<s>
The	O
Task	B-Operating_System
Control	I-Operating_System
Block	I-Operating_System
(	O
TCB	O
)	O
contains	O
the	O
state	O
of	O
a	O
task	O
in	O
,	O
e.g.	O
,	O
OS/360	B-Application
and	I-Application
successors	I-Application
on	O
IBM	B-Device
System/360	I-Device
architecture	I-Device
and	O
successors	O
.	O
</s>
<s>
In	O
OS/360	B-Application
,	O
OS/VS1	B-Application
,	O
SVS	B-Application
,	O
MVS/370	O
,	O
MVS/XA	O
,	O
MVS/ESA	O
,	O
OS/390	B-Application
and	O
z/OS	B-Application
,	O
the	O
TCB	O
contains	O
,	O
among	O
other	O
data	O
,	O
non-dispatchability	O
flags	O
and	O
the	O
general	O
and	O
floating	O
point	O
registers	O
for	O
a	O
task	O
that	O
is	O
not	O
currently	O
assigned	O
to	O
a	O
CPU	O
.	O
</s>
<s>
A	O
TCB	O
provides	O
an	O
anchor	O
for	O
a	O
linked	O
list	O
of	O
other	O
,	O
related	O
request	O
blocks	O
(	O
RBs	O
)	O
;	O
the	O
top-linked	O
RB	O
for	O
a	O
TCB	O
contains	O
the	O
Program	B-Device
status	I-Device
word	I-Device
(	O
PSW	O
)	O
when	O
the	O
task	O
is	O
not	O
assigned	O
to	O
a	O
CPU	O
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
MVS/370	O
and	O
successor	O
systems	O
,	O
a	O
whole	O
new	O
environment	O
was	O
introduced	O
:	O
the	O
Service	O
Request	O
Block	O
(	O
SRB	O
)	O
,	O
which	O
generally	O
has	O
a	O
higher	O
priority	O
than	O
any	O
Task	B-Operating_System
Control	I-Operating_System
Block	I-Operating_System
,	O
and	O
,	O
indeed	O
,	O
which	O
itself	O
has	O
two	O
distinct	O
priorities	O
:	O
a	O
Global	O
SRB	O
(	O
priority	O
over	O
all	O
local	O
address	O
space	O
SRBs	O
and	O
TCBs	O
)	O
and	O
a	O
Local	O
SRB	O
(	O
priority	O
over	O
only	O
the	O
local	O
address	O
space	O
TCBs	O
)	O
;	O
and	O
MVS	O
's	O
dispatcher	O
must	O
manage	O
all	O
of	O
these	O
with	O
absolute	O
consistency	O
across	O
as	O
many	O
as	O
two	O
processors	O
(	O
MVS/370	O
)	O
and	O
as	O
many	O
as	O
sixteen	O
processors	O
(	O
successor	O
systems	O
)	O
.	O
</s>
