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JCSM Shareware Collection 1996 September
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JCSM Shareware Collection (JCS Distribution) (September 1996).ISO
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LESSON2
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1990-02-09
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ELEMENTARY COMPUTER PROGRAMMING
Lesson 2
Copyright 1990
Castle Oaks Computer Services
Post Office Box 36082
Indianapolis, IN 46236-0082
TRAC, the hypothetical computer introduced in lesson 1, can be
simulated on an IBM PC compatible computer. (In fact, it can be
simulated on any MS-DOS computer even if it is not fully IBM PC
compatible.) Therefore, it is possible to execute TRAC programs
in order to gain a familiarity with coding in machine language.
In this lesson, the information needed to actually execute a TRAC
program will be given.
First, the problem must be coded according to the rules of the
language and the format given in Lesson 1. After coding the
problem on paper, then it must be entered into a disk file on the
computer. This package does not provide the software necessary
to do this, but you may use any suitable text editor or word
processor in non-document mode. At Castle Oaks we usually use
WORDSTAR (TM MicroPro International Corporation) in non-document
mode, but there are several shareware text editors that would
also be adequate. Any file name can be used for your file, but
we recommend that the name have an extension of 'OBJ'. The files
that you run with TRAC are called 'object' files and therefore it
is desirable to identify them as such. In the following lessons
we will discuss another type of file.
The general command line format for running TRAC is:
TRAC [object file] [trace file]
The parameters in brackets are optional. If the first parameter
is omitted, the program will prompt you for a file descriptor of
the object file to be run. This file descriptor may include a
drive letter and subdirectories, if required. If the second
parameter is also omitted from the command line, the program will
ask if you want the program traced; and, if so, will ask if you
want it traced on the screen or to a file. If to a file, you
will also be prompted for a file name. If the object file and
the trace file are both given on the command line, the program
will start to execute without any queries.
The purpose of tracing a program is to trouble shoot it in case
it does not function correctly. It is a good idea to trace the
exercises so you can analyze what is happening at each step in
order to gain a fuller understanding of the process. When trac-
ing a program, each instruction is displayed in the following
format:
II-1
LLLL T OO AAAA SVVVVVVVVV SAAAAAAAAA
Where LLLL is the location of the instruction;
T is the TAG;
OO is the operation code;
AAAA is the effective address of the operand;
SVVVVVVVVV is the value of the operand; and
SAAAAAAAAA is the current value in the accumulator.
Note that the resulting value of an operation does not appear in
the accumulator until the following line is printed. If you
trace your program on the screen, it may print too fast for you
to analyze. You may pause the display at any time by pressing ^S
(control-S) or you may send the screen display to the printer by
pressing ^P (control-P) before starting your program. If you
send it to a file, later you may either view it on the screen or
you can send it to the printer. It is suggested that when you
name a trace file that you give it an extension of 'LST' which
will help you identify it as a file to list.
Figure II-1 is the source code of a simple program to illustrate
the execution and tracing of a program. This program reads in
two numbers, finds their sum and difference and prints out the
numbers, and the results to the screen.
-----------------------------------------------------------------
0010 400100 Read X, Y into 100 and 101
0011 000100 Load X
0012 010101 Add Y
0013 030102 Store sum at 102
0014 410100 Print 100, 101, 102, (also 103 and 104)
0015 000100 Load X
0016 020101 Subtract Y
0017 030102 Store difference at 102
0018 410100 Print 100, 101, 102, (also 103 and 104)
0019 500000 Halt
9999 000010 Start program at 10
Figure II-1
-----------------------------------------------------------------
Figure II-2 shows what appears on the screen when this program is
executed.
II-2
-----------------------------------------------------------------
A:\TRAC
TRAining Computer (TRAC)
Version 1.0
Copyright 1990
CASTLE OAKS COMPUTER SERVICES
Post Office Box 36082
Indianapolis, IN 46236-0082
What is the name of your object code file?
FIGII-1.OBJ
Do you want to trace the program? (Y or N) Y
Do you want to trace it on screen (S) or on file (F)? S
10 0 40 100 0 0
? 123456789 876543210
11 0 0 100 123456789 0
12 0 1 101 876543210 123456789
13 0 3 102 0 999999999
14 0 41 100 123456789 0
123456789 876543210 999999999 0 0
15 0 0 100 123456789 999999999
16 0 2 101 876543210 123456789
17 0 3 102 999999999 -753086421
18 0 41 100 123456789 -753086421
123456789 876543210 -753086421 0 0
19 0 50 0 0 -753086421
Figure II-2
-----------------------------------------------------------------
If the program is run with the object file named on the command
line and with no tracing, the resultant screen display will be as
shown in Figure II-3.
-----------------------------------------------------------------
A:\TRAC FIGII-1.OBJ
TRAining Computer (TRAC)
Version 1.0
Copyright 1990
CASTLE OAKS COMPUTER SERVICES
Post Office Box 36082
Indianapolis, IN 46236-0082
Do you want to trace the program? (Y or N) N
? 123456789 876543210
123456789 876543210 999999999 0 0
123456789 876543210 -753086421 0 0
Figure II-3
-----------------------------------------------------------------
Note: Locations 103 and 104 print zero because zero was entered
into them implicitly during the read. Note also that TRAC first
clears all memory locations to zero before loading and executing
the program. Do not count on other computers doing this.
II-3
Note that in Figure II-2 the trace printout and the program were
both intermixed on the screen. This may be desirable or it may
be undesirable. If you do not want them together, you can send
the trace printout to a file and view it later; or you may send
your program output to the printer while tracing on the screen.
The remainder of this lesson will be devoted to showing some
other examples. The coding and tracing of these examples also
will be given.
The following Figure gives a flow chart for finding the factorial
of a number. A factorial is the product of all integers up to
and including the given number. Thus factorial 3 is 1*2*3=6;
factorial 4 is 1*2*3*4=24; etc.
-----------------------------------------------------------------
|
v
---------
/ Read N /<------------------+
--------- |
| |
v |
/ \ <= +------+ |
<N:0>----->| STOP | |
\ / +------+ |
+------------------+ | > ^ |
Note: N>12 is | v | |
not allowed | / \ | |
since the result | / \ >= | |
would exceed the | - - <N:13 >---------+ |
capacity of the | \ / |
A register. | \./ |
+------------------+ | < |
v |
+-------+ |
| F = 1 | |
| K = 2 | |
+-------+ |
| |
v |
+-------+ / \ > ------------ |
| F=F*K |-><K:N>----->/ Print N,F /--+
| K=K+1 | \ / ------------
+-------+ |
^ | <=
+--------+
Figure II-4
-----------------------------------------------------------------
Figure II-5 gives the coding for the flow chart of Figure II-4.
Note: In a flow chart, a diamond shaped box is used to indicate a
decision point. An expression such as K:N in a diamond means to
compare K and N (usually by subtraction) and then taking the
appropriate path based on the symbols shown.
II-4
-----------------------------------------------------------------
0010 401901 Read value for N
0011 001901 Load N into A
0012 240016 If negative go to stop
0013 250016 If zero go to stop
0014 021801 Subtract 13
0015 240017 If negative skip around stop
0016 500000 Stop
0017 001802 Load 1
0018 031902 Store at F
0019 001803 Load 2
0020 031804 Store at K
0021 021901 Subtract N
0022 240026 Jump, if negative, to update F
0023 250026 Jump, if zero, to update F
0024 411901 Print
0025 260010 Branch back to beginning
0026 001902 Load F
0027 151804 Multiply by K
0028 031902 Store at F
0029 001804 Load K
0030 011802 Add 1
0031 031804 Store at K
0032 260021 Branch to test if done
1801 13 Constant 13
1802 1 Constant 1
1803 2 Constant 2
9999 000010 End of code
Figure II-5
-----------------------------------------------------------------
There are a few features of this program that should be noted.
Almost any location can be picked to receive the value of N. In
this case, 1901 was chosen.
Constants can go almost anyplace. In this case, 1801, 1802, and
1803 were used for constants.
F could have been stored almost anyplace; but looking ahead, it
was noted that it was to be printed along with N. Therefore, at
print time they need to be in consecutive locations. Since N is
already in 1901 and 1902 was not used, it is logical to assign
1902 for F.
At location 0021 we have the beginning of a loop. Since location
0021 can be entered two ways, once from 0020 and later from 0032,
we must be careful about what is in the accumulator at the begin-
ning of this loop. Normally we would load the accumulator at the
beginning of the loop. But by careful planning we see that we
can have K already in the accumulator when we enter from either
direction. This saves the load instruction.
Figure II-6 shows what would appear on the screen if the program
is traced and the values 3 and 13 are entered for N.
II-5
A:\TRAC FIGII-5
TRAining Computer (TRAC)
Version 1.0
Copyright 1990
CASTLE OAKS COMPUTER SERVICES
Post Office Box 36082
Indianapolis, IN 46236-0082
Do you want to trace the program? (Y or N) y
Do you want to trace it on screen (S) or on file (F)? s
10 0 40 1901 0 0
? 3
11 0 0 1901 3 1
12 0 24 16 500000 3
13 0 25 16 500000 3
14 0 2 1801 13 3
15 0 24 17 1802 -10
17 0 0 1802 1 -10
18 0 3 1902 2 1
19 0 0 1803 2 1
20 0 3 1804 3 2
21 0 2 1901 3 2
22 0 24 26 1902 -1
26 0 0 1902 1 -1
27 0 15 1804 2 1
28 0 3 1902 1 2
29 0 0 1804 2 2
30 0 1 1802 1 2
31 0 3 1804 2 3
32 0 26 21 21901 3
21 0 2 1901 3 3
22 0 24 26 1902 0
23 0 25 26 1902 0
26 0 0 1902 2 0
27 0 15 1804 3 2
28 0 3 1902 2 6
29 0 0 1804 3 6
30 0 1 1802 1 3
31 0 3 1804 3 4
32 0 26 21 21901 4
21 0 2 1901 3 4
22 0 24 26 1902 1
23 0 25 26 1902 1
24 0 41 1901 3 1
3 6 0 0 0
25 0 26 10 401901 1
10 0 40 1901 3 1
? 13
11 0 0 1901 13 1
12 0 24 16 500000 13
13 0 25 16 500000 13
14 0 2 1801 13 13
15 0 24 17 1802 0
16 0 50 0 0 0
Figure II-6
II-6
Another simple problem is shown in Figure II-7. In this program
we will raise a number, X, to any positive power, N. We cannot
do a negative power since that would result in a fraction.
Therefore, we will use a negative N to terminate execution of the
program.
-----------------------------------------------------------------
|
v
-----------
/ Read N,X /<-----------------+
----------- |
| |
v |
+-------+ |
| F = 1 | |
| K = N | |
+-------+ |
+------->| |
| v |
+-------+ > / \ = -------------- |
| F=F*X |<-<K:0>--->/ Print N,X,F /--+
| K=K-1 | \ / --------------
+-------+ | <
|
v
+------+
| STOP |
+------+
Figure II-7
-----------------------------------------------------------------
The coding is shown in Figure II-8.
-----------------------------------------------------------------
1001 400011 Read N, X into 11, 12
1002 001501 Load A with a 1
1003 030013 Store F at 13
1004 000011 Load N into A
1005 031502 Store at K
1006 241017 Jump to stop instruction
1007 251015 Jump to print
1008 000013 Load F
1009 150012 Multiply by X
1010 030013 Store at F
1011 001502 Load K
1012 021501 Subtract 1
1013 031502 Store at K
1014 261006 Go back to start of loop
1015 410011 Print N, X, F
1016 261001 Jump to beginning of program
1017 500000 Stop the program
1501 1 Constant 1
9999 001001 End of code
Figure II-8
-----------------------------------------------------------------
II-7
Figure II-9 shows some sample results of the program as they
would appear on the screen.
-----------------------------------------------------------------
A:\>TRAC FIGII-8.OBJ
TRAining Computer (TRAC)
Version 1.0
Copyright 1990
CASTLE OAKS COMPUTER SERVICES
Post Office Box 36082
Indianapolis, IN 46236-0082
Do you want to trace the program? (Y or N) n
? 10 2
10 2 1024 0 0
? 10 4
10 4 1048576 0 0
? 3 1234
3 1234 879080904 0 0
? -1
A:\>
Figure II-9
-----------------------------------------------------------------
In this program there is no simple way to determine that the
answer will exceed the capacity of the A register. Therefore
there is a possibility of having an overflow. That happened in
the case of raising 1234 to the 3rd power. The correct answer is
1879080904.
Each of the example programs are included in the package. The
ones associated with this lesson are FIGII-1.OBJ, FIGII-5.OBJ,
and FIGII-8.OBJ. You should run each of these and experiment
with various inputs, tracing, etc. before trying one of your own
programs.
When you are comfortable with how to execute and trace a program
then you might experiment with the above programs such as chang-
ing from printing to the screen to printing to the printer. You
should try executing your Exercise I-1 now. Then you should code
and execute exercise II-1. The flow chart for this exercise
appears on the next page.
In this program, you are to enter 10 values (done in two reads
and the second read should use locations that immediately follow
those of the first read.) The program will do a "minimum-in-
pass" sort and the sorted array is printed. It will be necessary
to use two index registers.
II-8
|
v
--------------------------------
/ Read A(0),A(1),A(2),A(3),A(4) /
--------------------------------
|
v
--------------------------------
/ Read A(5),A(6),A(7),A(8),A(9) /
--------------------------------
|
v
+-----+
| J=0 |
+-----+
|
v
+---------+
| S=A(J) |<-------------+
| I=J+1 | |
+---------+ |
| |
v |
/ \ |
/ \ >= |
+----><A(I) >------+ |
| \ :S/ | |
| \ / | |
| | < | |
| v | |
| +---------+ | |
| | T=S | | |
| | S=A(I) | | | +-------+
| | A(I)=T | | | | STOP |
| +---------+ | | +-------+
| | | | ^
| v | | |
| +-------+ | | -------------------------
| | I=I+1 |<----+ | / Print /
| +-------+ | /A(5),A(6),A(7),A(8),A(9)/
| | | -------------------------
| v | ^
| / \ | |
| < / \ | -------------------------
+-----<I:10 > | / Print /
\ / | /A(0),A(1),A(2),A(3),A(4)/
\./ | -------------------------
| >= | ^
v < | |
+-------+ +-------+ / \ >= |
| A(J)=S|->| J=J+1 |-><J:9>-----------+
+-------+ +-------+ \./
Exercise II-1
II-9
Here (Figure II-10) is a method of multiplying two 9-digit numbers
to obtain an 18-digit product as promised earlier. The factors are
each divided into three 3-digit groups. Then individual products
are found and summed with proper attention to overflow digits. See
the comments for more detail. Try tracing this program (on disk as
DOUBLMPY.OBJ) using the factors 999888777 and 666555444. These
factors will help illustrate the isolation of the groups and how the
product is reassembled. Note that if the lower part of the product
has leading zeroes, TRAC does not print them. A shorter program for
doing this multiply could probably be devised but this method was
chosen because of its symmetry. If it is known that the product
will contain fewer than 18 digits, the program could be modified so
fewer steps would be required.
0010 401000 Read A and B 0050 151503 Times Z
0011 001000 Load A 0051 031507 V*Z --> T
0012 360006 Isolate high 3 0052 001506 Load W
0013 031501 Store as X 0053 151502 Times Y
0014 001000 Load A 0054 011507 Plus V*Z
0015 370003 Isolate 0055 011003 Plus LOWER
0016 360006 middle 3 0056 031003 V*Z+W*Y+???000
0017 031502 Store as Y 0057 001506 Load W
0018 001000 Load A 0058 151503 Times Z
0019 370006 Isolate 0059 031507 W*Z --> T
0020 360006 low 3 0060 360003 Isolate high 3
0021 031503 Store as Z 0061 011003 Plus LOWER
0022 001001 Load B 0062 031003 Save at LOWER
0023 360006 Isolate high 3 0063 360006 Isolate carry
0024 031504 Store as U 0064 031508 Save at CARRY
0025 001001 Load B 0065 001003 Load LOWER
0026 370003 Isolate 0066 370003 Shift off carry
0027 360006 middle 3 0067 031003 Save at LOWER
0028 031505 Store as V 0068 001507 Load W*Z
0029 001001 Load B 0069 370006 Isolate
0030 370006 Isolate 0070 360006 low 3
0031 360006 low 3 0071 011003 Plus LOWER
0032 031506 Store as W 0072 031003 Save at LOWER
0033 151501 Times X 0073 001504 Load U
0034 031507 X*W --> T 0074 151502 Times Y
0035 001505 Load V 0075 031507 U*Y --> T
0036 151502 Times Y 0076 001505 Load V
0037 011507 Plus X*W 0077 151501 Times X
0038 031507 X*W+V*Y --> T 0078 011507 Plus U*Y
0039 001504 Load U 0079 011002 Plus UPPER
0040 151503 Times Z 0080 011508 Plus CARRY
0041 011507 Plus X*W+V*Y 0081 031002 Save at UPPER
0042 031507 X*W+V*Y+U*Z --> T 0082 001504 Load U
0043 360003 Isolate high 3 or 4 0083 151501 Times X
0044 031002 Save in UPPER 0084 370003 Position U*X
0045 001507 Load X*W+V*Y+U*Z 0085 011002 Plus UPPER
0046 370006 Isolate 0086 031002 Save at UPPER
0047 360003 low 3 0087 411000 Print A,B,Product
0048 031003 Save in LOWER 0088 500000 Halt
0049 001505 Load V 9999 000010
Figure II-10
II-10
