Re: [hatari-devel] Wrong bus error stack frame with 68000 and JSR

[Toni Wilen <twilen@xxxxxxxxxx>]

>> On real TT HW, it gives a bus error as it seems to not switch to
>> supervisor mode when fetching the "invalid instruction" vector
>> (remember, address < 0x10000 can only be accessed in supervisor mode on
>> Atari machines). This would be a bug in the 68030.
>>
>> On Hatari, it gives an "invalid instruction" because bit S is set (which
>> should be the normal behaviour)
>
>Actually, no, on Hatari this invalid instruction ("3584 6900") runs
>without any exception.

That is quite odd one.. I also get bus error (which is not possible normally!) when running it on A500 with 68030 expansion.

Stack frame type: $B, vector offset: 8.

Special status word:  $030f (FC=7. Hmm.., Data fault)

Fault address: $10 (Right..)

Hangs on 68020. This is probable the reason I skipped these in tester because CPU hung. My decision at that time was to ignore undefined behavior that can't be explained fully because there is always chance it is CPU model or revision specific.

I guess instruction is not fully decoded and CPU jumps to some random microcode address.