HP Surestore 125ex Optical Drive and Library SCSI-2 Command Reference - Page 32
Identify 80H-FFH, Bit 7, Bit 5-3, Table 1-6, Head of Queue Tag, Ordered Queue Tag, Message Code 22H
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SCSI Command Overview Table 1-6 Table 1-7 SCSI Command Overview Drive Control Commands Head of Queue Tag Byte 7 6 5 4 3 2 1 0 0 Message Code (21H) 1 Queue Tag (00H-FFH) The Head of Queue Tag message specifies that the I/O process be placed first in the logical unit's command queue. When the drive receives a subsequent I/O process received with a Head of Queue Tag message, the I/O process that has been already done is not interrupted. Ordered Queue Tag Byte 7 6 5 4 3 2 1 0 0 Message Code (22H) 1 Queue Tag (00H-FFH) The Ordered Queue Tag message specifies that the I/O process be placed in the logical unit's command queue for execution in the order received. All queued I/O processes for the logical unit received prior to this I/O process are executed before this I/O process is executed. All queued I/O processes received after this I/O process are executed after this I/O process, except for I/O processes received with a Head of Queue Tag message. Identify 80H-FFH These messages are sent by either the initiator or the target to establish the physical path connection between initiator and target for a particular logical unit. Bit 7 This bit is always set to 1. Bit 6 This bit is set to 1 by the initiator to indicate that the initiator has the ability to accommodate the disconnection and reconnection. Bit 5-3 Reserved. Chapter 1 1-13