NOFAULT(3spp)       RISC/os Reference Manual        NOFAULT(3spp)



NAME
     nofault - fault handling for standalone programs

SYNOPSIS
     #include <saio/saioctl.h>

     int *nofault;

DESCRIPTION
     Nofault is a global variable that can be assigned the
     address of a jmpbuf (see setjmp(3spp)).  If nofault is not
     zero, a longjmp will be made via the jmp_buf pointed to by
     nofault. Nofault is typically used in code sequences like:

     #include <saio/setjmp.h>
     #include <saio/saioctl.h>

          jmp_buf jb;
          extern int *nofault;

          if (setjmp(jb)) {
               /*
                * control is transferred here after a fault,
                * place fault handling code here
                */
          } else {
               /*
                * setting nofault to address on jmp_buf enables
                * longjmp to fault handler above, should fault
                * occur
                */
               nofault = jb;
               /*
                * code that may cause a fault goes here
                */
          }
          /* cancel special fault handling */
          nofault = NULL;

     Nofault is cleared while performing the longjmp.  Once a
     fault has been taken, the fault handler must be re-enabled
     by assigning a value to the global variable nofault.  When a
     fault is taken, the standalone library switches to a second
     stack referred to as the "fault stack" and always saves a
     small amount of state for the fault handler to examine.  The
     following global integer variables can be examined by the
     fault handler to determine the nature of the fault:
          _epc_save           pc at time of fault
          _exc_save           exception vector type
                              Indicates if exception was via "utlbmiss", or
                              general exception.
                              Literals EXCEPTUTLB, and EXCEPTNORM are



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NOFAULT(3spp)       RISC/os Reference Manual        NOFAULT(3spp)



                              defined in <saio/saioctl.h>
          _badvaddr_save      system coproc. badvaddr register
          _cause_save         system coproc. cause register
          _sp_save            stack pointer at time of exception
          _sr_save            system coproc. status register
          _stack_mode         stack in use at time of exception
                              Literals MODENORMAL and MODEFAULT are
                              defined in <saio/saioctl.h>

     Additional state beyond that shown above is saved, if the
     global variable _stack_mode is equal to MODENORMAL at the
     time of a fault.  In this case, the contents of the regis-
     ters are saved into the global integer array _regs and
     _stack_mode is changed to MODEFAULT.  If the floating point
     coprocessor was marked as usable in the status register at
     the time of a fault on the normal stack, its state is also
     saved into _regs.  Further faults will not alter the array
     _regs until _stack_mode is reassigned to MODENORMAL.
     _stack_mode is initialized to MODENORMAL when a program is
     started.  The _regs array can be subscripted with the
     literals RR0 .. RR31.  These literals, and literals for
     offsets to other registers saved in the _regs array, are
     defined in <saio/saioctl.h>.  The context that is
     represented by the contents of _regs can be resumed by cal-
     ling the routine _resume(3spp).  Resume restores all regis-
     ters from the contents of _regs; floating point coprocessor
     registers will be restored if the coprocessor is marked
     usable in the status register saved in _regs.  Calling
     _resume resets _stack_mode to MODENORMAL.

SEE ALSO
     setjmp(3spp)























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