IBM 5110 User Manual page 117

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Unblocked and Unspanned Format
The unblocked and unspanned format is the basis for general
,exchange with other products using diskette storage. In this case, the
file name cannot exceed 8 alphameric characters, and the diskette
sector size must be 128 or 256 bytes.
The unblocked unspanned format allows only character scalars or
vectors to be written to the data file, with each record starting on a
sector boundary. Also, the records cannot exceed (span) a sector
boundary.
When the unblocked and unspanned format is used, all the records
'written to the data file must have the same shape (p DATA) as the
first record written to the data file. The records must have the same
shape because, when multiple records are read from a direct access
data file, the records are laminated together along a new first
dimension.
Since the unblocked unspanned format requires each record to start
on a sector boundary, to prevent wasted diskette storage the record
size should be as close as possible to the sector size. The unused
diskette storage in each sector is unavailable for storing data (see
Type U Data File in Chapter 8, Diskette Concepts).