<s>
A	O
Service	B-Application
Request	I-Application
Block	I-Application
(	O
SRB	O
)	O
is	O
a	O
data	B-General_Concept
structure	I-General_Concept
of	O
MVS/370	O
and	O
successor	O
versions	O
of	O
IBM	B-Device
mainframe	I-Device
operating	B-General_Concept
systems	I-General_Concept
employed	O
mainly	O
,	O
but	O
not	O
exclusively	O
,	O
by	O
the	O
Start	B-Application
Input/Output	I-Application
interface	O
.	O
</s>
<s>
An	O
SRB	O
may	O
be	O
considered	O
,	O
in	O
the	O
abstract	O
,	O
to	O
be	O
a	O
highly	O
optimized	O
Task	B-Operating_System
Control	I-Operating_System
Block	I-Operating_System
(	O
TCB	O
)	O
,	O
one	O
which	O
has	O
few	O
,	O
if	O
any	O
,	O
associated	O
resources	O
other	O
than	O
access	O
to	O
the	O
processor	O
itself	O
.	O
</s>
<s>
All	O
system	O
resources	O
which	O
are	O
utilized	O
under	O
an	O
SRB	O
must	O
be	O
accessed	O
through	O
the	O
use	O
of	O
"	O
branch	O
entries	O
"	O
,	O
some	O
of	O
which	O
are	O
new	O
entries	O
to	O
traditional	O
system	O
services	O
which	O
were	O
formerly	O
accessed	O
exclusively	O
using	O
SVC	B-Operating_System
instructions	O
(	O
which	O
an	O
SRB	O
may	O
not	O
employ	O
for	O
any	O
purpose	O
other	O
than	O
abnormally	O
terminating	O
itself	O
in	O
which	O
case	O
SVC	B-Operating_System
13	O
,	O
ABEND	O
,	O
may	O
be	O
used	O
,	O
however	O
the	O
"	O
branch	O
entry	O
"	O
to	O
ABTERM	O
is	O
really	O
more	O
appropriate	O
)	O
.	O
</s>
<s>
When	O
employed	O
by	O
the	O
Start	B-Application
Input/Output	I-Application
interface	O
,	O
an	O
SRB	O
is	O
always	O
paired	O
with	O
an	O
Input/Output	B-Application
Supervisor	I-Application
Block	I-Application
(	O
IOSB	B-Application
)	O
.	O
</s>
<s>
When	O
otherwise	O
employed	O
,	O
an	O
SRB	O
facilitates	O
inter-address-space	O
communication	O
in	O
general	O
,	O
and	O
inter-application	B-Operating_System
communication	I-Operating_System
in	O
particular	O
.	O
</s>
<s>
SRBs	O
may	O
also	O
be	O
employed	O
for	O
intra-address-space	O
processes	O
,	O
where	O
the	O
highest	O
possible	O
performance	O
is	O
required	O
,	O
and	O
in	O
this	O
case	O
the	O
necessary	O
resources	O
are	O
first	O
acquired	O
under	O
a	O
TCB	O
(	O
usually	O
the	O
"	O
job	O
step	O
"	O
TCB	O
)	O
,	O
before	O
the	O
SRBs	O
are	O
SCHEDULEd	O
(	O
i.e.	O
,	O
are	O
presented	O
to	O
the	O
system	O
dispatcher	O
to	O
compete	O
for	O
processor	O
resources	O
)	O
.	O
</s>
<s>
It	O
is	O
conceivable	O
that	O
an	O
address	B-General_Concept
space	I-General_Concept
may	O
have	O
but	O
one	O
TCB	O
(	O
again	O
,	O
the	O
"	O
job	O
step	O
"	O
TCB	O
)	O
but	O
tens	O
or	O
hundreds	O
or	O
even	O
thousands	O
of	O
SRBs	O
,	O
with	O
the	O
SRBs	O
performing	O
almost	O
all	O
of	O
the	O
work	O
in	O
the	O
address	B-General_Concept
space	I-General_Concept
,	O
and	O
the	O
TCB	O
merely	O
synchronizing	O
the	O
SRBs	O
and	O
responding	O
to	O
communications	O
from	O
the	O
system	O
operator	O
.	O
</s>
<s>
For	O
purposes	O
of	O
such	O
synchronization	O
,	O
the	O
TCB	O
will	O
usually	O
issue	O
a	O
WAITR	O
,	O
SVC	B-Operating_System
1	O
,	O
specifying	O
a	O
list	O
of	O
Event	O
Control	O
Blocks	O
(	O
ECBs	O
;	O
one	O
ECB	O
per	O
SRB	O
,	O
plus	O
one	O
for	O
the	O
system	O
operator	O
)	O
,	O
and	O
each	O
SRB	O
will	O
indicate	O
its	O
completion	O
to	O
the	O
TCB	O
by	O
using	O
a	O
"	O
branch	O
entry	O
"	O
to	O
the	O
POST	O
system	O
service	O
(	O
which	O
is	O
normally	O
SVC	B-Operating_System
2	O
,	O
but	O
in	O
this	O
special	O
case	O
would	O
be	O
a	O
call	O
to	O
the	O
address	O
contained	O
in	O
CVT0PT01	O
)	O
,	O
and	O
specifying	O
the	O
ECB	O
which	O
is	O
associated	O
with	O
its	O
SRB	O
,	O
and	O
possibly	O
a	O
"	O
message	O
"	O
to	O
the	O
TCB	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	B-Application
Request	I-Application
Block	I-Application
(	O
SRB	O
)	O
,	O
which	O
generally	O
has	O
a	O
higher	O
priority	O
than	O
any	O
TCB	O
,	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	B-General_Concept
space	I-General_Concept
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	B-General_Concept
space	I-General_Concept
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>
<s>
z/System	O
support	O
for	O
zIIP	B-Device
has	O
dramatically	O
increased	O
the	O
use	O
of	O
SRBs	O
(	O
over	O
and	O
above	O
their	O
traditional	O
Start	B-Application
Input/Output	I-Application
usage	O
)	O
.	O
</s>
<s>
An	O
SRB	O
is	O
a	O
highly-optimized	O
instance	O
of	O
a	O
Process	B-Operating_System
control	I-Operating_System
block	I-Operating_System
in	O
these	O
MVS/370	O
and	O
successor	O
systems	O
.	O
</s>
<s>
A	O
TCB	O
is	O
a	O
general-purpose	O
instance	O
of	O
a	O
Process	B-Operating_System
control	I-Operating_System
block	I-Operating_System
in	O
these	O
OS/360	O
and	O
successor	O
systems	O
.	O
</s>
