Why I owe @slugonamission a (or several) large drink(s)

gdb icon, created for the Open Icon Library
gdb icon, created for the Open Icon Library (Photo credit: Wikipedia)

I have been struggling with a problem with my Microblaze simulation on OVPsim all week.

I have been trying to make a start on implementing a simple demand paging system and to trigger the hardware exception that the Microblaze CPU manual says should happen when you either try to read from a mapped page that is not in the TLB (ie, a “minor” page fault) or has not been mapped at all (a “major” page fault).

My problem was that – despite having, I thought, specified all the correct parameters for the Microblaze and knowing that I had virtual memory mappings work well, it simply was not happening for me. Instead of the expected exception handling, the simulation reported an error and exited.

But Jamie Garside from the University of York’s computer science department saved my life by (correctly) pointing out that what I also needed to do was turn on ICM_ATTR_SIMEX. Otherwise the simulation will always exit on an exception (or halt in GDB).

I can see why that might make sense – at least the halting in the debugger bit: if an exception is about to be fired you can halt the machine while it is still executing in virtual, as opposed to real, mode and see what has caused it.

It was also a reminder to RTFM – in this case not just the Microblaze manual, but also the OVPsim manual. I cannot rely on Jamie doing that for me everytime.

So to fix my problem I ORed in the missing attribute:

#define SIM_ATTRS (ICM_ATTR_DEFAULT|ICM_ATTR_SIMEX)