• The 89c51 has 128 byte internal RAM and divided into 3 parts/area.

In first part :

• 32 bytes from addresses 00H to 1FH i.e. 32 registers organised in 4 banks. Each bank contains 8 registers (each of 8 bit) and available at each address.

• Registers are named as:

Bank 0 –R0, R1, R2, R3, R4, R5, R6, R7

Bank 1 –R0, R1, R2, R3, R4, R5, R6, R7

Bank 2 –R0, R1, R2, R3, R4, R5, R6, R7

Bank 3 –R0, R1, R2, R3, R4, R5, R6, R7

• If register banks are not selected, it can be used as general purpose area.

• On reset Bank 0 is selected.

• RS1 and RS0 bits (D3 and D4) of PSW register are used to select the register bank.