Table of Contents
I/O System
DATA AREA
I/O
QUEUE
L
I
I
I
I
I
I
tl
Interrupt
Linkage
Table
Device
Information
Table
Unit 0
Device
Information
Table
Unit 1
Device
Information
Table
Unit 2
Device
Information
Table
Unit 3
DEVICE
REFERENCE
TABLE
I/O
Prog Pointer
Ext.
Prog. Label
OBI
Reserved
DEVICE
CONTROLLER
Unit 0
Unit 1
Unit 2
Unit3
SOFTWARE
HARDWARE
Figure 7-3.
I/O System Fundamental Elements
some of the important elerne"lts of the I/O system.
(This figure is
by no means complete, but rather is intended to define the
chain
of
linkages that
are basic to the I/O system.)
For each Device
Controller
there is a
four-word entry
in the
Device Reference
Table (DRT).
The third word in
the four-word table
entry con-
tains a
pointer to a data
area
uniquely
associated with
that
table entry.
The data
area consists
of an
Interrupt Linkage
Table (ILT), one or more Device Information Tables (DIT)
(depend-
ing on how
many units the Device Controller is driving),
and an
I/O
program area.
Along with various
other information,
the
Driver Linkage Table (DLT) contains Code Segment Table (CST)
and
Segment
Transfer Table (STT) values for defining the location of
the dr iver
routines associated with that particular
Dev ice Con-
troller.
The DIT contains information
relevant to one physical
I/O device and is configured differently for each type of device.
In each case, however, the third word of this table points to
an
entry in the I/O Queue (IOQ) when a request is being made.
The IOQ is a single table (only
one
per
system)
containing
a
fixed number of entries having a fixed number of words per entry.
If there are no I/O requests pending in the system, none
of
the
DIT
entries
will be pointing to the IOQ.
In this case, all en-
tr ies of the IOQ are un used, and the second word
of
each
entry
points
to
the
first
word of the next entry.
Thus, all unused
entries are linked together.
Assume that the file system makes a
request to use Unit
I
of the Device Controller
shown
in
figure
7-3.
The I/O system will unlink the first free entry in the
IOQ
and fill it with information pertaining to the request (including
buffer address and logical device number).
Assume that the
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