Hi Marco,

my question was already the answer:

There is no main loop in your program !
So it can never work and your simulator seems very poor, that it never detect the fault.

I assume, it runs still from the reset vector again and again. Since your main runs into the interrupt handler and the RETI cause pushing an uninitialized return address.

Good simulators alarming, if variables read before written first.

Poor simulators still load all RAM with 00h but this was not real. Any RAM was loaded randomly after power on.

The smallest main loop:

jmp $


Peter