
Chapter 2. Processing Commands and Presenting Status
An understanding of the Enterprise Systems Architecture/390
channels is
assumed throughout this manual. The only points discussed are those points
requiring further clarity, or where the subsystem has an option.
The following I/O instructions in ESA/390 affect the subsystem:
Clear Subchannel
Halt Subchannel
Resume Subchannel
Start Subchannel
Test Subchannel
Reset Channel path
Channel Subsystem Call - Reset Control Unit.
The Manual
IBM Enterprise Systems Architecture/390 Principles of Operation
describes the above instructions.
Internal Disk Subsystem supports all count-key-data (CKD) and extended
count-key-data (ECKD) channel commands defined in this manual.
Input/Output Channel Interface
Command Processing
The Internal Disk Subsystem accepts and processes channel commands when:
The host system is powered on and initialized.
The logical path has been established between the Internal Disk Subsystem
and the host system.
The logical volume address is within the range supported by the control unit.
The physical RAID array associated with the logical volume address is installed
and on-line.
A Logical Volume has been configured at the device address.
If any of these conditions are not satisfied, the Internal Disk Subsystem will not
process the command. The subsystem response depends on which conditions are
not satisfied.
Status Presentation
Internal Disk Subsystem generates status conditions that it presents to the host
channel.
Status Byte
The eight bits of status information make up the status byte. The status byte
contains information that shows the status of the subsystem or logical volume. The
status byte contents are shown in Table 2-1 on page 2-2.
Copyright IBM Corp. 1999
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