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BAD144(8)										BAD144(8)

       bad144 - read/write DEC standard 144 bad sector information

       bad144 disktype disk [ sno [ bad ...  ] ]

       Bad144  can  be	used to inspect the information stored on a disk that is used by the disk
       drivers to implement bad sector forwarding.  The format of the information is specified by
       DEC standard 144, as follows.

       The  bad  sector  information  is located in the first 5 even numbered sectors of the last
       track of the disk pack.	There are five identical copies of the information, described  by
       the dkbad structure.  Only the first of these copies is used.

       Replacement  sectors  are  allocated  starting with the first sector before the bad sector
       information and working backwards towards the beginning of the disk.  A maximum of 126 bad
       sectors	can  be supported.  The position of the bad sector in the bad sector table deter-
       mines which replacement sector it corresponds to.

       The bad sector information and replacement  sectors  are  conventionally  only  accessible
       through the ``h'' file system partition of the disk.  If that partition is used for a file
       system, the user is responsible for making sure that it does not overlap  the  bad  sector
       information or any replacement sectors.

       The bad sector structure is as follows:

       struct dkbad {
	      long    bt_csn;		  /* cartridge serial number */
	      u_short bt_mbz;		  /* unused; should be 0 */
	      u_short bt_flag;		  /* -1 => alignment cartridge */
	      struct bt_bad {
		      u_short bt_cyl;	  /* cylinder number of bad sector */
		      u_short bt_trksec;  /* track and sector number */
	      } bt_bad[MAXBAD];

       Unused slots in the bt_bad array are filled with all bits set, a putatively illegal value.
       MAXBAD (in <sys/dkbad.h>) may be tuned locally to reduce the space required  to	hold  the
       bad-sector file in memory.  It may not be greater than 126, which uses the whole disk sec-
       tor.  Bad sectors past MAXBAD may be included by the formatter,	but  replacement  sectors
       will not be used until MAXBAD is increased.

       Bad144 is invoked by giving a device type (e.g. rk07, rm03, rm05, etc.), and a device name
       (e.g. hk0, hp1, etc.).  It reads the first sector of the last track of  the  corresponding
       disk  and  prints  out the bad sector information.  It may also be invoked giving a serial
       number for the pack and a list of bad sectors, and will then write the  supplied  informa-
       tion  onto  the same location.  Note, however, that bad144 does not arrange for the speci-
       fied sectors to be marked bad in this case.  This option should only be	used  to  restore
       known bad sector information which was destroyed.

       New bad sectors can be added by running the standard DEC formatter in section ``bad.''


       Not all drivers support bad-sector forwarding on the PDP-11.

       It should be possible to both format disks on-line under UNIX and to change the bad sector
       information, marking new bad sectors, without running a standalone program.

       The bootstrap drivers used to boot the system do not understand bad sectors or handle  ECC
       errors.	 This  means  that  none of these errors can occur when reading the file /unix to
       boot.  Sector 0 of the disk drive and the file /boot in the root file system of that drive
       must also not have any of these errors in it.

       The  drivers  that  write  a system core image on disk after a crash do not handle errors;
       thus the crash dump area must be free of errors and bad sectors.

3rd Berkeley Distribution								BAD144(8)
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