There are 21 special function registers (SFR) in 8051 microcontroller and this includes Register A, Register B, program status word (PSW) PCON, SCON, TCON, etc.

There are 21 unique location for these 21 special function registers and each of these registers is of 1 byte size some of these special function register are bit addressable (which means you can access and individual bits inside a single byte) other are only byte addressable.

SFR are accessed as if they were normal internal RAM. The only difference is that internal RAM is from address 00H thtough 7Fh whereas SFR registers exist in the address range of 80H throughFFh.

Each SFR has an addresses (80H through FFh) and a name. The address range of 80 H through FFh offer 128possible addresses there are only 121 SFR in standard 8051 All other addresses in the SFR range (80H through FFh) are consider invalid writing to or reading from these register may produce undefined values or behavior.