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CHAPTER 24
24.
SYSTEM DESCRIPTION
This
chapter
contains the detailed descriptions of the
structure of an MDOS diskette:
the structure
of
MDOS
files
and
the
i
l'
formats,
th e sy stem over lay s,
th
e
memory
map,
th e
command interpreter,
interrupt handlers,
the system
function
handler,
and
the
MDOS
eq,uate
file.
The subseq,uent three
chapters contain the detailed descriptions of the
individual
system functions
and
how they
are
parameterized.
24.1 Diskette Structure
MOOS
is
based
on
a
single-
or
double-sided,
single-densit~
flexible disk,
or diskette.
The
diskette
is
compact
in
size:
portable,· rairly
durable)
and
easily
inserted into and removed rrom the diskette drives.
Due
to
the
diskette's
portability
and
interchangeabilit~1
each
diskette is
treate~
by MDOS
as
a
complete,
self-contained
entity.
Each
diskette has its own system tables, operating
system, and riles.
Information on an MDOS diskette is stored in sectors 128
(decimal)
bytes
in
size.
As
the
diskette
turns,
the
read/write head in a stationary position will
pass
over
26
(decimal)
sectors
each
revolution.
The area accessible to
the stationary head on one side of the diskette is
called
a
track.
The
area
accessible to the stationary head on both
sides of the diskette is called
a
cylinder.
The
head can
be
p
0
sit ion e d
0
v e r
77
(d e c
i
ma I)
dis c
l'
e tee y
1
i
n d e
l'
s .
Th
us,
the
l'
e
are a
total
of 2002
(decimal) sectors
on
each
surface
Or
a
diskette.
A single-sided diskette only has one surface that
can be read Tram
and
written to.
A double-sided diskette has
two surfac es.
In
order
to minimize access time and yet provide ror a
dynamic allocation scheme, all
disk~tte
space
allocation
is
done
in
terms
of
clusters,
rather
than
sectors.
MDOS
clusters consist of rour,
physically sequential
sectors.
A
cluster is the smallest structural unit of information on the
dis
k
e t t e .
T
h us,
the s rna
11
est
p
os sib
1
e s
i
z
e t hat a
f
i 1
e
c an
have is one cluster.
The
statistics.
rollowing
table
summarizes
these
diskette
Page
24-01
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