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More about memory and information representation

All information inside the computer and on external storage devices is represented in a form related to the Binary number system. Binary is a number system which uses the base 2 instead of the base 10 decimal system that is used in normal life. In the decimal system a positional number system is used which allows all numbers to be expressed using only the digits 0-9. Successive digits from the right represent the number of the corresponding power of 10. Thus 123 in decimal is

$$1 \times 10^2+2 \times 10^1+3 \times 10^0$$

that is, one hundred, plus two tens, plus three units. Similarly the binary number system builds all numbers from the bits 0 and 1, and powers of 2. 101 in the binary number system represents the number with value

$$1 \times 2^2+0 \times 2^1+1 \times 2^0$$

which is of course the decimal number 5. Using the binary system allows all computation inside the computer to take place using cheap two-state electronic devices.

Modern computers organise information into small units called bytes which hold eight bits, each bit representing a 0 or a 1. Each byte may hold a single character of text or a small integer. Larger numbers, computer instructions and character strings occupy several bytes.

The main memory can be thought of as a series of bytes numbered from 0, 1, 2, 3, ...upwards, each byte containing a pattern of eight bits which can be accessed by the CPU when it supplies the number, or Address, of the byte required. For example consider the section of memory illustrated below:

Address	Contents
3168	10110111
3167	01000111
3166	01010101

The byte with the address 3167 contains the binary pattern 01000111. Depending on circumstances the CPU may interpret this as an instruction, as a number (or part of a number) or as a character. This pattern can represent the character G in the ASCII character code that is used almost universally now or it could represent the decimal number 71.

It is important to keep a clear distinction in your mind of the difference between the address of a memory location and the contents of that memory location.