Using port 0 in AT89C52 without pull up resistors
Those numbers actually have nothing to do with Port 0. They're actually manipulating the address bits on Port 2 &mdash; specifically, P2.0/A8 and P2.1/A9.

These bits are connected to the digit select lines of the display. Obviously, if you want to display something on both digits, you need to time-division multiplex them.


  I tried to use other adresses such as 0xFA00 and 0xFB00 and it didn't work.


I can't explain that. It should have worked just as well. Maybe you should show us your code.