<s>
In	O
the	O
IBM	B-Application
System/360	I-Application
storage	O
architecture	O
,	O
the	O
Volume	B-Application
Table	I-Application
of	I-Application
Contents	I-Application
(	O
VTOC	B-Application
)	O
,	O
is	O
a	O
data	O
structure	O
that	O
provides	O
a	O
way	O
of	O
locating	O
the	O
data	B-Application
sets	I-Application
that	O
reside	O
on	O
a	O
particular	O
DASD	B-Application
volume	B-Application
.	O
</s>
<s>
With	O
the	O
exception	O
of	O
the	O
IBM	O
Z	O
compatible	O
disk	O
layout	O
in	O
Linux	O
on	O
Z	O
,	O
it	O
is	O
the	O
functional	O
equivalent	O
of	O
the	O
MS/PC	O
DOS	B-Device
File	B-Application
Allocation	I-Application
Table	I-Application
(	O
FAT	O
)	O
,	O
the	O
NTFS	B-Application
Master	O
File	O
Table	O
(	O
MFT	O
)	O
,	O
and	O
an	O
inode	B-Application
table	O
in	O
a	O
file	B-Application
system	I-Application
for	O
a	O
Unix-like	B-Operating_System
system	I-Operating_System
.	O
</s>
<s>
The	O
VTOC	B-Application
is	O
not	O
used	O
to	O
contain	O
any	O
IPLTEXT	O
and	O
does	O
not	O
have	O
any	O
role	O
in	O
the	O
IPL	O
process	O
,	O
therefore	O
does	O
not	O
have	O
any	O
data	O
used	O
by	O
or	O
functionally	O
equivalent	O
to	O
the	O
MBR	O
.	O
</s>
<s>
It	O
lists	O
the	O
names	O
of	O
each	O
data	B-Application
set	I-Application
on	O
the	O
volume	B-Application
as	O
well	O
as	O
size	B-General_Concept
,	O
location	O
,	O
and	O
permissions	O
.	O
</s>
<s>
Additionally	O
,	O
it	O
contains	O
an	O
entry	O
for	O
every	O
area	O
of	O
contiguous	O
free	O
space	O
on	O
the	O
volume	B-Application
.	O
</s>
<s>
The	O
third	O
record	O
on	O
the	O
first	O
track	B-Device
of	O
the	O
first	O
cylinder	O
of	O
any	O
DASD	B-Application
(	O
e.g.	O
,	O
disk	O
)	O
volume	B-Application
is	O
known	O
as	O
the	O
volume	B-Application
label	O
and	O
must	O
contain	O
a	O
pointer	O
to	O
the	O
location	O
of	O
the	O
VTOC	B-Application
.	O
</s>
<s>
The	O
location	O
of	O
the	O
VTOC	B-Application
may	O
be	O
specified	O
when	O
the	O
volume	B-Application
is	O
initialized	O
.	O
</s>
<s>
For	O
performance	O
reasons	O
it	O
may	O
be	O
located	O
as	O
close	O
to	O
the	O
center	O
of	O
the	O
volume	B-Application
as	O
possible	O
,	O
since	O
it	O
is	O
referenced	O
frequently	O
.	O
</s>
<s>
A	O
VTOC	B-Application
is	O
added	O
to	O
a	O
DASD	B-Application
volume	B-Application
when	O
it	O
is	O
initialized	O
using	O
the	O
Device	O
Support	O
Facilities	O
program	O
,	O
ICKDSF	O
,	O
in	O
current	O
systems	O
.	O
</s>
<s>
When	O
in	O
OS/360	B-Application
and	I-Application
successors	I-Application
allocates	O
a	O
data	B-Application
set	I-Application
,	O
it	O
generally	O
searches	O
the	O
catalog	O
to	O
determine	O
the	O
volumes	O
on	O
which	O
it	O
resides	O
.	O
</s>
<s>
When	O
a	O
program	O
opens	O
a	O
Direct	B-Application
Access	I-Application
Storage	I-Application
Device	I-Application
(	O
DASD	B-Application
)	O
dataset	O
,	O
the	O
OPEN	O
routine	O
searches	O
the	O
VTOC	B-Application
index	O
(	O
VTOCIX	O
)	O
if	O
there	O
is	O
one	O
,	O
or	O
directly	O
searches	O
the	O
VTOC	B-Application
if	O
there	O
is	O
no	O
VTOCIX	O
.	O
</s>
<s>
The	O
VTOC	B-Application
consists	O
of	O
a	O
sequence	O
of	O
140-byte	O
records	O
known	O
as	O
Data	B-Application
Set	I-Application
Control	I-Application
Blocks	I-Application
(	O
DSCBs	O
)	O
.	O
</s>
<s>
+	O
DSCB	O
types	O
in	O
a	O
VTOC	B-Application
DSCB	O
format	O
type	O
Purpose	O
Notes	O
0	O
Empty	O
entry	O
1	O
Data	B-Application
set	I-Application
primary	O
Describes	O
first	O
three	O
extents	B-Application
of	O
a	O
data	B-Application
set	I-Application
(	O
Starting	O
and	O
ending	O
cylinders	O
and	O
tracks	O
)	O
2	O
ISAM	O
data	B-Application
set	I-Application
Describes	O
ISAM	O
data	B-Application
sets	I-Application
3	O
Data	B-Application
set	I-Application
extension	O
Describes	O
data	B-Application
set	I-Application
extents	B-Application
after	O
the	O
third	O
4	O
VTOC	B-Application
Describes	O
volume	B-Application
attributes	O
5	O
Free	O
space	O
26	O
extents	B-Application
on	O
non-index	O
volumes	O
6	O
Shared	O
Cylinder	O
Allocation	O
In	O
OS/360	B-Application
,	O
Format	O
6	O
DSCB	O
is	O
used	O
for	O
Shared	O
Cylinder	O
Allocation	O
,	O
which	O
enabled	O
multiple	O
datasets	O
to	O
be	O
interleaved	O
across	O
a	O
range	O
of	O
cylinders	O
(	O
such	O
that	O
,	O
for	O
example	O
,	O
the	O
first	O
half	O
of	O
each	O
cylinder	O
was	O
allocated	O
for	O
data	B-Application
set	I-Application
A	O
,	O
and	O
the	O
second	O
half	O
for	O
data	B-Application
set	I-Application
B	O
)	O
.	O
</s>
<s>
7	O
Free	O
space	O
Extension	O
of	O
Format	O
4	O
DSCB	O
8	O
Data	B-Application
set	I-Application
primary	O
EAVExtended	O
Address	O
Volume	B-Application
,	O
a	O
volume	B-Application
with	O
more	O
than	O
65,520	O
cylinders	O
.	O
</s>
<s>
The	O
VTOC	B-Application
must	O
reside	O
within	O
the	O
first	O
64K	O
tracks	O
on	O
the	O
volume	B-Application
,	O
and	O
The	O
first	O
DSCB	O
in	O
the	O
VTOC	B-Application
is	O
always	O
a	O
format	O
4	O
DSCB	O
which	O
describes	O
the	O
VTOC	B-Application
itself	O
and	O
attributes	O
of	O
the	O
DASD	B-Application
volume	B-Application
on	O
which	O
this	O
VTOC	B-Application
resides	O
.	O
</s>
<s>
The	O
second	O
DSCB	O
is	O
always	O
a	O
format	O
5	O
DSCB	O
which	O
describes	O
free	O
space	O
within	O
the	O
VTOC	B-Application
.	O
</s>
<s>
Normally	O
,	O
the	O
rest	O
of	O
the	O
VTOC	B-Application
will	O
contain	O
format	O
0	O
DSCBs	O
,	O
which	O
are	O
empty	O
entries	O
,	O
and	O
format	O
1	O
or	O
format	O
3	O
DSCBs	O
,	O
which	O
describe	O
the	O
extents	B-Application
of	O
data	B-Application
sets	I-Application
,	O
giving	O
their	O
start	O
address	O
and	O
end	O
address	O
of	O
up	O
to	O
16	O
such	O
extents	B-Application
on	O
disk	O
.	O
</s>
<s>
The	O
initial	O
part	O
of	O
a	O
data	B-Application
set	I-Application
is	O
described	O
by	O
a	O
format	O
1	O
DSCB	O
.	O
</s>
<s>
If	O
necessary	O
,	O
format	O
3	O
DSCBs	O
are	O
used	O
to	O
describe	O
further	O
extents	B-Application
of	O
the	O
data	B-Application
set	I-Application
.	O
</s>
<s>
When	O
a	O
data	B-Application
set	I-Application
is	O
deleted	O
,	O
its	O
format	O
1	O
DSCB	O
is	O
overwritten	O
to	O
become	O
a	O
format	O
0	O
DSCB	O
,	O
and	O
the	O
format	O
3	O
DSCB	O
,	O
if	O
one	O
exists	O
,	O
is	O
similarly	O
deleted	O
.	O
</s>
<s>
Originally	O
,	O
a	O
VTOC	B-Application
search	O
was	O
a	O
sequential	O
scan	O
of	O
the	O
DSCBs	O
,	O
stopping	O
when	O
the	O
correct	O
format	O
1	O
DSCB	O
was	O
found	O
or	O
the	O
end	O
of	O
the	O
VTOC	B-Application
was	O
reached	O
.	O
</s>
<s>
As	O
DASD	B-Application
volumes	O
became	O
larger	O
,	O
VTOC	B-Application
search	O
became	O
a	O
bottleneck	O
and	O
so	O
a	O
VTOC	B-Application
index	O
was	O
added	O
.	O
</s>
<s>
This	O
VTOC	B-Application
entry	O
describes	O
a	O
dataset	O
and	O
defines	O
its	O
first	O
three	O
extents	B-Application
.	O
</s>
<s>
This	O
is	O
the	O
format	O
of	O
the	O
DSCB	O
from	O
OS/360	B-Application
Release	O
21.7	O
in	O
1973	O
,	O
prior	O
to	O
changes	O
for	O
Y2K	B-Error_Name
.	O
</s>
<s>
44	O
1	O
Format	O
identifier	O
,	O
'	O
1	O
 '	O
for	O
Format-1	O
DSCB	O
(	O
character	O
)	O
45	O
6	O
Volume	B-Application
serial	O
number	O
of	O
first	O
DASD	B-Application
volume	B-Application
fot	O
this	O
dataset	O
(	O
alphanumeric	O
)	O
51	O
2	O
Volume	B-Application
sequence	O
number	O
of	O
this	O
volume	B-Application
for	O
multi-volume	O
datasets	O
(	O
binary	O
)	O
53	O
3	O
Dataset	O
creation	O
date	O
(	O
binary	O
)	O
one	O
byte	O
for	O
year	O
of	O
century	O
and	O
two	O
bytes	O
for	O
day-of-year	O
56	O
3	O
Dataset	O
expiration	O
date	O
(	O
same	O
format	O
as	O
creation	O
date	O
)	O
99365	O
indicates	O
that	O
this	O
dataset	O
will	O
never	O
expire	O
.	O
</s>
<s>
59	O
1	O
Number	O
of	O
extents	B-Application
for	O
this	O
dataset	O
(	O
binary	O
)	O
60	O
1	O
If	O
this	O
dataset	O
is	O
a	O
partitioned	O
dataset	O
this	O
is	O
the	O
number	O
of	O
bytes	O
used	O
in	O
the	O
last	O
directory	O
block	O
.	O
</s>
<s>
(	O
bitfield	O
)	O
85	O
1	O
Option	O
code	O
,	O
other	O
options	O
specified	O
when	O
the	O
dataset	O
was	O
created	O
(	O
bitfield	O
)	O
86	O
2	O
Block	O
size	B-General_Concept
(	O
fixed	O
size	B-General_Concept
,	O
or	O
maximum	O
size	B-General_Concept
for	O
files	O
of	O
variable-length	O
records	O
)	O
(	O
binary	O
)	O
88	O
2	O
Record	O
length	O
(	O
fixed	O
size	B-General_Concept
or	O
maximum	O
length	O
for	O
variable	O
length	O
records	O
)	O
(	O
binary	O
)	O
90	O
1	O
Key	O
length	O
if	O
this	O
file	O
has	O
recorded	O
record	O
keys	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
the	O
dataset	O
is	O
created	O
with	O
the	O
DD	O
card	O
specification	O
SPACE	O
=(	O
CYL	O
,	O
(	O
4	O
,	O
2	O
)	O
)	O
the	O
dataset	O
is	O
initially	O
allocated	O
four	O
cylinders	O
,	O
and	O
an	O
additional	O
two	O
cylinders	O
,	O
not	O
necessary	O
contiguous	O
,	O
will	O
be	O
allocated	O
each	O
time	O
more	O
space	O
is	O
needed	O
,	O
up	O
to	O
fifteen	O
additional	O
extents	B-Application
.	O
</s>
<s>
98	O
3	O
DASD	B-Application
address	O
of	O
last	O
block	O
written	O
in	O
the	O
dataset	O
.	O
</s>
<s>
101	O
2	O
Number	O
of	O
bytes	O
remaining	O
on	O
last	O
track	B-Device
following	O
last	O
block	O
indicated	O
above	O
.	O
</s>
<s>
105	O
10	O
Description	O
of	O
first	O
or	O
only	O
extent.Type	O
of	O
extent	O
(	O
one	O
byte	O
,	O
bitfield	O
)	O
,	O
extent	O
sequence	O
number	O
(	O
one	O
byte	O
,	O
binary	O
)	O
,	O
extent	O
starting	O
cylinder	O
and	O
track	B-Device
(	O
four	O
bytes	O
,	O
binary	O
)	O
,	O
</s>
<s>
extent	O
ending	O
cylinder	O
and	O
track	B-Device
(	O
four	O
bytes	O
,	O
binary	O
)	O
.	O
</s>
<s>
135	O
5	O
Pointer	O
to	O
Format-2	O
(	O
index	O
)	O
DSCB	O
in	O
VTOC	B-Application
in	O
format	O
cylinder/head/record	O
if	O
this	O
is	O
indexed	O
dataset	O
.	O
</s>
<s>
Pointer	O
to	O
Format-3	O
(	O
extension	O
)	O
DSCB	O
if	O
this	O
dataset	O
has	O
more	O
than	O
three	O
extents	B-Application
.	O
</s>
