Actually, the arithemetic trap has nothing to do with alignment.
The program "burst" was actually delivered a SIGFPE. I don't off
hand recall what summary==4 means. That is for debugging only,
and really should be removed at this point.
As for the free page warning, you were running low on physical
memory. It is primarily a debugging message. The kernel should
have recovered, either by swapping, or killing processes. The
fact that there was no oops after the message indicates that it
succeeded.
r~