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Bytes 10, 11, and 12
indicate the address of the data extent block.
The request element is normally 12 bytes in length; however, in
a
multitasking
environment,
it
includes
4
wore bytes that
indicate the address of the TCB.
• Register 2 contains the address of the input/output block (lOB).
• Register 3 contains the address of the data extent block (DEB).
• Register 4 contains the address of the data control block (DCB).
• Register 7 contains the address of the unit control block (UCB).
The system places, into register 14, the address of the
location
in
the
input/output
supervisor
to
which control is to be returned after
execution of the appendage.
When passing control from an
appendage
to
the
system, you may use displacements to the return address in register
14 for optional return procedures.
Some of these procedures
differ
in
their
treatment
of
the
request
element
associated with the channel
prograrr.
You may not change register 1 in an appendage.
Register 9, if
used,
must
be
set
to
binary zero before control is returned to the system.
All other registers, except those indicated in the descriptions of
each
appendage, must be saved and restored if they are used.
The
types of appendages are listed in the following paragraphs, with
explanations of when they are entered, how they return
control
to
the
system, and which registers they may use without saving and restoring.
start Input/Output (SIO) Appendage
This appendage is entered before the input/output supervisor issues a
start input/output (SIO) instruction for an I/O operation.
If the return address in register 14 is used to return control to the
input/output
supervisor,
the
I/O operation is executed normally.
You
may optionally bypass the SIO instruction and prevent execution
of
the
channel
progra~
by
using
the
contents
of register 14 plus 4 as the
return address.
In
this
case,
the
thannel
program
is
not
posted
complete, but its request element is made available.
You
may
use
registers
10 and 11 in a start input/output appendage
without saving and restoring their contents.
Program Controlled Interruption (PCI) AEEendage
This appendage is entered
when
a
program
controlled
interruption
occurs.
At
the
time of the interruption, the contents of the channel
status word will not have been placed in the "channel status word" field
of the input/output block.
The channel status word can be obtained from
location 64.
You must use the return address in register
14
to
allow
the system to proceed with normal interruption processing.
You
may
use
registers
10
through
13
in
a
program
controlled
interruption appendage without saving and restoring their contents.
End-of-Extent Appendage
This appendage is entered when the
seek
address
specified
in
the
input/output
block
is outside the allocated extent limits indicated in
the data extent block.
82
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