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I/O SYSTEM
I~
This section contains an overview of the
computer's
I/O
system
which
includes
discussions on file system operation, I/O system
q>eration, I/O instructions,
and I/O system hardware.
In
addi-
tion, this section contains principles of operation and servicing
information for
the
computer
system's
Input/Output
Processor
(lOP),
Multiplexer
Channel,
Port
Controller,
and
Selector
Channel.
7-1. INTRODUCTION
The general purpose of any computer system is to input,
process,
and output information.
Under MPE, this information may be cre-
ated and used
by the operating system
itself,
by compilers
or
other systems,
by
user
programs,
or by users themselves.
To
handle all information in a uniform, efficient way, MPE treats it
as groups of data called files.
Specifically,
a file is a col-
lection of information or data identified by a name recognized by
MPE.
MPE uses media such as discs, cards,
and tapes for storing
the information.
On any of these media,
a file may contain MPE
commands, s ys tern or user p rogr ams, or da ta ;
alone or in any com-
bination.
Within a file, all information is organized into units
of related data called logical records that for most applications
are similar in form,
purpose,
and content.
The records in the
file
can be arranged in almost any order; alphabetically, numer-
ically,
chronologically,
by subject matter,
etc.
The logical
record
is the smallest grouping of data that MPE can address di-
rectly;
you specify its length when you create the file.
Indi-
vidual subsystems and user programs,
however, also can recognize
fields for data items within each record.
In addition, programs
can also
recognize and
manipulate
individual words,
eight-bit
bytes, and bits within a byte.
Data is
transferred to and
from files in
units called
blocks.
These are the basic units that are physically transferred between
Main Memory and the peripheral device on which the file
resides.
On
disc and magnetic tape files, a block consists of one or more
logical records;
on files of
other media,
a block
normally is
equivalent to one logical record (unless you request input/output
under the multi-record mode).
To summarize the interrelation
of
files,
logical records,
and blocks:
a file is a
collection of
records
treated as a unit
and recognized by
a name;
a logical
record is a collection of fields treated as a unit, residing in a
file;
and a block
is a
group of
one or
more logical
records
transmitted
to or from a file by an input/output operation.
The
purpose of the I/O system,
then, is to
perform actual
.physical
input/output
operations for the file system of the MPE operating
system. The user normally does not interact directly with the I/O
s ys te m;
0
n 1yind ire c tl y v i a the
f i 1 e s ys te mas shown
in fig u r e
7-1.
Normally,
all I/O operations
are invisible
to the
user.
However,
as shown in figure 7-1, privileged users may access the
I/O system directly.
7-1
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