8051 is a 40 Pin microcontroller consisting 4 ports each port consists 8 pin i.e. a total of 4X8=32 pins. These pins are used to input/output operation some Pins have dual functions like timer interrupts etc. Among these 32 pins two pins are for power supply and ground (pin 40 and 20) two pins to connect crystal oscillator (pin 18 19) 3 pins to connect external memory (pin 29 30 31) and one pin for Reset function (Pin 9). It has two types of memory RAM and Flash memory (EEPROM) they have different sizes depending on the microcontroller you choose.