IBM System/360 System Programmer's Guide

4. Issue an EXCP roacro-instruction to pass the address of the IOB to
the routines that initiate and supervise the I/O operations.
The input/output supervisor will then:
5. Schedule the I/O request.
6. Issue a start input/output (SIO) instruction to activate the I/O
device.
7. Process I/O interruptions and schedule error recovery procedures,
when necessary.
8. Place a completion code in the event control block after the
channel program has been executed.
The programmer is not concerned with these procedures and does not
know the status of I/O operations until they are completed. Device-
dependent operations are limited to those provided by the macro-
instructions of the particular access method selected.
PROGRAMMER USE OF EXCP
If you wish to issue the EXCP macro-instruction directly, you must
perform the procedures that the access methods perform, as summarized in
items 1 through 4 of the preceding discussion. You must, in addition to
constructing and opening the data control block with the DCB and OPEN
macro-instructions, construct a channel prograrr, an input/output block,
and an event control block before you can issue the EXCP macro-
instruction. The input/output supervisor always handles items 5 through
8.
After issuing the EXCP rr.acro-instruction, you should issue a WAIT
macro-instruction specifying the event control block to determine
whether the channel program has terminated. If volume switching is
necessary, you rrlust issue an EOV macro-instruction. When processing of
the data set has been completed, you must issue a CLOSE macro-
instruction to restore the data control block.
EXCP REQUIREMENTS
This section describes the channel prograrr
1
that you must provide in
order to issue thE EXCP macro-instruction. The control blocks that you
must either construct directly, or cause to be constructed by use of
macro-instructions, are also described.
CHANNEL PROGH.AM
The channel program supplied Ly you and executed through EXCP is
composed of channel command words (CCWs) on double-word boundaries.
Each channel command word specifies a corrmand to be executea and, for
commands initiating data transfer, the area to or from which the data is
to be transferrert. Channel corrmand word formats used with specific I/O
devices can be found in IBM Systems Reference Library publications for
each device. All channel comrrand words described in these publications
can be used, with the exception of REWIND and UNLOAD (RWU).
Data and Command Chaining
Chaining is the successive loading of channel command words into a
channel from contiguous douDle-word locations in main storage. Data
chaining occurs when a new channel command word loaded into the channel
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IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English