Hello Mr. ravindra gurjar,

The comparator used in the 89C2051 works the same as any analog comparator (i.e. lm311, lm393 etc). The following site give you information on how comparators work.
http://home.cogeco.ca/~rpaisley4/Comparators.html

You can only poll the p3.6 by using the jnb p3.6, address XXX or jb p3.6, address XXX (jump if bit is less or jump if bit is greater)

Normally one of the inputs to a comparator is used as a reference voltage (i.e., zener diode) and the other as an input which is compared to the reference to determine if greater or less than.

Good luck,

Charles Bannister