI commonly delivers 5 MB per second, data transfer rate, Fast SCSI (SCSI-2) up to 10 MB/s and SCSI Fast and Wide (SCSI-3) up to 40 MB/s.

SCSI is the choice for disk storage media on high end, multimedia and database development systems. The main reason all of you do not have SCSI drives in your computers is the cost. Typically SCSI drives cost three to four times that of an equivalent IDE and there is the initial additional cost of the interface card, where as IDE controllers are built-in to most standard motherboards. (The one's that most people get).

Bus Mastering

Bus Mastering hard drive controllers were introduced and solved several issues. Before bus mastering, if two E-IDE devices, such as a CD-ROM and a hard disk were connected to the same interface, they operated in the mode of the slowest device. Bus mastering enabled each device attached to an interface to operate at its maximum transfer rate. Bus mastering also enables booting from any device attached to any interface such as a CD-ROM drive or SCSI drive on the primary, secondary, master or slave.

Bus mastering as a feature enables a controller to communicate directly with other devices such as a sound or graphics cards, without going through the CPU, hence improving performance.

The Basic IO Ports

The Power Supply (PSU)