IBM System/360 System Programmer's Guide

defines a new storage area for the original I/O operation. Command
chaining occurs when the new channel comwand word specifies a new I/O
operation. For detailed information about chaining, refer to the IBM
System/360: principles of Operation publication (Form A22-6821).
To specify either data chaining or commanQ chaining, you must set
appropriate bits in the channel command word, and indicate the type of
chaining in the input/output ~lock. Both data and command chaining
should not be specified in the same channel comwand word; if they are,
data chaining takes precedence.
When a channel program includes a list of channel command words that
chain data for reading operations, no channel command word may alter the
contents of another channel command word in the same list. (If such
alteration were allowed, specifications could be placed into a channel
command word without being checked for validity. If the specifications
were incorrect, the error could not be detected until the chain was
completed. Data. could be read into incorrect locations and the systew
could not correct the error.)
CONTROL BLOCKS
When using the EXCP macro-instruction, you wust be familiar with the
function and structure of an input/output block (lOB), an event control
block (ECB), a data control block (DCB), and a data extent block (DEB).
Brief descriptions of these control blocks follow. Their fields are
illustrated in the section "EXCP Programming Specifications."
Input/Output Block (lOB)
The input/output block is used for corrmunication between the problew
program and the system. It provides the addresses of other control
blocks, and maintains inforwation abcut the channel prograro, such as the
type of chaining and the progress of I/O operations. You ITust define
the input/output block and specify its address as the only parameter of
the EXCP macro-instruction.
Event Control Block (ECB)
The event control block provides you with a completion code that
describes whether the channel prograrr was completed with or without
error. A WAIT macro-instruction for synchronizing I/O operations with
the problem program must be directed to the event control block. You
must define the event control block and specify its address in the
input/output block.
Data Control Block (DCB)
The data control block provides the system with information about the
characteristics and processing requirements of a data set to be read or
written by the channel program. A data control block must be produced
by a DCB macro-instruction that includes parameters for EXCP. You
specify the address of the data control block in the input/output block.
Data Extent Block (DEB)
The data extent block contains one or more extent Entries for the
associated data set, as well as other control information. An extent
defines all or part of the physical tounjaries cn an I/O device occupied
by, or reserved for, a particular data set. Each extent entry contains
the address of a unit control block (UCB), which provides information
about the type and location of an I/O device. More than one extent
entry can contain the same UCB address. (Unit control blocks are set up
at system generation time and need not concern you.) For all I/O
Execute Channel Program (EXCP) Macro-Instruction 77

IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English