MEMORY LOCATIONS & ADDRESSES

Memory locations consist of millions of storage cells which can store a binary digit or bit having value 0 or 1.

Group of n bits, called as a word of information can be stored and retrieved in a single basic operation.

Accessing the main memory requires address for each word location 0 to 2k – 1.

Main memory of this computer can have up to 2k  words.

For example a 24 bit address generates an address space of 224 (16777216) locations ≡16M 

and a 32 bit address creates an address space of 232   or 4G.

Address	n bits				Word
0					word 0
1					word 1
	. . . .
i	bn-1	.  .  .  .  . .	b1	b0	word i
	. . . .
2k -1					word 2k-1

Representation of Numbers in main memory

Consider a 32 bit pattern is used to represent a signed integer

                                                                                                                 

b31

b30

b1

b0

            Sign bit           b31       = 0 for positive numbers

                                                 = 1 for negative numbers

            Magnitude       = b30 . 230 + b29 . 229  + . . . . . + b1 . 21  + b0 . 20

  Magnitude can range from 0 to 231 – 1 and the numbers are said to be in binary 

  positional notation.

 The above encoding format is called Signed Magnitude representation.

Representation of Characters in main memory

Characters can be letters of the alphabet, decimal digits, punctuation marks etc. 

They are represented by codes that are usually 6 – 8 bits long.

Shows how 4 characters in ASCII can be stored in a 32 bit word.                                      

8 bits	8 bits	8 bits	8 bits
ASCII Character	ASCII Character	ASCII Character	ASCII Character

Representation of Instructions in main memory

A main memory word can also be used to represent an instruction. One part of the word specifies the operation to be performed and the other part specifies operand address. 

Each of these parts are called as ‘field’.                                                     

8 bits	24 bits
Operation field	Addressing information

Here the 8 bit operation field can encode 28 (256) distinct instructions.

• The different ways in which operands can be named in machine instructions are called addressing modes.
• Memory words whose addresses are specified by the instructions are interpreted as operands. Whether an operand is a character or a numeric data item is determined by the operation field of the instruction.
• An operand may be either shorter or longer than one word. An operand length of 8 bit is convenient, because this size is used to encode character data. An 8 bit data is called a byte.