TAP (V)                      6/12/72                      TAP (V)

NAME            tap -- DEC/mag tape formats

DESCRIPTION     The DECtape command tap and the magtape command

                mt dump and extract files to and from their re-

                spective tape media.  The formats of these tapes

                are the same except that magtapes have larger di-


                Block zero of the tape is not used.  It is avail-

                able to contain a boot program to be used in a

                stand-alone environment.  This has proved valu-

                able for DEC diagnostic programs.

                Blocks 1 through 24 for DECtape (1 through 146

                for magtape) contain a directory of the tape.

                There are 192 (resp. 1168) entries in the direc-

                tory; 8 entries per block; 64 bytes per entry.

                Each entry has the following format:

                   path name       32 bytes

                   mode            1 byte

                   uid             1 byte

                   size            2 bytes

                   time modified   4 bytes

                   tape address    2 bytes

                   unused          20 bytes

                   check sum       2 bytes

                The path name entry is the path name of the file

                when put on the tape.  If the pathname starts

                with a zero word, the entry is empty.  It is at

                most 32 bytes long and ends in a null byte.

                Mode, uid, size and time modified are the same as

                described under i-nodes (see file system (V)) The

                tape address is the tape block number of the

                start of the contents of the file.  Every file

                starts on a block boundary.  The file occupies

                (size+511)/512 blocks of continuous tape.  The

                checksum entry has a value such that the sum of

                the 32 words of the directory entry is zero.

                Blocks 25 (resp. 147) on are available for file


                A fake entry (see mt(I), tap(I)) has a size of


SEE ALSO        filesystem(V), mt(I), tap(I)