IBM System/360 System Programmer's Guide

COMPLETION OF CHANNEL PROGRAM
The system considers the channel program completed when it receives
an indication of a channel end conditien. When channel end occurs, the
request element for the channel prograrr is ~ade available, and a
completion code is placed into the event contrcl block. The completion
code indicates whether errors are associated with channel end. If
device end occurs simultaneously witl) channel end, errors associated
with device end (i.e., unit exception or unit check) are also accounted
for.
Device End Errors
If device end occurs after channel end and an error is associated
with device end, the completion code in the event control block does not
indicate the error. However, the status of th~ unit and channel is
saved in the unit control block (UCB) fer the device, and the UCB is
marked as intercepted. The input/output block for the next request
directed to the I/O device is also rrarked as intercevted. The error is
assurLied to be permanent, and the completion code in the event control
block for the intercepted request indicates interception. The IFLGS
field of the data control block is also flagged to indicate a permanent
error. It 3hould be noted that when a Write T2pe Mark or Erase Long Gap
CCW is the last (or only) CCW in your channel progran', the I/O
Supervisor will not attempt recovery procedures for Device End errors.
In these circumstances, cOITrrand chaining a NOPCCW to your Write Tape
Mark or Erase Long Gap CCW ensures initiation of device end error
recovery procedures.
To be prepared for device end errors, you should be familiar with
device characteristics that can cause such errors. After one of your
channel programs has terminated, you should not release buffer space
until you have determined that your next request for the device has not
been interceptea. You may reissue an intercepted request.
INTERRUPTION HANDLING AND ERROR RE:CCVERY PROCEDURES
An I/O interruption allows the CPU to respond to signals from an I/O
device which indicate either terrrination of a phase of I/O operations or
external action on the device. A complete explanation of I/O interrUp-
tions is contained in the IBM Systerr./360: Principles of Operation
publication. For descriptions of interruptions by specific devices,
refer to IBM Systerrs Reference Library publications for each device.
If error conditions are associated with an interruption, the
input/output supervisor schedules the appropriate device-dependent error
routine. The channel is then restarted with another request that is not
related
1
to the channel program in error. If the error recovery
procedures fail to correct the error, the system places ones in the
first two bit positions of the IFLGS field of the data control block.
You are inforwed of the error by an error code that the system places
into the event control block.
Error Recovery Procedures for Related Channel Programs
Related channel programs are requests that are associated with a
particular data control block and data extent block in the same job
step. They must be executed in a definite erder, i.e., the order in
which the requests are received by the input/output supervisor. A
channel program is not started until all previous requests for related
1Related channel programs are discussed in the next section.
Execute Channel Program (EXCP) Macro-Instruction 79

IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English