HP Surestore 125ex Optical Drive and Library SCSI-2 Command Reference - Page 120
Read Group 1 Command 28H, Rel Adr, Transfer, Length
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SCSI-2 Drive Commands NOTE NOTE SCSI-2 Drive Command Set Read (Group 1) Command (28H) FUA Rel Adr Transfer Length PBA A Force Unit Access bit of 1 indicates that data is read directly from the disk. A Force Unit Access bit of 0 indicates that the drive may access the cache memory. A relative address bit of one indicates that logical block address specified is a two's complement offset from the last logical block address accessed. A value of 0 indicates that the logical block address is an absolute address. The Transfer Length field specifies the number of contiguous logical blocks of data to be transferred. A value of 0 indicates that no blocks will be transferred and is not considered an error. A Physical Block Address bit of 1 indicates that physical block addressing is used. A PBA bit of 0 indicates that logical block addressing is used. For write-once media, blank checking is performed following a failed read. If the sector is blank, the sense key/additional sense code will be set to BLANK CHECK/Blank Sector Detected. When this command is used with 14X 512 or 1024 byte/sector media, setting the PBA bit will cause the drive to return a CHECK CONDITION status. The sense key/additional sense code will be set to ILLEGAL REQUEST/INVALID FIELD in CDB. Chapter 2 2-83