All volumes have a minimum of one partition called partition 0, the default data partition. For devices which support only one partition, the beginning-of-partition zero (BOP 0) may be equivalent to the beginning-of- medium and the end-of-partition zero (EOP 0) may be equivalent to the end- of-medium.
When a volume is mounted, it is logically positioned to beginning of the default data partition (BOP 0). When a REWIND command is received in any partition (x), the device positions to the beginning-of-partition (BOP x).
Partitions on a volume do not need to be recorded in any defined order, nor do all partition numbers in a sequence need to be present on a volume. It is sufficient for a device to be able to locate a partition, given its code value, or determine that it does or does not exist on the volume. For interchange, information about which partitions are present on a volume may be stored on the volume in a device-defined area (possibly unavailable to the initiator) or the information may be an intrinsic attribute of the device implementation.
Figure 24 shows a possible partition implementation for a four-track serpentine recording device, assuming that each track group defines a partition.
Figure 24 - Partitioning example - one partition per track group
Another possible partition implementation for this four-track serpentine recording device is shown in figure 25, using two track groups to define each partition.
Figure 25 - Partitioning example - one partition per two track groups
The previous examples show the beginning and ending points for a partition aligned with physical bounds of the medium. This is not a mandatory requirement for partitioning; it is sufficient for a device to be able to locate to and stay in any partition bounded by a BOP x and EOP x. In this case, a recorded mark or some other device-recognizable attribute could be used to delineate the partitions. Figure 26 shows a possible two- partition implementation for a device with only one track group.
Figure 26 - Partitioning example - two partitions per track group
Three methods are defined in the MODE SENSE and MODE SELECT commands for managing partitions; each method is progressively more difficult to implement and manage: