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CDS Resident Software Development Aids _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 67
line is a
printable
character (including space or
blank). Every non-printing character code represents
a control character. A control code may be generated
on the keyboard either by hitting an appropriately
marked key (e.g., RETURN, ESC, etc.) or by
depression of the CTRL button while hitting another
key. The printer reacts to the receipt of a control
character in one of several possible ways. Some
control characters (such as carriage return, line feed,
bell,
'
etc.) cause the printer to execute a specific
control function. Other control codes either are
ignored by the printer or may cause the equivalent of
a space on the printed line.
A line in a file may contain control characters
(with certain restrictions to be discussed later). If it
does, it is quite possible that its printed record will not
completely reflect the true contents of the line. CRE
treats most control characters which it encounters
within a line in the same manner as it treats printing
characters. However, certain control characters have
special meaning to CRE.
The proper format for tape files is shown in Fig. 27.
Each line is terminated with a CARRIAGE
RETURN (CR), LINE FEED (LF) pair, followed
by a field of six nulls. The NULL character (hex code
00) is ignored by the system. A set of nulls appears
after each CR, LF, pair merely to provide a sufficient
time delay for the printer carriage to settle to the new
line 'When the tape contents are being printed. Note
that the last line on the tape should be followed by a
"
dummy
"
line containing only the single data
character DC. DC3 is a special control character
(generated on the keyboard by hitting CTRL and S).
lt
acts as an END OF FILE indicator. Note also that
if the file is stored on paper tape, it is normally
preceded by a leader of nulls and followed by a trailer
of nulls. The null leader permits arbitrary initial
positioning of the tape in the reader.
Tape records read by CRE are deposited into the
buffer as they appear on the tape, but with all LF's
and NULL's ignored. (Incidentally, the DEL or
RUBOUT character, hex code 7F, is also ignored on
tape input) While CRE operates on the data in its
buffer, it specifically uses the CR character as an
indicator of the end of a line. (Recall that a line has a
variable length.) A new line is assumed to start with
the next character in the buffer. The buffer format is
shown in Fig. 28.
L
-_ _
--1
L . . U _ ~ _ H _ ~ O _ R A _ 7 g _ T
_
~
_
~ I
_ A_'__.L__L
_ _
__'__ _ _ _ _
...J_J{
I
I
DC3
CR
I
LF
I
Ntl~LS
I
' - -__ ---'
1----1.
,..-.I,
II
·1
PAPER TAPE
ONLY
FIRST LINE
t----r-------l·
1
1-
1.
--r-----+
- I-· ---.----!-I
11- - - - - - - 1 '
---i-I
SECOND LINE
(FOLLOWED BY
LF
+
6 NULLS)
LAST LINE
END OF FILE
(FOLLOWED BY
"DUMMY" LINE
LF
+
6 NULLS)
Fig.
27 -
Tape file format.
PAPER TAPE
ONLY
9
2C
M-
2
8
2
15
,---I
DATA_CHARAC_
TERS
- - , - -
CR
~I
ji
I
lEG
1---
1
. ------,----I---r----l . 1
~1. ~
J
L-J
LINE I
LINE 2
LINE m
.
L
PRE~
ENT
ONLY IF LINE m IS THE
~
LINE IN THE INPUT FILE
Fig. 28 - Memory buffer format.
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