What is mean by Special Function Registers? 

Special Function Registers (SFR): 

8051 has a set of Special Function Register (SFR) which may be addressed like Internal RAM using addresses from 80H - FFH Some SFRS are bit addressable This feature allows programmer to change any specified bit. 


All the addresses from 80H - FFH are not used. SFRS are used by their function name in some Instructions; whereas they can be addressed by their 8-bit addresses.