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SYSTEM DESCRIPTION
24.3 -- Record Structure
checksum fields.
The first
ASCII
record
in
a
rile
begins
with
the first data character of
a
file and is terminated by
the first carriage return.
All other
ASCII
records
in
the
file begin with the. First data
character
Following a
carl"ia'ge
return.
When
ASCII
records
are
copied
to
non-diskette
devices,
the
terminating
carriage
return
is
actually
a
combination of three control
characters:
carriage
return,
1
ine
feed,
and
null.
ASCI I
records
should contain only
displayable characters.
When
MDOS
writes
ASCII
records
to
diskettel
they
normally
contain
space
compression
characters to conserve
diskette space.
A
space compression character
is
indicated
by
a
data byte having the sign bit (bit
7)
set to a one.
The
remaining bits (0-6) contain
a
binary number representing the
number
or
spaces
($20)
to
be
inserted
in
place of the
compressed
character"
MOOS
automatically
expands
these
characters
into
spaces when such files are read.
MDOS will
also automatically. create these
compressed
characters
~hen
such riles are written.
Since
MDOS
maintains the logical end-of-file
indicato~
in a file's
RIE,
no
ASCII
EOF
record
will
be
seen
in
a
diskette
filei
however,
when
ASCII
record riles are written
to a non-diskette device,
~he
~ollowing
EOF
record
will
be
supplied:
lA : CR :
where
the
"1A"
symbol represents the end-or-file indicator.
It is the hexadecimal value
$lA
or
SUB
control
character
(CTL-Z)'
The
CR
symbol is the carriage return,
line
reed,
and null sequence.
If
ASCII
record files generated on another
syst~m
are to
be
processed
by
MDOSJ
it
is
important that the carriage
return,
line reed,
and null sequence
be
present at the end of
each
record.
Otherwise,
i t
is possible for each data record
to lose one or two characters from its beginning.
24.3.3
ASCII-converted-binary records
A
special form of the binary record exists when
copying
to a non-diskette device that can only accept seven-bit data.
This record rormat is usually never kept
in a
diskette
rile.
The
rormat
of
the ASClI-converted-binary record
is
identical
to the binary record;
howeverl
each byte, with the
exception
of
the special header character and the terminating carriage
return,
line feed,
and null
sequenc~,.
is converted
into
two
eight-bit
bytes
with
bit
seven
set
to
zero.
This
is
accomplished by taking
each
haIr of
the
original
byte
and
Page
24-13
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