There is no actual harm due to the undefined values, we just want to know the facts. What we want to know further is whether there are other places with initial states different from the datasheet besides this register.
The reason for this inquiry is as follows. We believe that the correct sequence is to set the PCIMSK0 register before enabling the Valid bit of PCIBR0, but a software bug was found that sets them in the opposite order. There are cases where it starts up normally and cases where it does not, and we believe that the undefined initial value of PCIMSK0 is the cause, which is why we are confirming this time. (If the initial value is All0, it should never start up)
