How Data is Written to Tape

Data is written to tape in blocks.  Most tapes are “fixed block“, meaning all the blocks in a file are of a fixed size, usually an integer multiple of the record size (see Blocking Factor), but there are also “variable block” formats.  (This is not to be confused with fixed-block drives, which can only write one block size. These mainframe tape drives always operate in variable block mode, and “fixed block” here means all the blocks are of the same size.)

There are two common formats: Labeled and Unlabeled.  Both can use either EBCDIC or ASCII coding.   Labeled tapes contain the Data Set Name, or DSN (file name), record length, block size, and other information about each file in the label.  Unlabeled tapes contain only the raw data.

A tape can contain multiple files, or Data Sets and files can span tapes, creating a multivolume set of tapes.

There are many other labeled tape formats as well.

The content of these tapes can vary considerably, but fixed-length records are the most common.  All of the above result in many combinations, but the most common are:

• Unlabeled Fixed-block EBCDIC
• Unlabeled Fixed-block ASCII
• IBM Standard Label Fixed-block EBCDIC
• ANSI Standard Label Fixed-block ASCII
• IBM Standard Label Variable block EBCDIC

DISC can convert all these formats and several other combinations as well.