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Chaining
When the channel has performed the transfer of
information specified by a CCW, it can continue
the activity initiated by START I/O or START
I/O FAST RELEASE by fetching a new C CW.
Such fetching of a new CCW is called chaining,
and the CCWs belonging to such a sequence are
said to be chained.
Chaining takes place between CCWs located in
successive double word locations in storage.
It
proceeds in an ascending order of addresses; that
is, the address of the new CCW is obtained by
adding 8 to the address of the current CCW. Two
chains of CCWs located in noncontiguous storage
areas can be coupled for chaining purposes by a
transfer-in-channel command. All CCWs in a
chain apply to the I/O device specified in the
original START I/O or START I/O FAST
RELEASE.
Two types of chaining are provided: chaining of
data and chaining of commands. Chaining is
controlled by the chain-data (CD) and chain-
command (CC) flags in conjunction with the
suppress-length-indication (SLI) flag in the CCW.
These flags specify the action to be taken by the
channel upon the exhaustion of the current CCW
and upon receipt of ending status from the device,
as shown in the figure "Channel-Chaining Action."
The specification of chaining is effectively
propagated through a transfer-in-channel
command. When in the process of chaining a
transfer-in-channel command is fetched, the CCW
designated by the transfer in channel is used for
the type of chaining specified in the CCW
preceding the transfer in channel. The CD and CC
flags are ignored in the transfer-in-channel
command.
Chapter 12. Input/Output Operations
12-31
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