Labeled Tapes

Labeled Tapes
In the two years I have been working with eMag in USA, I have become aware of how much more common labeled tape is in USA, than Europe. Despite the fact we can not agree on how to spell labeled, it is a very common, universal standard, with both ANSI and ISO standards, as well as the backing of IBM.

The labels are typically 80 bytes, and the typical sequence is VOL1 HDR1 HDR2, followed by data, followed by EOF1 EOF2 . There is a tapemark () separating the data from the labels.

The labels are used to define the tape, and also the files. They also have a built in mechanism for handling multi-volume tapes.

The labels can be either in EBCDIC or ASCII, but most labeled tapes only contain straight text in the labels.

The volume label is typically used just to tie up the tape with physical label, or barcode on the tape. The headers store details such as a 17-character file name and file sequence, volume sequence, and generation number. They also store details of block and record size, along with attributes such as fixed, variable, undefined block size. The creation end expiry dates are also included.

The EOF labels will give a block count of the number of data blocks, which is a useful check to ensure all data has been read.

The original specs for labeled tapes had a maximum of 1 million blocks for the block count, and 100K for the block size. These sizes have recently been increased by allocating otherwise un-used bytes in the headers.

The multi-volume aspect is handled very easily. Rather than ending a file with EOF, it is terminated with EOV (end of volume), and this indicates that the data file is not complete, and is stored on the next tape volume.

With MediaMerge/PC it is possible to view these labels, but the simplest way to understand the values is to do a Scan of the tape. This displays the values with descriptions beside them making them easy to understand. MM/PC will also automatically handle both ASCII and EBCDIC labels.