Computers and information

To understand how computers use transistors in their operation, we briefly describe how information can be represented and manipulated via switches. An early illustration of this idea is the Morse code - letters and numbers are represented by a particular series of dots and dashes. In computers dots and dashes aren't used, but rather electrical signals, but the idea is the same - certain sequences represent particular letters and numbers. For example, to represent the numbers we might use the following sequence of signals:

 

Table 12.2: Binary representation of the first ten numbers

Number	0	1	2	3	4	5	6	7	8	9
Binary form	0	1	10	11	100	101	110	111	1000	1001

 

In this table, a ``1'' means the presence of a signal, and a ``0'' means the absence of a signal.

Information content is measured in bits (binary digits), with one bit being able to represent two possibilities (on or off). For the numbers in the table above we needed 4 bits to represent each number, but if we wanted to include all the letters of the alphabet, both upper and lower case, as well as a few other special symbols, we would need more bits. In the standard ASCII character set 256 characters are used, and so each such character would need 8 bits (2⁸ = 256) of information to specify it. One defines 1 byte of information to be equal to 8 bits.

Let us estimate the information content of a typical text, assuming it is made up of only the ASCII characters. If one character represents one byte, then a typical word (of 6 letters long) represents 6 bytes. If there are 10 words per line, and 30 lines per page, then each page represents about 1800 bytes. A 500 page text thus represents approximately 900,000 bytes, or just under 1 Megabyte (MB), of information. This can can stored on a standard 1.44 MB floppy disc. A CD-ROM, with a storage capacity of about 1 Gigabyte (GB), can store about 1000 of these texts.

This information is stored and accessed on a computer in a number of different ways. On floppy discs, hard discs, magnetic tapes, and magnetic stripes on the back of credit cards, it is in the arrangement of the magnetic fields of the atoms within the media: one particular orientation of the magnetic field might represent an on switch, and the opposite orientation an off switch. In a CD-ROM, grooves are etched in the media and a laser scans the disc; this light then gets reflected back, and so interferes with the incoming light. The interference pattern depends on whether the light hit a groove or not, and so the grooves can be interpreted as representing on-off switches (the same principle works for bar code readers, but instead of grooves the two different interference patterns are caused by alternating dark and light bands). Finally, in the ROM (read only memory) and RAM (random access memory) of a computer, transistors and related electronic devices are used as switches to represent information.

Transmission of this information occurs either via a series of electrical pulses, as over electric or telephone lines, or via the faster and more efficient fiber optic cables, where pulses of light are used.

    Information in the form of on-off switches is then fed into the computer where it is manipulated by the CPU (central processing unit) according to some predefined instructions or algorithms. These manipulations are again handled by transistors arranged in special combinations called gates. These days millions of such transistors constitute an integrated circuits contained within a microchip, which can be smaller than the tip of your finger.