September 2002
Intermediate to advanced
332 pages
11h 1m
English
Content preview from IBM eServer zSeries 900 Technical Guide
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60 IBM eServer zSeries 900 Technical Guide
The logical flow of an I/O operation is illustrated in Figure 2-18 on page 61, using a
CCW-based I/O operation as an example.
A CCW-based I/O operation is always initiated by a CP or an IFL. The operating system
issues a Start Subchannel (SSCH) instruction to pass the I/O request to the z900 Channel
Subsystem. The I/O request is placed in the supporting I/O subchannel, using an Unit Control
Word. UCWs are stored in the Hardware System Area of central storage and are moved to a
channel subsystem during execution of an I/O operation.
One of the System Assist Processors, a processing unit running specialized Licensed Internal
Code, is informed that there is an I/O request. The SAP then selects one of ...
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