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SYSTEM DESCRIPTION
24. 1 -
Diskette Structure
sector that is loaded contains information regarding the size
or the resident operating system.
From this information.
the
Bootblock program configures the diskette controller to
load
into memory the actual resident operating system.
24.2 File. Structure
While
the
contents
of
a
rile
can
be
thought
of as a
logically
contiguous
block
or
information,
the
actual
diskette
area
allocated
to
the
file
mayor
may not be
physically contiguous.
Space can be allocated to one or more
groups
of
physically
contiguous
clusters on the diskette.
Each contiguous group of clusters is called
a
segment.
This
segmentation
allows
the dynamic allocation and deallocation
or
space
to
occur
without
having
to
move
any
of
the
information contained in the file
Eac
h
r
i 1
e
must,
th ereforel
have a tab
1
e that
d
escri bes
which segments are allocated to the file.
This table is kept
in
the
first physical sector of each file and is called the
Retrieval Information Block
(RIB),
It
is
the address of
the
RIB
that is contained in the directory entry or a file.
MDOS
accesses
sectors
within a file by logical sector
numbe~
(LSN)'
Since the first physical sector of a
rile
is
not
really
a
data
sector)
the
RIB
is given an LSN
Or
minus
one
($FFFF)'
Therefore,
logical sector zero of
a
rile
(the
first
data sector) is actually the second phqsical sector of
the file.
Logical
sector
numbers
for
data
sectors
are
numbered sequentially beginning with zero.
Thus, even though
a file may be segmented (not
physically
contiguous
on
the
diskette),
it is treated as a logically contiguous collection
of sectors when
accessed
by
logical
sector
number.
The
system
I/O
functions decode the
LSN
into the actual
PSN.
24.2.1 Retrieval Information Block
For
all
files,
the
RIB contains a series of two-byte
entries called segment descriptor words
(SDWs).
A
special
SDW
is
used
as
a
terminator
to
indicate the end of the
segment descriptors within the
RIB.
Each SDW (other than the
terminator)
contains two pieces of information:
the cluster
number of the first cluster in the segment, and the length of
the
segment.
Since
each
segment
consists
of physically
contiguous clusters,
this information is all that
is
needed
to
describe
where
a
segment of the file is located on the
diskette.
A RIB
can contain a maximum or
57
(decimal)
SDWs
and one terminator.
The
RIB
of
a
memory-image file contains
some
additional
information that describes where the contents of the file are
to
be
loaded
in
memory.
This
info~mation
includes the
Page
24-07
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