IBM System/360 System Programmer's Guide

EXECUTE CHANNEL PROGRAM (EXCP) MACRO-INSTRUCTION
Execute Channel Program (EXCP) is a macro-instruction of System/360
Operating System that causes a supervisor-call interruption to pass
control to the input/output supervisor. EXCP also provides the
input/output supervisor with control information regarding a channel
program to be executed. When the IBM standard data access methods are
being used, the access method routines are responsible for issuing BXCP.
If you are not using the standard access methods, you may issue EXCP
directly. Direct use of EXCP provides you with device dependence in
organizing data and controlling I/O devices.
You issue EXCP primarily for I/O programming situations to which the
standard access methods do not apply. When you are writing your own
data access methods, you must include EXCP for I/O operations. EXCP
must also be used for processing of nonstandard labels, including the
reading and writing of labels and the positioning of magnetic tape
volumes.
To issue EXCP, you must provide a channel program (a list of channel
command words) and several control blocks in your program area. The
input/output supervisor then schedules I/O requests for the device you
have specified, executes the specified I/O commands, handles I/O
interruptions, directs error recovery procedures, and posts the results
of the I/O requests.
USE OF EXCP IN SYSTEM AND PROBLEM PROGRAMS
This section briefly explains the procedures performed by the system
and the programmer when the EXCP macro-instruction is issued by the
routines of the standard data access methods. The additional procedures
that you must perform when issuing the EXCP macro-instruction yourself
are then described by direct comparison.
SYSTEM USE OF EXCP
When using a standard data access method to perform I/O operations,
the programmer is relieved of coding channel programs, and of
constructing the control blocks necessary for the execution of channel
programs. To permit I/O operations to be handled by an access method,
the programrr.er need only issue the fcllowing macro-instructions:
• A DCB macro-instruction that produces a data control block (DCB) for
the data set to be retrieved or stored.
• An OPEN macro-instruction that initializes the data control block
and produces a data extent block (DEB) for the data set.
• A macro-instruction (e.g. GET, WRITE) that requests I/O operations.
Access method routines will then:
1. Create a channel program that contains channel commands for the I/O
operations on the appropriate device.
2. Construct an input/output block (lOB) that contains information
about the channel program.
3. Construct an event control block (ECB) that is later supplied with
a completion code each time the channel program terminates.
Execute Channel Program (EXCP) Macro-Instruction 75

IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English