The subdirectories here contain images of old DECtapes given to Warren
Toomey by Dennis Ritchie in 1997. The .tp and .tap files are the original tape
images, which are in the tp(1) and tap(1) format, respectively. The contents
of each tape is listed in a 'dir' file. The tapes have also been converted to
the POSIX USTAR tar(1) format for reading using modern operating systems. The
slack space, boot block and unused blocks of each tape have been extracted and
packed into <name>_slack.tar (where <name> is the name of the tape), for data
recovery and forensic analysis purposes (there are interesting things in the
unused blocks).

The tapes in tap(1) format expressed time with 1/60-second granularity, which
is not supported by POSIX USTAR, and therefore those timestamps have been
rounded down to the nearest whole second in the converted tar(1) archives. The
tap(1) format also did not support group IDs, so 0 was used as the placeholder
value.

The 'dmr' tape was originally written by UNIX V1-V3, however, some files were
added later by running tap(1) under UNIX V4. The 'nsys' tape was created using
tap(1) running under UNIX V4. UNIX V4 introduced the modern UNIX timestamp and
access mode format, as well as the concept of groups and group IDs. tap(1) did
not support any of that, therefore it truncated the access mode from 16-bit to
8-bit and neglected the group ID when writing those files. See the 'warnings'
file in the respective directories for files affected. In the converted tar(1)
archives, the timestamps of those files have been corrected, reasonable
attempts have been made to restore the 3 lost bits of the original access
modes (by setting SUID, SGID to 0, and UREAD to 1) and the lost group IDs have
all been set to 0.

The 's1' tape is a 'UNIX INIT DECtape' used for cold booting UNIX, from a
version of UNIX between V1 and V2. It was created by the rather poorly
documented maki command, and therefore not readable by most tools. The format
consists of a boot block, a dump of the last 32 KiB of the RF disk /dev/rf0
and a small set of important programs. The boot block is the vcboot program
described in bproc(7). The 32-KiB dump consists of the bos, Warm UNIX, Cold
UNIX and an unassigned program, with the bos occupying 2 KiB, Warm and Cold
UNIX occupying 12 KiB each and the unassigned program occupying 3 KiB. The
remaining 3 KiB is slack space from the RF disk. The files on the tape are
stored in a way similar to tar(1), albeit in a more primitive way. All files
are stored consecutively with a directory block followed by file data. A
directory block consists of a 16-bit file size field, an 8-bit mode field, an
8-bit user ID field and a null-terminated path field. End of directory is
marked by setting the file size field to 0. Given the limitation of this
format and the fact that group IDs did not exist back then, timestamps and
group IDs are all set to 0 in the converted tar(1) archive.

Dennis' original description of the contents of the tapes is in the top-level
'list' file.
