There are four ports in the microcontroller 8051 but only port o is known as bidirectional port.
On most microcontrollers, I/O port pins can be input-only, output-only or bidirectional. There are physical differences between each of these types of pin; the data sheets typically show the configuration of the output stage so you can see exactly what you are dealing with. For bidirectional ports, there is normally a data direction register which you load with values indicating how you want to use each pin on a given port (generally defaulting to input for safety). Read through the data sheet; it will give you detailed information on the electrical and logical interfaces to each pin.
The microcontroller port may be input, output or input/output port. this difference cab be obvious from its datasheet. an input/output port can be programmed to be an input or an output according to your need by control word.
You can program any port (0,1,2,3) as bidirectional port by writing 0xFF -- Input, and 0x00-Output. All port functions both as input and as output. Port function also depend on the external interface with it, to successfully work as input or output.
But as per datasheet and my practical usage, all port have the capability to function as Input and Output.