Eradicate Null Values From Stored Procedure Input

my stored procedure is

create procedure t1 (@a int,@b int,@c int,@d int,@e int,@f int)
as
begin
select no,name,department from emp where a = @a and b =@b orc =@c or
d = @d or e = @e or f = @f
end

my problem is while executing i may get null values as input to the stored procedure .
how to validate them ? any ideas .
are there any arrays.
if c and d are null then my condition would be
where a = @a and b =@b or e = @e or f = @f
how many loops shall i have to write.