In this case, you should & with 0xff

Actually, I think it's sufficient to use $? & 0x7f (or $? & 127 in decimal) to test for abnormal termination.  That's what the code snippet in system suggests. Also, the system header files responsible for the exit status handling (e.g. /usr/include/bits/waitstatus.h in my case) usually define those macros (among others):

#define __WTERMSIG(status)      ((status) & 0x7f)

/* Nonzero if STATUS indicates normal termination.  */
#define __WIFEXITED(status)     (__WTERMSIG(status) == 0)
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(from that I would conclude that the core-dumped bit ($? & 128) doesn't occur in isolation (i.e. without WTERMSIG being non-zero) )