RESTART(5spp)       RISC/os Reference Manual        RESTART(5spp)



NAME
     restart - description of standalone communication block

SYNOPSIS
     #include <prom/entrypt.h>

DESCRIPTION
     The restart block is used to communicate information between
     various components of the MIPS standalone system.  Informa-
     tion conveyed in the restart block allows the PROM monitor
     to warm start existing program images, and dbgmon to commun-
     icate with its clients.

     The format of a restart block is:
     struct restart_blk {
          int  rb_magic; /* magic pattern */
          int  (*rb_restart)();/* restart routine */
          int  rb_occurred;   /* to avoid loops on restart failure */
          int  rb_checksum;   /* checksum of 1st 32 wrds of restrt */
          char *rb_fbss; /* start of PROM bss and stack area */
          char *rb_ebss; /* end of PROM bss and stack area */
          /*
           * These entries are for communication between the debug monitor
           * and the client process being debugged
           * NOTE: a return value of -1 from (*rb_vtop)() is distinguished
           * to indicate that a translation could not be made.
           */
          int  (*rb_bpaddr)();     /* breakpoint handler */
          int  (*rb_vtop)();  /* virtual to physical conversion rtn */
     };
     Constants that are used in conjunction with the restart
     block are:

     #define   RESTART_MAGIC  0xfeedface
     #define   RESTART_CSUMCNT     32        /* cksum 32 words of restart routine */
     #define   RESTART_ADDR   0xa0000400     /* PROM looks for restart block here */

     The block is always located at physical address 0x400, and
     is referenced by the PROM monitor via the k1seg address
     0xa0000400.  The fields in the restart block have the fol-
     lowing uses:

     rb_magic
          This field is checked by the PROM monitor at the time
          of a warm start to determine the validity of the res-
          tart block.  In order for a warm start to occur, this
          field must be equal to the value RESTARTMAGIC.  This
          field must be assigned RESTARTMAGIC by the standalone
          program to indicate its desire to permit warm starts on
          its image.

     rb_restart



                         Printed 1/6/92                    Page 1

RESTART(5spp)       RISC/os Reference Manual        RESTART(5spp)



          This field indicates the address to transfer to in
          order to initiate a warm start on the current memory
          image.  This field must be assigned by the standalone
          program.

     rb_occurred
          This field must be zero to allow a warm start, this
          field is set non-zero by the PROM monitor once it has
          attempted a warm start to avoid infinite loops should
          the warm start fail.

     rb_checksum
          This field should be assigned the 32 bit, 2's comple-
          ment sum of the first 32 words starting with the
          address in rbrestart.  The PROM monitor calculates the
          sum of the first 32 words of the warm start routine and
          compares its sum with the value in rbrestart to ensure
          that the restart code is valid.

     rb_fbss
          The address of the first word of memory used by PROM
          for its bss segment.  This address is a "k1seg"
          address.  Note that the PROM also "uses" the utlbmiss
          and general exception vectors along with the restart
          block itself, all of which lie outside of its bss
          region.

     rb_ebss
          The address of the first work beyond the PROM bss seg-
          ment.

     rb_bpaddr
          This location is non-zero if the debug monitor,
          dbgmon(1spp), is present.  The non-zero contents of
          rbbpaddr is the address of the debug monitor entry
          point for breakpoint handling.  If rbbpaddr is non-
          zero, standalone programs that field their own excep-
          tions should transfer to this location if they receive
          a breakpoint exception via a "break 1" instruction.
          Before transferring to this location, the standalone
          program should restore all registers except "at" and
          "k0" to the contents the registers had at the time of
          the exception.  The "k0" register should be set to the
          contents of the "at" register at the time of the fault,
          and the "at" register should be loaded with the con-
          tents of rbbpaddr and a jump register performed to
          enter the debug monitor breakpoint handler.  If
          rbbpaddr is zero, the debug monitor is not present;
          the breakpoint exception should then be reported and
          the program terminated.

     rb_vtop



 Page 2                  Printed 1/6/92

RESTART(5spp)       RISC/os Reference Manual        RESTART(5spp)



          A standalone program may offer to translate "kuseg" or
          "k2seg" addresses to k0seg or k1seg addresses by
          assigning rbvtop to the address of a routine that pro-
          vides this service.  This routine is called with a sin-
          gle parameter, which is a kuseg or k2seg address; it
          should return a corresponding address in k0seg or
          k1seg.  The translation should be based upon the
          current process identifier in the R2000 tlbhi register.
          If for some reason the translation can not be made, the
          routine should return -1.

     NOTE: currently the dbgmon command "dump" does not call the
     rbvtop entry point for kuseg or k2seg addresses; the dump
     command must be given k0seg or k1seg addresses.

SEE ALSO
     dbgmon(1spp), prom(1spp), warm(1spp)






































                         Printed 1/6/92                    Page 3