INTERACTIVE VISUAL SIMULATORS
F O R B E G I N N I N G P R O G R A M M I N G STUDENTS
Stuart C. S h a p i r o
Douglas P. W i t m e r
C o m p u t e r Science D e p a r t m e n t
Indiana University
Bloomington, Indiana
i.
Introduction
complexities of a m o d e r n c o m p u t e r w o u l d
obscure the points to be made.
Therefore,
various "hypothetical" computers have been
i n v e n t e d that consist of a basic set of
operations and registers.
Students generally write programs for these h y p o t h e t i c a l
computers and find out if their p r o g r a m s
are correct either from a human i n s t r u c t o r
or by e x e c u t i n g them using a s i m u l a t o r in
a b a t c h environment.
In either case, it
is d i f f i c u l t for them to u n d e r s t a n d the
s e q u e n t i a l operation of their programs or
the effect of any bugs that might have
been in them.
In order to provide a b e t t e r way for
i n t r o d u c t o r y c o m p u t i n g students to gain
this basic u n d e r s t a n d i n g , we have w r i t t e n
an i n t e r a c t i v e s i m u l a t o r of one h y p o t h e tical computer.
This s i m u l a t o r displays
all the storage and active registers of
the h y p o t h e t i c a l c o m p u t e r on the screen of
a CRT terminal, allows the student to load
and p a t c h programs, load data, and execute
the program.
The s i m u l a t o r traces the
p r o g r a m by changing the i n f o r m a t i o n on the
screen at a slow enough rate that the student can watch how his p r o g r a m works~
He
can then make any m o d i f i c a t i o n s he feels
are n e c e s s a r y and see the effect of those
changes.
This p a p e r discusses two p r o g r a m s
that have been w r i t t e n to be aids to
introductory p r o g r a m m i n g students.
They
both embody the b e l i e f that C o m p u t e r
A s s i s t e d I n s t r u c t i o n can be a w o r t h w h i l e
aid to students when p r o p e r l y used and
that one of the best uses is to present
visually to the student a process that he
has some control over and which he w o u l d
not otherwise be able to observe.
Section
2 of this paper discusses HYCOMP1, an
interactive visual computer simulator.
Section 3 discusses IVF, the Interactive
Visual FORTRAN interpreter.
They were
b o t h w r i t t e n in SNOBOL4 ± and run under
the KRONOS Time Sharing S y s t e m on a CDC
6600 using an Applied Digital Data
Systems, Inc. ADDS Consul 880 terminal,
which is an ASCII terminal with a CRT
display and an a d d r e s s a b l e cursor.
2.
An Interactive V i s u a l C o m p u t e r
Simulator 2
2.1 I n t r o d u c t i o n
It is often felt desirable to include in an i n t r o d u c t o r y c o m p u t i n g
course a small section of m a t e r i a l on
machine language, even when the course
will mostly use a language such as BASIC,
FORTRAN or ALGOL.
The purpose of the
section on machine language is to give
the students an u n d e r s t a n d i n g of what a
programmable, digital, s e q u e n t i a l comp u t e r is, what its basic operations are,
and how a program, made up of very basic
operations can effect a fairly complicated calculation.
To a c c o m p l i s h this
purpose, it is not n e c e s s a r y to use a
real c o m p u t e r as an example.
Indeed, the
2.2
The simulator, H Y C O M P I was w r i t t e n to
simulate the HYCOMP c o m p u t e ~ of Terry
W a l k e r ' s i n t r o d u c t o r y text. ~ W a l k e r ' s
H Y C O M P has 1000 words of memory, each
h o l d i n g a sign plus five decimal digits.
Its i n s t r u c t i o n code ignores the sign,
uses the two high o r d e r digits for an
o p e r a t i o n code and the r e m a i n i n g t h r e e
digits for an address.
In order to display all of memory on the CRT, we implem e n t e d only 100 words for HYCOMP1, and use
the two low order digits for the address,
i g n o r i n g the middle digit.
HYCOMP and
i Griswold, R.E., Poage, J.F., Polonsky,
I.P., The SNOBOL4 P r o g r a m m i n g Language,
2nd edition, Prentice-Hall, E n g l e w o o d
Cliffs, N.J., 1971.
2
HYCOMPI
3 Walker, Terry M., I n t r o d u c t i o n to Computer Science: An I n t e r d i s c i p l i n a r y Approach,
Allyn and Bacon, Inc., Boston, 1972.
H Y C O M P I was p r o g r a m m e d by David A.
Grace.
ii
HYCOMPI have the following active registers: a sign plus 5 digit arithmetic register (KEG); a 5 digit control unit (CU);
a program instruction counter (IC) which
is three digits in HYCOMP and two digits
on HYCOMPI.
They also have an overflow
(OF) switch, underflow (UF) switch, and
an end-of-file switch (EOF).
Floating
point numbers are stored using the two
high order digits as a biased exponent.
There are forty nine alphanumeric characters coded two digits per character and
stored two characters per word in the low
order four digits.
The 24 instructions
include numeric and alphanumeric I/O, integer and floating point arithmetic and
test and branch instructions.
HYCOMP does
not have index registers, but in the
future, they may be added to HYCOMPI using
the middle digit of instructions for the
index register field.
2.3
HYCOMPI simulator gives the student the
same options he would have if he were
sitting at a HYCOMP1 console, but in
addition, he can observe the execution of
his program.
There are two commands with which the
student can execute his HYCOMPI program RUN and CYCLE.
With RUN, the student
specifies the starting address of his program, and it is executed until it halts or
produces an execution error.
If the student uses CYCLE, one machine cycle is
executed so that he can study the results
of each instruction at his own speed.
A machine cycle consists of the following steps:
la. The CRT cursor underlines the contents of the word whose address is
in the IC.
b. That instruction is copied into the
CU and displayed there on the screen.
2.
IC is incremented by i, the new
value being displayed properly on
the screen.
3. The instruction in the CU is executed.
This may ~ause various
things to happen on the screen, for
example:
a. A test instruction causes the displayed contents of REG to blink for
a short time.
b. A branch instruction changes the IC.
Usin~ H YCOMPI 4
When the student executes HYCOMPI,
the screen is cleared and written on as
shown in fig. i. He may then load his
program (only machine language is accepted
by HYCOMPI) and his data and run the program.
He may then patch or change his
program and continue trying and changing
it until he is satisfied or tired.
The
O0:
07:
14:
21:
28:
35:
42:
49:
01:
08:
15:
22:
29:
36:
02:
09:
16:
23:
30:
37:
43:
50:
44:
56:
63:
70:
77:
57:
64:
71:
78:
51:
58:
65:
72:
79:
84:
85:
86:
91:
92:
99:
93:
OF-
98:
03:
I0:
17:
04:
24:
31:
25:
32:
05:
12:
19:
26:
33:
38:
45:
39:
46:
40:
47:
34:
41:
48:
52:
59:
66:
73:
53:
60:
54:
61:
55:
62:
67:
74:
81:
88:
68:
69:
75:
76:
80:
87:
94:
UF-
11:
18:
06:
13:
20:
27:
82:
~:
83:
90:
97:
95:
CU"
IC"
REG"
EOF1
TO SEE A LIST OF AVAILABLE COMMANDS, TYPE HELP
TO PROCEED AFTER AN ERROR MESSAGE APPEARS, HIT NEW LINE
TO LEAVE A MODE, JUST TYPE NEW COMMAND. ? MEANS READY
INPUT APPEARS ON LINE 22, OUTPUT ON LINE 23
?
Fig.
4
A user
Stu&rt
to the
No. 8,
Univ.,
!
i.
CRT screen after initialization
C. Whenever data is read, either from
KEG, a memory word or on the Input
llne, the cursor underlines the data.
d. Data may be written into REG, a
memory word or the Output llne.
e. A floating point instruction may set
s guide is available as: Shapiro,
C. and Grace, David A., "A Guide
Use of HYCOMPl",~,Technical Report
Computer S c i e n c e D e p t . Indiana
Bloomington, In., Dec. 10, 1973.
12
UF or OF.
f. An input i n s t r u c t i o n may set EOF.
If an e x e c u t i o n e r r o r occurs, an a p p r o p r i ate m e s s a g e is w r i t t e n on the screen.
W h e n the student is ready, he can c l e a r
this m e s s a g e and e n t e r a d d i t i o n a l commands.
The valid commands are:
±nnnnn - The s i g n e d five digit n u m b e r
is put into the w o r d whose address is
the current value of IC and IC is inc r e m e n t e d by 1.
If the sign is
omitted, + is assumed.
LOAD nn - IC is set to nn, OF, UF, and
E O F are i n i t i a l i z e d to 0, and the input line is p o s i t i o n e d at the first
"card" of data.
If nn is omitted, 00
is assumed.
RUN nn - IC is set to nn and the program is run to t e r m i n a t i o n or until
an e x e c u t i o n e r r o r occurs.
If nn is
omitted, the current value of IC is
retained.
IC XX - XX is a two digit number.
IC
is set to XX.
UF X - X is 0 or 1.
UF is set to X.
OF X - X is 0 or 1.
OF is set to X.
E O F X - X is 0 or 1.
EOF is set to X.
REG n - n is an optionally s i g n e d integer with no more than five digits.
REG is set to n.
CY - One m a c h i n e cycle is executed.
QUIT - The H Y C O M P 1 s i m u l a t o r is terminated.
DATA - Allows data to be entered, one
"card" at a time.
A card w i t h # in
column 1 signals the end of data
entry and serves as the E O F flag.
DATA ADD - Allows data to be added to
the end of the e x i s t i n g data "deck".
The e x i s t i n g E O F card is a u t o m a t i cally removed.
HELP - A list and b r i e f d e s c r i p t i o n of
the commands is displayed.
2.4
able task for the b e g i n n i n g student.
He
must not only m e m o r i z e the key words and
s t a t e m e n t syntax a s s o c i a t e d w i t h the language, but must learn to assemble m e a n i n g ful p r o g r a m s from these b u i l d i n g blocks.
In this regard, it is e s s e n t i a l for the
student to be able to visualize the stepb y - s t e p o p e r a t i o n of his programs.
The
I n t e r a c t i v e Visual FORTRAN I n t e r p r e t e r
(IVF) was d e s i g n e d as an aid to such a
visualization.
3.2
Summary
The I n t e r a c t i v e
preter
VIsualFORTRAN
Inter-
3.1 I n t r o d u c t i o n
L e a r n i n g to p r o g r a m
language such as F O R T R A N
Description
The IVF accepts a FORTRAN p r o g r a m and
g r a p h i c a l l y s i m u l a t e s its e x e c u t i o n on an
i n t e r a c t i v e t i m e - s h a r i n g t e r m i n a l with a
cathode ray tube (CRT) screen.
D u r i n g the
s i m u l a t i o n the e x e c u t a b l e statements are
d i s p l a y e d on the screen.
An arrow is
caused to point at each statement in the
order in w h i c h e x e c u t i o n w o u l d n o r m a l l y
take place.
Current values of variables
are d i s p l a y e d in the unused space to the
right of the program, and are u p d a t e d w i t h
each move of the arrow.
Thus the immediate
state of the p r o g r a m is v i s u a l l y apparent
at all times in terms of current values
of variables and flow of control.
Figure 2 shows a short p r o g r a m as it
w o u l d a p p e a r d u r i n g s i m u l a t i o n on the IVF.
The arrow indicates that e x e c u t i o n of the
statement "X = (I ** 2) / 2" is b e i n g
simulated.
The current values of "I" and
"X" are d i s p l a y e d on the right side of the
screen.
In this example the arrow will
next m o v e to the "IF" statement.
Simulation of the "IF" s t a t e m e n t succeeds and
the arrow moves to the statement w h os e
label is "4".
Now the value of "I" is inc r e m e n t e d and the new value is d i s p l a y e d
on the right.
The s i m u l a t i o n p r o c e ed s in
this m a n n e r until e x e c u t i o n of the "IF"
statement is s i m u l a t e d with the current
value of "I" equal to I0.
At this time
the arrow moves to the "END" statement and
the s i m u l a t i o n is terminated.
Upon t e r m i n a t i o n of the e x e c u t i o n
phase, the IVF displays an a p p r o p r i a t e
message and the user is given the option to
exit the IVF or attempt a n o t h e r simulation.
The FORTRAN p r o g r a m Just s i m u l a t e d is not
lost in e i t h e r case.
The user may have it
s i m u l a t e d again (as d i s c u s s e d later) or
dispose of it in some o t h e r fashion.
The IVF assumes a naive user, and is
therefore self-explanatory.
Instructions
may be r e c e i v e d at the b e g i n n i n g of each
session by r e s p o n d i n g a p p r o p r i a t e l y to the
p r o g r a m ' s inquiries.
In addition, all
p o r t i o n s of the p r o g r a m r e q u i r i n g a user
response w i l l recognize errors and prompt
as necessary.
A l t h o u g h the IVF is p r i m a r i l y an aid
to the v i s u a l i z a t i o n o f FORTRAN p r o g r a m
e x e c u t i o n , it is also an i n t e r a c t i v e FORTRAN s t a t e m e n t a c c e p t e r with a n u m b e r of
e r r o r c h e c k i n g facilities.
The IVF will
accept FORTRAN lines and c o n t i n u a t i o n lines
until a complete s t a t e m e n t has b e e n
entered.
It then checks for syntax errors,
HYCOMP1 has not yet had e x t e n s i v e
c l a s s r o o m use.
We did have several students from an i n t r o d u c t o r y p r o g r a m m i n g
class use it d u r i n g the latter stages of
its development.
E a r l i e r in the semester,
these students had s t u d i e d H Y C O M P and had
w r i t t e n several H Y C O M P programs w h i c h w e r e
graded by a h u m a n grader.
Their reaction
to H Y C O M P 1 was e x t r e m e l y favorable and they
r e p o r t e d that it w o u l d have b e e n v e r y helpful in their study of m a c h i n e language.
One student finally u n d e r s t o o d that there
is no inherent d i f f e r e n c e b e t w e e n i n s t r u c tions and data.
This k i n d of insight, if
not gained i n t e l l e c t u a l l y , could hardly be
gained by any means o t h e r than an interactive, visual simulation.
3.
Basic
in a h i g h e r level
can be a formld-
13
.>
I-0
I-I+
1
X " (I * * 2) / 2
IF(I .LT. i0) GO TO 4
END
I:
1
X:
0.4999999999
Figure 2
S i m u l a t i o n of a FORTRAN p r o g r a m is
a c c o m p l i s h e d by first d i s p l a y i n g its
e x e c u t a b l e s t a t e m e n t s on the CRT screen,
and then e x e c u t i n g the c o m p i l e d S N O B O L
statements d i s c u s s e d above.
D u r i n g simulation the IVF displays output from the
FORTRAN p r o g r a m on the b o t t o m line of the
screen, and a c c e p t s i n p u t on the line
above.
The p o s i t i o n of the. e x e c u t a b l e
FORTRAN s t a t e m e n t s on the CRT screen is
n e v e r changed.
The CRT's cursor (which
u n d e r l i n e s the next p r i n t a b l e position) is
m o v e d about as n e c e s s a r y to prevent erasure
of v a l u a b l e i n f o r m a t i o n and to facilitate
input and output.
errors in "DO" loop nesting, and m u l t i p l y
d e f i n e d s t a t e m e n t numbers.
If the IVF
detects an error, the statement is reJ e c t e d and an a p p r o p r i a t e error message is
displayed.
The user is thus made aware of
his m i s t a k e i m m e d i a t e l y and is g i v e n
a n o t h e r o p p o r t u n i t y to enter a correct
statement.
3.3
A d d i t i o n a l Features
A user may o p t i o n a l l y bracket a
portion of his FORTRAN p r o g r a m by
p l a c i n g a "+" in the c o n t i n u a t i o n field
of a n o n c o n t i n u a t i o n line to turn the
"STEPPED MODE" on.
The "STEPPED MODE"
is turned off in an analogous fashion
w i t h a "-".
W i t h i n the b r a c k e t e d section the user must press the "RETURN"
key after the e x e c u t i o n of each F O R T R A N
line has b e e n simulated.
This permits
a s l o w i n g of the s i m u l a t i o n over a
p o r t i o n of the user's p r o g r a m for m o r e
careful study.
The IVF saves a copy of the FORTRAN p r o g r a m simulated, and makes it
available on a file r e s e r v e d for that
purpose.
The u s e r may o p t i o n a l l y submit complete p r o g r a m s to the IVF
rather than enter them one line at a
time.
E r r o r c h e c k i n g is p e r f o r m e d on
each s t a t e m e n t as with on-line entry.
The complete p r o g r a m may have o r i g i n a t e d
from any source, i n c l u d i n g a session w i t h
the IVF.
3.4
3.5
Summary
V i s u a l i z a t i o n of the e x e c u t i o n of his
p r o g r a m s is of utmost importance to the
b e g i n n i n g programmer.
It is h o p e d that the
IVF will serve as an e f f e c t i v e aid to such
visualization.
It is f u r t h e r h o p e d that
the IVF w i l l be found flexible in its features and yet simple enough for use by a
b e g i n n i n g programmer.
IVF can readily be a d a p t e d to run on
a n u m b e r of c o m p u t i n g systems w h i c h support
t i m e - s h a r i n g terminals and p r o v i d e interactive execution.
The p r i m a r y r e s t r i c t i o n s
are the n e e d for a CRT display at the
t i m e - s h a r i n g terminal and a s u f f i c i e n t l y
large c h a r a c t e r set to support control of
the display.
The IVF is w r i t t e n in a straightforward m a n n e r w i t h n u m e r o u s C o m m e n t s .
An
e x p e r i e n c e d p r o g r a m m e r should, t h e r e f o r e
be able to e x p a n d the scope of the IVF to
suit the needs of his local users.
He
might even cause the IVF to simulate some
language other than FORTRAN.
Im~lementatlon
The IVF is w r i t t e n in the SNOBOL4
p r o g r a m m i n g language.
Each FORTRAN statement a c c e p t e d by the IVF is t r a n s l a t e d
into a s e q u e n c e of S N O B O L statements.
Steps are taken to adjust o p e r a t o r
p r e c e d e n c e and make type c o n v e r s i o n s as
necessary.
This is b r a c k e t e d by statements w h i c h move the arrow and update disp l a y e d i n f o r m a t i o n d u r i n g the simulation.
This b l o c k of S N O B O L statements is conc a t e n a t e d into a string of such blocks for
later compilation, u t i l i z i n g the CODE
function.
14