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Draft Document for Review April 7, 2004 6:15 pm 6947ch04.fm
Chapter 4. Channel Subsystem
This chapter describes how the Channel Subsystem (CSS) is implemented on the z990. Each
server has a Channel Subsystem. Its role is to control communication of internal and external
channels to control units and devices. The configuration definitions of the CSS define the
operating environment for the correct execution of all system Input/Output (I/O) operations.
The CSS provides the server communications to external devices via channel connections.
The channels permit transfer of data between main storage and I/O devices or other servers
under the control of a channel program. The CSS allows channel I/O operations to continue
independently of other operations within the central processors.
The architecture and functionality of the CSS is significantly enhanced on the z990. The
implementation of the Channel Subsystem is significantly different from those on previous
zSeries servers.
This chapter introduces the concept of multiple Logical Channel Subsystems (LCSSs)
implemented on the z990. The goal of this chapter is to make you familiar with the technology,
terminology, and implementation aspects of the z990 Channel Subsystem design. The
multiple Logical Channel Subsystem (LCSS) and related components are described in:
򐂰“Multiple Logical Channel Subsystem (LCSS)” on page 110
򐂰“Logical Channel Subsystem structure” on page110
򐂰“LCSS configuration management” on page115
򐂰“LCSS-related numbers” on page117
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