THE SBC GAZETTE
Mastering The Ohio Scientific Challenger 1P, A Learn-By-Doing Approach
by Keith Russell and Dave Schultz
Total Information Services
Los Alamos, NM
The Ohio Scientific Challenger 1P (C1P) is the SUPERBOARD II single board computer in a cabinet with a power supply. It is destined to become a very popular personal computer. Its low cost, graphics features, and powerful BASIC language make it an attractive machine for many people. For most purchasers, the C1P will be your first computer. With little background or experience with computers, you will need some help and instruction on how to make the C1P work for you.
First, let's define some notation. We will use a consistent notation in this article to indicate what is to be typed on the keyboard (T:), what appears on the TV display (R:), and what indicates that blanks are to be typed (b). For example:
T: info (‘RETURN’ key) means to type the characters contained on the line after the colon (:) and then type ‘RETURN.’
R: response means that the C1P will display this information on the TV after you type the previous line.
Blanks are important in some cases. When they are important they are specified by b. For example:
T: ?"ABbC" (‘RETURN’ key) means that you should type ?, then ", then the letter A, then the letter B, then a space, then the letter C, then " followed by a ‘RETURN’.
Now let's run that example all together:
T: ?"ABbC" (‘RETURN’ key)
The ‘RETURN’ key must be pressed at the end of each line. We will assume that you know that and will not use (‘RETURN’ key) in any more examples.
Now lets tackle how the C1P represents and stores numbers. You will learn how the C1P represents numbers by experimenting systematically. Each experiment (exercise) shows you what to type as input to your computer and the expected reply.
DATA REPRESENTATION ON THE C1P
A. Largest Numeric Value To find the largest floating-point number that the C1P can handle, run the following program.
Exercise: Test for the floating-point maximum.
T: NEW T: 200 I = 1 T: 210 I = I * 2 T: 220 PRINT I T: 230 GO TO 210 RUN
A long series of numbers starting with
R: 2 R: 4 R: 8 R: 16
will be printed out; the last number will be
R: 4.2535E + 37
R: ?0-ERROR IN 210
This means that 8.50706 E + 37 was too large to represent.
Exercise: Modify the program above to display large negative floating-point numbers to determine the largest negative number that the C1P can represent. If the sign of the result alternates between plus and minus, you should try a different modification. Your result should show that the sign is independent of the maximum size permitted.
T: T: R: ... R:
The largest negative number is _______.
B. Memory Space Used Different types of data require different amounts of memory. To determine the memory space required for each type, we will use the FRE function. FRE returns the amount of space remaining.
Exercise: Determine the amount of space used by a single floating-point value.
T: NEW T: DIM A(100) T: ?FRE(0) R: 2913
This determines the amount of memory space left after reserving 100 floating-point locations. Next, determine the amount of space left after reserving 101 floating-point locations.
T : NEW T : DIM A(101) T : PRINT FRE(0) R : 2909
The difference (4) is the number of bytes that one floating-point variable requires.
C. Number of Significant Digits The CIP can display six digits in a number even though more information can be stored in memory.
Exercise: Determine the number of digits displayed for floating-point numbers.
Note: The values given are the responses from FRE on a 4K machine. If you have an 8K machine, note the values. The difference should show the same result for the size of the variable.
T : PRINT 123456 R : 123456
This indicates that six digits can be displayed.
T : PRINT 1234567 R : 1.23457E + 06
This shows that the seventh digit is not displayed.
Exercise: Show how many digits of information can be saved in memory.
T : A = 1234560 : PRINT A R : 1.23456E + 06 T : B = 1234561 : PRINT B R : 1.23456E + 06
Even though the numbers entered differ by 1 in the seventh digit, the display shows that they are equal. Are they really?
Exercise: Test if the numbers entered are really equal.
T : IF A < > B THEN PRINT "< >" R : < >
The response shows that the C1P can distinguish the difference of 1 in the seventh digit, even if it doesn't display the difference. Can the C1P distinguish a difference of 1 in the eighth digit?
T : C = 12345670 : PRINT C R : 1.23457E + 07 T : D = 12345671 : PRINT D R : 1.234567E + 07
The displayed values appear to be equal. Are they?
T : IF C < > D THEN PRINT "< >" R : < >
Since the C1P can distinguish between numbers that differ by one in the eighth digit, let's try a difference of 1 in the ninth digit.
T : C = 123456780 : PRINT C R : 1.23457E + 08 T : D = 123456781 : PRINT D R : 1.23457E + 08 T : IF C = D THEN PRINT " = " R : =
It appears that the C1P cannot distinguish between two numbers that differ by one in the ninth digit.
The number of significant digits displayed is six and the number of digits saved in memory is approximately eight.
D. Rounding Since the C1P stores approximately eight digits in memory and only displays six digits, it must "round" to decide what to display.
Exercise: Determine how the C1P rounds numbers for display.
T : NEW T : 100 A = 1234510 T : 110 FOR I = 1 TO 15 T : 120 B = A + 1 T : 130 PRINT "A + "; I ;" = " ;B T : 140 NEXT I T : RUN R : A + 1 = 1.23451E + 06 ... R : A + 5 = 1.23452E + 06 ... R : A + 15 = 1.23453E + 06
From this display, you can see that 1234515 was rounded up to 1.23452E + 06 and 1234525 was rounded up to 1.23453E + 06. The C1P rounds the sixth digit up whenever the seventh digit is 5 (or more) and rounds it down whenever the seventh digit is 4 (or less).
These simple exercises show that you can learn quite a bit about your C1P (or any other personal computer) by doing some well chosen experiments. You can master your C1P. All it takes is a little experimenting.
This material is exerpted from the Total Information Services (TIS) Workbook 101. Getting Started with Your OSI C1P. (Copyright © 1979 Total Information Services) Reprinted with permission of TIS.
Total Information Services (TIS) has been publishing tutorial workbooks for the Commodore PET since March 1978. They have now expanded their workbook series to include the Challenger 1P from Ohio Scientific.
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