hi,
this schematic has some problems if be compared with external interrupt controller.
The main bad thing I see is that this schematic does not lock interrupt events. As result, it may not be used to control of edge-triggered interrupts.
For example, an external hardware produces short pulse which comes to a port pin and via 8-gate to /INT0 as well. So its ISR executes and we read port state in it. But if the pulse is short then all we obtain is just 0xFF value. Now we know that it was interrupt event but cannot determine which one. Here we have two problems with it:
- due multiply pulses of button`s contacts (remember about debounce?) our ISR will be executed many times but there is no warranty that it will be able to recognize which button is pressed;
- if at least one button is pressed and then keeps hold then we not be able to recognize other buttons` state changes without using polling port method.

So think twice before use this scheme.

Regards,
Oleg