IBM System/360

If the nam4ed data set is stored on more than 20 vol urnes, bytes
253-255 of the block contain the relative track address of the next
block of volume pointers. Byte 256 contains a binary zero.
Example: In the following example, the list of volumes that contain
data set A.B is read into main storage. The search for the volume-list
block starts on the system residence volume.
r---------T----------T-------------------------------------------------,
I Name I Operation I Operand ~
~---------+----------+-------------------------------------------------i
I I LOCATE I INDAB READ VOLUME-LIST BLOCK FOR I
I I Check Exceptional Returns CATALOGED DATA SET A.B INTOI
I
INDAB I CAMLST I NAME,AB"LOCAREA MAIN STORAGE AREA NAMED I
I
AB I DC I CL44'A.B' LOCAREA. LOCAREA ALSO I
I LOCAREA I DS I OD CONTAINS 3-BYTE TTR AND I
I
I DS I 265C 6-BYTE SERIAL NUMBER I
L _________ ~ ___________ ~ _________________________________________________ J
The LOCATE macro-instruction points to the CAMLST macro-instruction.
NAME, the first operand of CAMLST, specifies that the system is to
search the catalog for a volume-list tlock by using the name of a data
set. AB, the second operand, specifies the main storage location of a
44-byte area into which you have placed the fully qualified name of a
data set. LOCAREA, the fourth operand, specifies a 265-byte area you
have reserved in main storage.
After execution of these macro-instructions, the 265-byte area
contains: the 256-byte volurre-list block for data set A.B, the 3-byte
relative track address (TTR) of the block following the one read into
main storage, and the 6-byte serial number of the volume on which the
block was found ..
Specifying the Name of a Generation Data Set
You specify the name of a generation data set by using the fully
qualified generation index name and the relative generation number of
the data set. The value of a relative generation number reflects the
position of a data set in a generation data group. The following values
can be used:
• Zero - specifies the latest data set cataloged in a generation data
group.
• Negative nt~ber - specifies a data set cataloged before the latest
data set.
• Positive number - specifies a data set not yet cataloged in the
generation data group.
When you use zero or a negative number as the relative generation
number, a volume-list block is read into main storage and the relative
generation number is replaced by the absolute generation name.
When you USE~ a positive number as the relative generation number, an
absolute generation name is created and replaces the relative generation
number. A volume-list block is not read, since none exists for these
data sets.
Example: In t.he following example, the list of volumes that contain
generation data set A.PAY(-3) is read into main storage. The search for
the volume-list block starts on the system residence volume.
Maintaining the Catalog and the Volume Table of contents 11