Header segment and bad-sector map¶

The header segment describes the tape: its format, its geometry, its history, and which sectors the format excluded. Tapewyrm reads it before anything else, because placing sectors beyond the start of track 0 and correcting any segment both depend on it.

Source: QIC-80-MC Rev N §7 (qic80n.pdf). QIC-3010-MC Rev H and QIC-3020-MC Rev H §7 use the same layout with the differences listed on QIC-3010 and QIC-3020.

Location¶

The first defect-free segment on the tape is the header segment. The second defect-free segment is a duplicate. Segments before the first copy and between the two copies are written with deleted-data marks (Rev N §7).

Rev N Appendix B shows a typical layout: segment 0 damaged, segments 1 and 2 the header copies, segment 3 the volume table, file sets from segment 4.

Format parameter record¶

Sector 0, bytes 0–255. Words and doublewords are little-endian. Rev N §7.1:

Packed dates (Rev N §7.1, also QIC-113's "Short Date/Time"): bits 31–25 are year − 1970; bits 24–0 hold SC + 60 × (MN + 60 × (HR + 24 × (DY + 31 × MO))), with month and day counted from 0.

Bad-sector map, format code 4¶

From offset 256 of the header segment, a list of 3-byte entries in ascending order (Rev N §7.2):

An entry of 00 00 00 ends the list. The map can hold 314,016 entries and address 16,777,215 sectors.

Bad-sector map, fixed formats¶

Fixed-format QIC-80 tapes (format codes 2, 3 and 5) use a different map. Rev N refers to Rev K for these and does not describe them, and Rev K is not in the standards library. Tapewyrm reads them as follows, from the bench:

This is the only reading under which the excluded segments of a bench tape (code 5) decode, with the QIC-113 byte offsets of the neighbouring segments agreeing exactly on both sides. Every slot of an excluded sector still carried a valid ID on that tape: the sector is recorded but is not part of the codeword.

Other fields of fixed-format headers match the Rev N layout byte for byte on the bench tapes (segments per track, tracks, name, dates, counters). One factory-preformatted code-2 tape has non-zero bytes at 78 and 144–145, which Rev N calls unused; their meaning is unknown.

Tapewyrm notes¶

Module. qiclib.header: parse_header_data decodes the record and the map, locate_header finds the header and re-places every sector under its geometry, and apply_bsm marks excluded sectors before correction.

Finding the header. locate_header walks segments in order and takes the first whose data starts with the signature. If sector 0 of a segment is unreadable, it tries a Reed-Solomon correction of that segment before moving on, and if the first copy is lost it finds the duplicate one segment later.

Geometry from the header. Tracks, segments per track, and floppy tracks per side (maximum floppy track + 1) all come from the header. A fixed-format tape can record a maximum floppy track of 149, not 254; see QIC-80-MC.

Where the map starts. Rev N puts the parameter record in bytes 0–255, so the code-4 map starts at 256. An earlier version started at 128 and read the lifetime segment counter and the initial format date as two bad sectors.

The map describes the tape, not the read. Excluded slots are computed for every segment, including segments that were not captured, so a lost segment's real data length is known when a volume is extracted.

Format code 6 (QIC-3020 only) is not handled; see QIC-3010 and QIC-3020.

See also¶

• Error correction
• Volume table
• qicsilver identify