BAD144(8)						      System Manager's Manual							 BAD144(8)

NAME

       bad144 - read/write DEC standard 144 bad sector information

SYNOPSIS

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

DESCRIPTION

       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 begin-
       ning of the disk.  A maximum of 126 bad sectors can be supported.  The position of the bad sector in the bad sector table determines  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 sector.  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 sec-
       tor 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 information onto the same location.	Note, however, that bad144
       does not arrange for the specified 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.''

SEE ALSO

       badsect(8)

BUGS

       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)