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WAIT
(User
Option)
I
I
BLOCKED I/O
USER
PROCESS
UNBLOCKED I/O
USER
1-----_
PROCESS
I/O System
Figure 7-8.
Blocked and Unblocked I/O
bit is examined.
If the awaken process is specified, the process
will be reactivated if it has put itself into the I/O wait state.
If no action is specified,
presumably the process has
continued
to execute without any wait,
or will be reactivted by some other
process.
In any case, the process checks for I/O completion.
1-12. I/O HARDWARE ELEMENTS
The I/O hardware elements are responsible for a large portion
of
the execution of an I/O request.
When software passes control to
hardware, the I/O hardware elements assume full control fram that
point while the
software performs other functions.
The comput-
er's I/O
hardware elements are the I/O Processor (lOP),
part of
the Module
Control Unit,
the Multiplexer Channel,
and Selector
Channel.
The following paragraphs
contain detailed discussions
for each of these elements.
1-13. I/O Processor
In addition to interrupting the CPU on behalf of the Device
Con-
trollers
(Section
VIII),
the lOP performs three specific func-
tions relating to the three different I/O transfer modes shown in
figure 7-9.
For direct I/O transfers,
the lOP executes the di-
rect I/O instructions
(RIO,
WIO, TIO,
CIa, SIN, and SMSK)
and
transfers data,
device status,
and control
information between
the
CPU
and a Device Controller.
For programmed I/O transfers
via a Multiplexer Channel,
the lOP transfers
I/O
program words
between
Main Memory
and
the MUltiplexer Channel
and transfers
data between
Main Memory
and the
Device Controller.
For pro-
grammed I/O transfers via a Selector Channel, the lOP only trans-
fers initialization
information to the Device Controller and
is
7-15
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