Its very simple my dear frnd vishal,
To be explained to you in a general language,initially the shares use to be in the physical form,but now completely it has been made in a demat form.The person who has purchased shares initially,now has to convert it in to demat form.
Now the shares are available only in demat form.So as per the Income Tax Act,1961,when the shares are sold,for the purpose of computation of capital gain,it has to be calculated on the basis of FIFO method.
EX:Y purchased 100 shares in1990 in physical form and converted in to demat A/C in 2011.Mean while he putchased 200 shares in 2010 in demat form.In 2012 he sold 150 shares.Now how to calulate capital gain?
Ans:The capital gain shall be calculated on the shares which has been purchased in 2010 i.e,in Demat A/C.Its not that you have purchased 1st shares in 1990 and you have to take it as a basis for computing C.G.
The answer would be different if the shares purchased in 1990 converted in to Demat form in 2005 i.e, here for the purpose of computing C.G,you have to take this shares as a basis.
In simple terms,the shares which has been 1st credited in to Demat A/C should be considered for computing C.G