But actually the pulses are accepted in both directions. In one direction (required dir) all pulses are accepted and in the other only few are accepted.

say in required direction if I get 500 pulses for one revolution then in other direction I will get may be only 6-7 pulses.

Do a search, this has been discussed ad nauseam.
You can not make your code work, 2 reasons
1) the a= clock, B= dir fail in one of 8 cases of direction change.
2) when the encoder stop right over a boundary, you can get pulses of several MHz which your little '51 can not even attempt to keep up with.

Erik