How Found

Chained from a format 1, format 2 or format 9 DSCB that represents the data set. In the case of a PDSE, HFS data set, sequential extended-format data set, or VSAM data set, the chain also can be from a preceding format-3 DSCB.

Table 1 shows the contents of a format-3 DSCB.

Table 1. DSCB Format-3
Offset Dec (Hex)	Type	Length	Name	Description
0(X'00')	Bitstring	4	-	Key identifier (X'03030303').
4(X'04')	Bitstring	40	DS3EXTNT	Four extent descriptions.
		1		Extent type indicator. (See DS1EXT1 in Table 1.)
		1		Extent sequence number.
		4		Lower limit (CCHH).
		4		Upper limit (CCHH).
44(X'2C')	Character	1	DS3FMTID	Format identifier (X'F3').
			DS3IDC	Constant value of X'F3' in DS3FMTID.
45(X'2D')	Bitstring	90	DS3ADEXT	Nine additional extent descriptions.
135(X'87')	Bitstring	5	DS3PTRDS	Pointer (CCHHR) to next format-3 DSCB, or zero.
140(X'8C')			DS3END	-