Tech support week two notes.

Digital Information Basics

The basic element of digital information is called the "bit", which is the contraction of the two words "binary digit". A bit represent the one single smallest piece of information a computer can store or process.

A bit can be imagined as a switch that can be in one of two possible positions: on or off. The transistors inside the computer's CPU ("central processing unit") actually function like this. They are either "On" or "Off", but never in between.

Otherwise, a bit can be represented as either a 1 (one) or a 0 (zero). In digital pictures, a bit is either a black or a white pixel. No grays allowed.

Present-day computers are based on what is known as eight bit code which means that all the code is divided into sections eight bits in length. That means that the zeros and ones (bits) form what are known as bytes, which look like this: 01001010 - or some other order of these same digits.

Once you reach a group of 1024 bytes, we call that amount of information a kilobyte. Just as in the metric system, kilo stands for "thousand". One thousand twenty-four bytes is also known as "1 Kb".

Once you have 1024 kilobytes, we call that amount of information a megabyte. Just as in the metric system, mega stands for "million". One thousand twenty-four kilobytes, or one million bytes, is also known as "1 Mb".

Once you have 1024 megabytes, we call that amount of information a gigabyte. Just as in the metric system, giga stands for "billion". One thousand twenty-four megabytes, or one billion bytes, is also known as "1 Gb".

Once you have 1024 gigabytes, we call that amount of information a terabyte. Just as in the metric system, tera stands for "trillion". One thousand twenty-four gigabytes, or one trillion bytes, is also known as "1 Tb".

Hard drives

A modern hard disk drive consists of several rigid metal, glass or ceramic platters sealed inside a dustproof enclosure, an electric motor and spindle and a magnetic head that rides on a cushion of air 2 / 1 000 000" (two one millionths of an inch) thick, above the surface which is coated with a magnetic film. Most hard drives have at least two platters, some up to eight. Data can be written to both sides of the platter. The coating is often an alloy about three millionths of an inch thick.

The read / write head travels across the disk surface, while the disk spins below it at a rate of 7200 rotations per minute (for standard drives) or about 300 miles per hour. The head must never come into contact with the platter, otherwise it would gouge into it and destroy any data on the part of the disk it crashed into. Technically speaking, only these kinds of mechanical failures can be described by the expression "my computer crashed", however many other technical failures have come to be described by "crash".

The hard drive works by using the properties of magnetism. When you zero all data on your drive, the magnetic read/write head passes all over the drive (which takes hours) and writes zeros to every possible location on the drive. This gets rid of all the information on the drive - whereas a format or low level format would not. Formatting a drive only rewrites the address tables.

During the zero all data process, what is actually taking place is that the head is using its magnetism to align all the magnetic particles in the coating of the platter in the same direction. A bit like making needles turn towards a magnet, the read / write head forces all the particles to align their poles in one direction. This is like having everyone lay down with their head pointing towards the north pole.

Data locations on the hard drive are determined by a set of sectors and tracks which divide the surface of the disk into equal parts. The tracks are concentric circles around the disk while the sectors are segments within each circle. To be able to know where to find a certain piece of data, the operating system and disk drive must record the track and sector address of each part of the file data.

A sector that cannot be used due to a physical flaw on the disk is called a bad sector. During the formatting process, the surface of the disk is examined and any bad sectors are noted and their locations written to the address tables, and marked so that no data will ever be sent to that sector.

FATs: your computer's white pages

Once the computer has readied the disk for reading and writing by dividing the surface into addressable sectors, it needs a way to be able to track the information itself. This is the "address tables" referred to in the previous paragraphs.

Since it would be much too long for the computer to look up every address and then figure out if what it was looking for was there or not, the computer writes an index of what is written and where it is on the disk. A database of what on the drive, essentially. It is much more efficient to query the database and then go read the information from the referred-to sector.

The type of database varies from operating system to operating system. The most often used formats are the following:

See: Wikipedia list of file systems and Wikipedia comparison of file systems