1) the number of bit ADDRESSABLE spaces do not vary from derivative to derivative, it is a fundamental part of the '51.
2) any bit anywhere can be tested, manipulated or otherwise processed, it just takes a little longer
e.g. to test a bit
mov a,byte_of_bits
jb xxx
3) If you are using "C" the compiler will do it for you.
4) If you need full optimal processing (bit addressability) of all 256, look at the Philips XA.

Erik