Describe PMBR in terms of syslinux GPT extension.

boot.c

 /*-
  * Copyright (c) 2007 Yahoo!, Inc.
  * All rights reserved.
  * Written by: John Baldwin <jhb@FreeBSD.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 161 matching lines @@
         close(bfd);
 
 #ifdef LINUX
         /* We use the syslinux GPT new protocol. This requires the GUID
            of the boot partition to be at offset 424 in the mbr. See the
            doc in the syslinux source: src/doc/gpt.txt */
         if (gpt_find(boot_entry, &boot_uuid, &gptboot) != 0) {
                 warnx("unable to find partition to make bootable.");
                 return;
         }
-        le_uuid_enc(((unsigned char*)mbr + 424),
+        le_uuid_enc(&mbr->boot_guid,
                     &((struct gpt_ent*)(gptboot->map_data))->ent_uuid);
+        mbr->magic_number = htole16(MBR_MAGIC);
 #endif
         gpt_write(fd, pmbr);
 
 #ifndef LINUX
         /* Third step: open gptboot and obtain its size. */
         bfd = open(gptboot_path, O_RDONLY);
         if (bfd < 0 || fstat(bfd, &sb) < 0) {
                 warn("unable to open GPT boot loader");
                 return;
         }
@@ skipping 101 matching lines @@
                         continue;
                 }
 
                 boot(fd);
 
                 gpt_close(fd);
         }
 
         return (0);
 }