Evolution Of Microprocessor

1.1         DATA SIZE : NIBBLE, BYTE, WORD, LONG WORD.

     The capacity of a microprocessor is normally referred to how many bits of data can be handled at one time, or what is the memory size (the amount of data cells of memory, determined by the amount of address lines/bits) accessible by the system.  Thus it is important to have a great understanding about data size.

     Data size is a mean of measure to determine how much data can be stored in a single cell of memory.  Imagine you are looking at the post boxes at a post office.  Each box can store a certain amount of letter (assumed all letters have equal size).  The size of each post box the greater numbers of letter can be stored.  Similarly in memory storage, a memory storage can be distributed into many single cell with equal data size.  In digital form, the smallest size of a single cell is called 1 bit.

     If a single cell can store 4 bit of data, the cell size is called Nibble. Subsequently 8 bits is called Byte, 16 bits is Word and 32 is Long Word.

     A single cell sized 1 bit can store either logic 0 or logic 1.  In other word, two different situations can be stored or represented.  Thus the range of data is 0-1.

Data size : n = 1

Data capacity : 2ⁿ = 2¹ = 2

Range : 0  - 1 

A single cell sized 4 bit (Nibble) can store 16 possible situations.

Data size : n = 4

Data capacity : 2ⁿ = 2⁴ = 16

Range : 0  - 15 

    Figure above shows the different data sizes in a graphic manner.  Please note that the data size is determined by the number of bit (n), and is label from 0 to n-1.  For the data type Byte, Word and Long Word allocate the MSB as the sign bit, to determine that value of remaining bits is positive or negative.

     For the data which has more bits, it is easier to divide it into dual-half portions i.e. upper (MSB section) and lower portions (LSB section).

ü  1 Byte consists of two nibbles, upper nibble and lower nibble.

ü  1 Word consists of two bytes, upper byte and lower byte.

ü  1 Long Word consists of two word, upper word and lower word.