What is the use of ic D8255AC-2?

Answer 1

It's a programmable, general-purpose input/output device designed for the Intel 8080A/8085A processor lines. Basically it's a chip you can use as interface to various other electronics - storage, displays, ports etc. It has some features (like the ability to drive printer and display interfaces directly) that make it a one-stop solution to interfacing with many devices (that is why it's general-purpose).

What you do is send data from the CPU to this chip, and then switch its communications to another chip - the D8255AC has various interface options (and so is programmable) for different methods of communication that it will handle for you. It can be an input, an output, or both. You can hook up many devices (other chips) to it at the same time and communicate with them accordingly.

The D8255AC-2 is the 5MHz system clock variant (there is a 4MHz variant, the D8255AC-5).

Please keep in mind that you still have to make input/output levels (voltages) match, by using other chips in-line if required.