Thanks to Stefan Roese, I think I definitely solved this bothering problem. It seems the cause is a "hidden" memory access to an invalid location (for example to an address in the SDRAM region before memory controller has been initialized). When stepping through the MTMSR instruction, it seems GDB generates some of these accesses even if the user does not perform a memory read (I suspect such an access is performed to the address contained in the EVPR register). I solved the problem by initializing the SDRAM controller even if the code to be debugged does not need it. This way all accesses to locations ranging from 0x00000000 to 0x08000000 are "well-behaved" and don't cause any exception.

llandre