Ticker problem on ppc405

[Gene Smith]

I have tried to run the ticker example with no changes. I can see that 
it installs a interrupt handler for a Programmable Interval Timer (PIT) 
called from the typical location 0x1000 (with EVPR set to 0). However, 
when it actually runs I rarely see a vector to 0x1000. The first 
interrupt that occurs is usually to location 0x200 which is a machine 
check (bus error). When the machine check tries to branch to its actual 
handler (in _ISR_Handler) it looks like it sees a zero pointer which in 
turn causes a "Program" interrupt at location 0x700, which in turn 
causes a branch to zero and another Program interrupt, ad infinitum.

I only see explicit handlers being installed for the UART (an external 
interrupt which begins at location 0x500) and the PIT interrupt off of 
location 0x1000. I don't see any real handler installed for Machine 
Check or Program. I would assume these might be caught by a default 
spurious handler but the "blrl" at irq_stub.S:150 branches to location 0 
which just results in another Program exception.

The first Machine Check interrupt that gets the whole process going 
stores in register SRR2 the value 0x5938 which is supposed to be the bus 
error location. Location 0x5938 is actually an instruction in irq_stub.S 
(part of _IRQ_Handler) but I don't see a problem there and putting a 
breakpoint before location 0x5938 never hits before the first Machine 
Check at 0x200.

One other observation: In old_exception_processing/cpu.c:508 where the 
actual stwu_r1 value is compared to a template value they do appear to 
be equal but for some reason the else condition is taken and 
*old_handler is set to zero. This appears to be a possible compiler 
error. (I am checking this with no optimization -O0 in powerpc-rtems-gcc.)

If anyone has an understanding of this code I would appreciate any 
suggestions as to what is going wrong.

Thanks,
-gene
P/S: I can step through the code and see what is happening using the 
Macraigor JTAG debugger with gdb/insight.
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