Fujitsu M2488 manual Read Position Return Data Description, Byte BIT Description

M2488 PRODUCT GUIDE TAPE UNIT SCSI COMMANDS

		Table 4-71. READ POSITION Return Data Description
BYTE	BIT	DESCRIPTION
		The Block Position Unknown (BPU) bit, if one, indicates that the first and last locations could
		not be determined by the tape unit; in this case the First Block Location field and the Last Block
0	2	Location field do not contain valid information. If the BPU bit is zero then the First and Last
		block locations contain valid information. The reported BPU bit will be one if no tape cartridge is
		loaded.
		The End-of-Partition (EOP) bit, if one, indicates that the tape unit is logically positioned between
0	6	early-warning (LEOT) and the Physical End of Tape. If this bit is zero then the tape unit is posi-
		tioned previous to early-warning.
		The Beginning-of-Partition (BOP) bit, if one, indicates that the tape unit is logically positioned at
0	7	the beginning of the tape; if the bit is zero then the tape unit is not logically positioned at begin-
		ning of tape. The tape unit is logically positioned at beginning of tape if and only if the next
		block to be written or to be read (forward) is block 0 (the first block on tape).a
1	0-7	The Partition number field is always set to 0. This tape unit only has one partition; the identifica-
		tion number of this partition is 0.
		The First Block Location field indicates the current logical position. The value in this field is the
4-7		block address of the next block to be transferred between the initiator and the tape unit if a READ
		or WRITE command is issued; the format of the block address reported is dictated by the BT bit
		setting as explained above.
		The Last Block Location field indicates the physical position of the tape. The value in this field is
		the block address of the next block to be transferred between the buffer and the [tape] medium;
		the format of the block address reported is dictated by the BT bit setting as explained above.
		The current logical position and the physical position of tape can be different because blocks can
8-11		be buffered both when reading and when writing. When writing, the First Block Location will be
		greater than or equal to the Last Block Location, the difference is the number of blocks that are in
		the buffer waiting to be written.b When reading forward, the Last Block Location will be greater
		than or equal to the First Block Location, the difference is the number of blocks that are in the
		buffer waiting to be read.
		The Number of Blocks in Buffer field equals the number of blocks that are in the buffer waiting
13-15		to be written to the medium. This field is set to zero if the buffer does not contain blocks to be
		written to tape.
		The Number of Bytes in Buffer field equals the total number of data bytes that are in blocks in
		the buffer waiting to be written. This field is set to zero if the buffer does not contain blocks to be
16-19		written to tape. When writing with compression enabled, the buffer actually contains the com-
		pressed bytes for the blocks waiting to be written; however the number of bytes reported in the
		Number of Bytes in Buffer field will always indicate the number of uncompressed bytes.

a.If the tape is positioned at Physical End of Tape then the BOP bit reported will necessarily be zero because no more blocks can be read or written; so if the BOP bit is sampled following a successful ERASE with the Long bit set to one, the BOP bit will be zero even if the ERASE was started from the beginning of tape.

b.When computing the difference between the First and Last Block Locations, only the logical block position portion of the Block IDs should be used if the BT bit is set to one.

April 1997 CG00000-011503 REV. A 4-77