11/3/71                                                  CORE (V)

NAME            format of core image

SYNOPSIS        --

DESCRIPTION     Three conditions cause UNIX to write out the core

                image of an executing program: the program gener-

                ates an unexpected trap (by a bus error or ille-

                gal instruction); the user sends a "quit" signal

                (which has not been turned off by the program); a

                trap is simulated by the floating point simula-

                tor.  The core image is called "core" and is

                written in the current working directory (pro-

                vided it can be; normal access controls apply).

                It is exactly 8192+64 bytes long.  The first 8192

                represent the actual contents of memory at the

                time of the fault; the last 64 are the contents

                of the system's per-user data area for this pro-

                cess.  Only the first word of this area will be


                When any trap which is not an I/O interrupt oc-

                curs, all the useful registers are stored on the

                stack.  After all the registers have been stored,

                the contents of sp are placed in the first cell

                of the user area; this cell is called u.sp.

                Therefore, within the core image proper, there is

                an area which contains the following registers in

                the following order (increasing addresses):










                        pc (at time of fault)

                        processor status (at time of fault)

                The last two are stored by the hardware.  it fol-

                lows that the contents of sp at the time of the

                faul were (u.sp) plus 22(10).

                The t-bit (trap bit) in the stored status will be

                on when a quit caused the generation of the core

                image, since this bit is used in the implementa-

                tion of quits.

FILES           --

SEE ALSO        --


BUGS            --

OWNER           ken, dmr