Specifications

ATAPI For Streaming Tape QIC-157 Rev B

For some pre-formatted volumes, the identifiers are associated with physical blocks. In variable-block

size implementations, the identifier can be associated with a physical block when the logical block and

the physical block have a one-to-one relationship on the medium.

Some formats may carry both physical and logical block identifiers recorded on the medium. When a

logical block is split over more than one physical block, or multiple logical blocks are concatenated to

form a physical block, the logical block identifier and the physical block identifier are not the same.

Filemarks may or may not have recorded identifiers, but if identifiers are used in the format, then each

mark is assigned a value even if it is not explicitly recorded.

5.6. Positioning

The READ POSITION and LOCATE commands use four-byte fields to hold these format dependent

identifiers. For some implementations, this value may correspond to a real physical location; however,

it is sufficient for the Device to map the identifier to a value representing the unique recorded element.

With this capability, the READ POSITION command may be used to report a Device-defined block

identifier and the Host may use this value with a LOCATE command to position to the same location

at some future time (provided the volume has not been rewritten in the interim).

5.6.1. Direction and Position Definitions

For Devices, positioning has the connotation of logically being in, at, before, or after some defined

place within a volume. This definition means the position is capable of being repeated under the same

circumstances. The orientation of usage for the four words (in, at, before, or after) is in one direction,

from BOPx toward EOPx. All positioning defined below is worded from this perspective.

The forward direction is defined as logically progressing from BOPx toward EOPx. The reverse

direction is defined as logically progressing from EOPx toward BOPx. In serpentine Devices, the

logical forward or reverse direction has an alternating relationship to the physical motion of the

medium.

The concept of being “in” some position means not being outside a defined region. The definition

allows the position to be on the boundary of a defined region. When a volume is first loaded, the

logical position is always at the beginning of the default data partition (BOP0). Whenever a volume is

mounted and the medium motion is stopped, the position is in some partition. While moving between

partitions, there is no stable position.

The concept of being “at” some position indicates being positioned to a logical or physical extremity

of a partition. A Streaming Tape Device may be positioned at beginning-of-medium, at BOPx, at

end-of-data (EOD), at EOPx, or at end-of-medium (EOM), since these are stable positions at

extremities of a partition.

The concept of being “before” some position indicates that there is some element (data block,

filemark, or other defined point) which may be encountered when moving toward EOPx, if the proper

commands are issued. Being positioned before a particular data block means that if the Device

receives a valid READ command, the data block is transferred to the Host. This position may also be

before Ewx and EOPx, since these are defined points within any partition. However, if data has not

been written to the end-of-partition, these points may not be accessible by the Host.

The concept of being “after” some position indicates that there is some element (data block, filemark,

or other defined point) on the BOPx side of the current position that may be encountered if the proper