Nested interrupts The AVR hardware clears the global interrupt flag in SREG before entering an interrupt vector. Thus, normally interrupts will remain disabled inside the handler until the handler exits, where the RETI instruction (that is emitted by the compiler as part of the normal function epilogue for an interrupt handler) will eventually re-enable further interrupts. For that reason, interrupt handlers normally do not nest. For most interrupt handlers, this is the desired behaviour, for some it is even required in order to prevent infinitely recursive interrupts (like UART interrupts, or level-triggered external interrupts). In rare circumstances though it might be desired to re-enable the global interrupt flag as early as possible in the interrupt handler, in order to not defer any other interrupt more than absolutely needed. This could be done using an sei() instruction right at the beginning of the interrupt handler, but this still leaves few instructions inside the compiler-generated function prologue to run with global interrupts disabled.
The compiler can be instructed to insert an SEI instruction right at the beginning of an interrupt handler by declaring the handler the following way:

ISR(XXX_vect, ISR_NOBLOCK)
{
...
}

bo przecież procedura przerwania nie zaczeka w kolejce na odblokowanie przerwań(a może jednak zaczeka...?).

i wychodzi na to że jednak zaczeka. Jeżeli wystąpi zbocze na INT0/INT1 nawet w trakcie wykonywania innego przerwania, zostanie ustawiona flaga INTFx i potem, jak bit I w rejestrze SREG zostanie ponownie ustawiony to przerwanie INT zostanie obsłużone....