The AT89C51 from ATMEL is the most commonly used variation of the 8051 microcontroller because of the built-in Flash which makes programming / burning very easy. The 8051 has a total of 4 ports for input / output operations which means you can transfer data in or out of the microcontroller through these ports. Let's have a brief introduction to these ports.
UPDATE: You can also click this link for an updated version of this post.
UPDATE: You can also click this link for an updated version of this post.