Block zero contains a copy of a stand-alone bootstrap program. See reboot (8).
Blocks 1 through 24 for DECtape (1 through 62 for magtape) contain a directory of the tape. There are 192 (resp. 496) entries in the directory; 8 entries per block; 64 bytes per entry. Each entry has the following format:
""struct { char pathname[32]; unsigned short mode; char uid; char gid; char unused1; char size[3]; long modtime; unsigned short tapeaddr; char unused2[16]; unsigned short checksum; };
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, gid, size and time modified are the same as described under i-nodes (see file system fs (5)). 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 above 25 (resp. 63) are available for file storage.
A fake entry has a size of zero.