What an instruction essentially consists of ?

Basically An instruction comprises of two parts ,those are 


1. operation code (called ‘opcode’) and 
2. the address of the data(called ‘operand’), on which the opcode operates. 

This is the structure on which an instruction is based. The opcode specifies the nature of the task to be performed by an instruction.

                          
                     Operation code                                Address of data

Dipending upon the Instruction Word we can Devide the instuction set in three Catagory 

                                                                         EXAMPLE  

 1-byte instruction :                                            ADD B 
 2-byte instruction :                                            MVIC, 07
 3-byte instruction :                                            LDA 4400

In 1-byte instruction, the opcode and the operand are in the same byte i.e.,

A 2-byte instruction looks like this:
                                         1st byte
                         2nd byte
While a 3-byte instruction looks like the following:
                                        1st byte
                        2nd byte
                       3rd byte