Fujitsu M2488 manual Description of Block ID Format, Block ID Field Description, Format Codes

TAPE UNIT SCSI COMMANDS								M2488 PRODUCT GUIDE
4-3.17.3	Description of Block ID Format
					BITS
BYTES	7	6	5	4		3	2		1	0
0	Wrap				Physical reference
1	Format code				Logical block position (MSB)
2	MSB			Logical block position
3										LSB

			Table 4-72. Block ID Field Description
BYTE	BIT	VALUE	DESCRIPTION
0	0-6	1-95	The Physical reference field will be in the range 1 to 95. The values in the range 1 to
			95 indicate an approximate physical location on tape close to the target position.
			The Wrap field indicates whether the target position is in the first wrap or second
0	7	0	wrapa of tape. If Wrap equals 0 then the target position is in the first wrap.
		1	If Wrap equals 1 then the target position is in the second wrap.
			The Format code field indicates the format of the tape cartridge. The meaning of the
1	6-7		possible values are shown in Table 4-73. When writing the format code reported will
			be 01b (36 track, packeted.) When reading, the format code reported will be the format
			code contained in the tape being read.
			A unique Logical block position is associated with each data block and filemark writ-
1	0-5		ten on a tape. The first filemark or data block on tape is assigned Logical block posi-
2-3			tion 0; the Logical block position increases by 1 for each data block and filemark
			thereafter. The maximum Logical block position is 3FFFEFh.

a.A 36 track tape consists of two interleaved groups of 18 tracks; each group is called a wrap. The first wrap is written first and runs from Physical BOT towards Physical EOT. The second wrap is written second and runs from Physical EOT towards Physical BOT. The tape unit hides the transition from the first wrap to the second wrap so that the user sees an [abstract] continuous length of tape running from Logical BOT (the beginning of the first wrap) to Logical EOT (the end of the second wrap).

Table 4-73. Format Codes

FORMAT CODE VALUE	FORMAT
00b	18 track, non-packeted
01b	36 track, packeted
10b	18 track, packeted
11b	reserved

4-78 CG00000-011503 REV. A April 1997