How Found

The first format-5 DSCB on the volume is always the second DSCB of the VTOC. If there is more than one format-5 DSCB, it is chained from the previous format-5 DSCB using the DS5PTRDS field.

Table 1 shows the contents of a format-5 DSCB.

Table 1. DSCB Format-5
Offset Dec (Hex)	Type	Length	Name	Description
0(X'00')	Bitstring	4	DS5KEYID	Key identifier (X'05050505').
4(X'04')	Bitstring	5	DS5AVEXT	Available extent.
		2		Relative track address of the first track in the extent. Relative to the beginning of the volume.
		2		Number of unused cylinders in the extent.
		1		Number of additional unused tracks.
9(X'09')	Bitstring	35	DS5EXTAV	Seven available extents.
44(X'2C')	Character	1	DS5FMTID	Format identifier (X'F5').
45(X'2D')	Bitstring	90	DS5MAVET	Eighteen available extents.
135(X'87')	Bitstring	5	DS5PTRDS	Pointer (CCHHR) to next format-5 DSCB, or zero.
140(X'8C')			DS5END	-