What mean by quasi-bidirectional and true bidirectional IO ports in the 8051 microcontrollers?
What is the difference between these two type of io ports?

 

The definitions are included in the descriptions. True bidirectional port bits can be set to being imputed in which case they die a high impedance and are not internally driven (when set as outputs),

A quasi bidirectional port, when set to being an input,has a pull-up current source connected to it all the time it is used as an input.

In low power applications a quasi directional port will usually cause a higher current drain than the true bidirectional port. The trade-off is that the quasi bidirectional port does not need external pull-ups when reading switched to ground or open collector outputs,