Specifications

ATAPI For Streaming Tape QIC-157 Rev B
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already retrieved the MODE data or determined the density and block size but shall not use this
information in this case.
5.3. Logical Elements within a Partition
The area between BOPx and EOPx on a typical recorded volume contains at least two types of Host
accessible elements, data blocks and tape marks. These elements are controlled and transferred
between the Host and the medium using READ, WRITE, and WRITE FILEMARK commands.
5.3.1. Blocks
A unit of data supplied or requested by the Host is called a logical block. Logical blocks are stored
according to the specifications of the format for the volume and may be recorded as one or more
physical blocks on the medium. When the physical block and the logical block are not recorded in a
one-to-one relationship, it is the responsibility of the Device to perform all blocking, de-blocking,
padding, stripping, splitting or rebuilding of the logical data block(s) sent by the Host. If the Device is
unable to provide the physical to logical translation, as determined by the Capabilities and Mechanical
Status Page, only the SPACE command operates using logical blocks but SPACE may not support the
reverse direction. As physical blocks may not be sequential, this requires saved READ POSITION
information and LOCATE commands for the Host to navigate the medium.
5.3.2. Filemarks
Filemarks are special recorded elements containing no user data. The filemark format is defined in
some American National Standards. Hosts traditionally use filemarks to separate user data from labels
and logical groupings of data from each other. Since some format standards do not define an explicit
end-of-data (EOD), Host software has often used conventions with filemarks to represent an EOD
indication. At least one American National Standard specifically defines filemark use for this purpose.
5.3.3. Gaps
Inter-block gaps, the gaps between blocks and filemarks, are introduced on the medium at the time a
block or mark is written without explicit action by the Host. Minimum and maximum lengths for
inter-block gaps are defined in some American National Standards.
An erase gap may be a length of erased medium or a recorded pattern not distinguishable as a block or
mark. Minimum and maximum lengths for erase gaps are defined in some American National
Standards while some Devices may have no implementation of an erase gap. For this specification,
gaps are defined by the Device.
5.3.4. Blank
After writing data from BOPx, the medium is considered to be a contiguous grouping of blocks,
filemarks, and gaps. Certain American National Standards define gap lengths which, if exceeded, are
to be considered as having reached blank medium. Depending on the format, this blank medium may
be treated as an end-of-data indication or an unrecoverable medium error causing an interchange
error. Unrecorded volumes (new or erased) may exhibit blank medium characteristics if an attempt is
made to read or space the volume before data has been written.
For SCSI standards, the device may have been capable of supporting fixed or variable length blocks.
The concept of fixed or variable mode for writing and reading blocks only indicates the method by