Thanks a lot GARTHWILSON!!! My intention was mostly to understand some things rather than just do my homework! The problem is that nobody at the college cares if you learning! So i have to search and ask a professional ! thanks again!

OK! Let me say it again:
First of all this is an exercise from my college that i have to solve!
this exercise wants to program VIA when a button is connected to port B of VIA( no the whole keyboard) . Furthermore, it asks to program the VIA in order to cause an IRQ interrupt through CA2 line. When ...

I believe that your solution helped me a little, but still I have not understood where you program the port Α of 6522 and the CA2 in order to cause an IRQ interrupt to the CPU please read my solution and tell me where is the mistake:

LDA #0
STA PCR \\an negative edge of CA2 request interrupt ...

A 4x4 keyboard is wired with the port B of a 6522. When a button is being pressed , it will cause an IRQ interrupt through the CA2 line of VIA.
WRITE A PROGRAM which will read from port A of VIA the button which is pressed and save the button's code in the 0050H,0051H,..... until the button "ENTER ...