Chapter 4. Command Descriptions
This section describes the channel command word and the valid channel
commands for the Internal Disk.
Channel Command Word
The channel command word (CCW) specifies the command, and designates the
processor storage area associated with the operation and the action taken when
the operation completes.
CCWs have either format 0 or format 1. The formats do not differ in the
information contained in the CCW but only in the arrangement of the fields within
the CCW, and the size of the data address field. The two formats are shown in
Table 4-1 and Table 4-2.
The description of the flags follows:
Chain Data (CD):
Bit 32 (format 0) or bit 8 (format 1), when ‘1’, specifies chaining
of data. It causes the storage area designated by the next CCW to be used with
the current I/O operation.
Chain Command (CC):
Bit 33 (format 0) or bit 9 (format 1), when ‘1’, and when
the CD flag and S flag are both zero, specifies chaining of commands. It causes
the operation specified by the command code in the next CCW to start on normal
completion of the current operation.
Suppress Length Indication (SLI):
Bit 34 (format 0) or bit 10 (format 1) controls
if an incorrect length indication is to be sent to the program. When this bit is ‘1’
and the CD flag is ‘0’, incorrect length is suppressed. When both the CC and SLI
flags are ‘1’, and the CD flag is ‘0’, command chaining takes place, regardless of
the presence of an incorrect length condition.
Skip (SKIP):
Bit 35 (format 0) or bit 11 (format 1), when ‘1’, specifies the
suppression of transfer of information to processor storage during read and sense
operations.
Program-Controlled Interruption (PCI):
Bit 36 (format 0) or bit 12 (format 1),
when ‘1’, causes the channel to generate an intermediate interruption condition by
using the appropriate subchannel when the CCW takes control of the I/O operation.
Table
4-1. Format 0 — 24 Bit Addressing (System/370)
0–7
Command
Code
8–31
Data Address
32–39
Flags
40–47
Not Used
48–63
Count
Table
4-2. Format 1 — 31 Bit Addressing (370-XA and ESA/370)
0–7
Command
Code
8–15
Flags
16–31
Count
32–63
Data Address
Copyright IBM Corp. 1999
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