# The Essentials of Computer Organization and Architecture. Linda Null and Julia Lobur Jones and Bartlett Publishers, 2003

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1 The Essentials of Computer Organization and Architecture Linda Null and Julia Lobur Jones and Bartlett Publishers, 2003 Chapter 2 Instructor's Manual Chapter Objectives Chapter 2, Data Representation, provides thorough coverage of the various means computers use to represent both numerical and character information. Addition, subtraction, multiplication, and division are covered once the reader has been exposed to number bases and the typical numeric representation techniques, including one s complement, two s complement, and BCD. In addition, EBCDIC, ASCII, and Unicode character representations are addressed. Fixed and floating point representation are also introduced. Codes for data recording and error detection and correction are covered briefly. This chapter should be covered after Chapter 1, but before Chapters 4 through 11. Lectures should focus on the following points: Number systems. Most students have been exposed to positional number systems and different bases. However, these concepts are crucial to understanding the remainder of Chapter 2, so they should be covered in detail. Decimal to binary conversions. Because the binary number system translates easily into electronic circuitry, it is important to become familiar with how computer represent values. Signed versus unsigned numbers. Representing unsigned numbers in binary form is much less complicated than dealing with signed numbers. Signed integer representation. There are basically three methods for representing signed numbers: signed magnitude, one's complement, and two's complement. Each of these methods should be covered, with the focus on signed magnitude and two's complement notations. Binary arithmetic. Although people do not often add binary values, performing binary addition and subtraction helps to reinforce the concepts necessary for understanding data representation. In particular, these operations illustrate the dangers of overflow conditions. Floating point representation. Computers must be able to represent floating point numbers, and there are numerous possible formats for doing so. Potential errors that may result from the limitations of the representation are also important to discuss. Character representation. ASCII, EBCDIC, Unicode and BCD are all important character codes. Lectures should emphasize the similarities and differences among these codes. Codes for data recording and transmission. When binary data is written to some sort of medium or transmitted over long distances, the binary one's and zero's can become Page 1

2 blurred. Some sort of encoding is necessary to ensure that characters are properly encoded in these situations. Error detection and correction. Regardless of the coding method used, no communications channel or storage medium is error-free. Although simple parity bits can help to detect errors, more complicated codes, including cyclic redundancy checks and Hamming codes, and are often necessary for sophisticated error detection and correction. Required Lecture Time Chapter 2 can typically be covered in 6 lecture hours, depending on how detailed one wishes to go into recording and transmission codes and error detection and correction. We suggest that the focus be on integer, floating-point, and character representation, with emphasis given to complement notation. If time permits, data recording and transmission codes and error detection and correction codes can be covered. Lecture Tips Teachers should spend time explaining the ranges allowed by the different representation formats. For example, if we are using 4 bits to represent unsigned integers, we can represent values from 0 to 15. However, if we are using signed magnitude representation, the bits are interpreted differently, and the possible range is from -7 to +7. Make sure students understand why it is not possible to represent +9 or +10, as these will be seen as -1 and -2 respectively. Converting unsigned whole numbers tends to be relatively straight-forward, but signed numbers and fractions tend to be more difficult. In particular, complement systems confuse students. They often think that all numbers have to be negated to be represented. For example, if a student is asked how a computer, using two's complement representation, would represent -6 (assuming 4-bit values), they answer: 1010 (take 0110, toggle each bit and add 1). This is, of course, the correct answer. However, if that student is asked how the computer using two's complement representation would represent +6, they will often do exactly the same thing, giving the same answer. Teachers need to be sure that students realize representing positive numbers does not require any "conversion". It is only when a number is negative that two's complement, signed magnitude, and one's complement representations are necessary. Instructors should spend time on overflow and look at several examples so that students can fully appreciate the consequences. Students find it difficult to understand why one time a carry results in overflow, but yet another carry may not result in overflow. Floating point representation is fairly straight-forward, but students often have difficulty with the format and the bias. Answers to Exercises 1. Perform the following base conversions using subtraction or division-remainder: a = 3 b = 5 Page 2

3 c = 7 d = 9 a b c d Perform the following base conversions using subtraction or division-remainder: a = 3 b = 5 c = 7 d = 9 a b c d Convert the following decimal fractions to binary with a maximum of six places to the right of the binary point: a b c d a b c d Convert the following decimal fractions to binary with a maximum of six places to the right of the binary point: a b c d a b c d Represent the following decimal numbers in binary using 8-bit signed magnitude, one's complement and two's complement: a. 77 b. -42 c. 119 d. 107 a. Signed magnitude: 0100 One's complement: 0100 Two's complement: 0100 b. Signed magnitude: One's complement: 0101 Two's complement: 0110 c. Signed magnitude: 0111 One's complement: 0111 Two's complement: 0111 Page 3

4 d. Signed magnitude: 1011 One's complement: Two's complement: Using a "word" of 3 bits, list all of the possible signed binary numbers and their decimal equivalents that are representable in: a. Signed magnitude b. One's complement c. Two's complement a. 011 to 111, or +3 to -3 b. 011 to 100, or +3 to -3 c. 011 to 100, or +3 to Using a "word" of 4 bits, list all of the possible signed binary numbers and their decimal equivalents that are representable in: a. Signed magnitude b. One's complement c. Two's complement a to 1111, or +7 to -7 b to 1000, or +7 to -7 c to 1000, or +7 to From the results of the previous two questions, generalize the range of values (in decimal) that can be represented in any given x number of bits using: a. Signed magnitude b. One's complement c. Two's complement a. -(2 x-1-1) to +(2 x-1-1) b. -(2 x-1-1) to +(2 x-1-1) c. -(2 x-1 ) to +(2 x-1 ) 9. Given a (very) tiny computer that has a word size of 6 bits, what are the smallest negative numbers and the largest positive numbers that this computer can represent in each of the following representations? a. One's complement b. Two's complement a. Largest Positive: (31) Smallest Negative: (-31) b. Largest Positive: (31) Smallest Negative: (-32) 10. You have stumbled on an unknown civilization while sailing around the world. The people, who call themselves Zebronians, do math using 40 separate characters (probably because there are 40 stripes on a zebra). They would very much like to use computers, but would need a computer to do Zebronian math, which would mean a computer that could represent all 40 characters. You are a computer designer and decide to help them. You decide the best thing is to use BCZ, Binary Coded Zebronian (which is like BCD except it Page 4

5 codes Zebronian, not Decimal). How many bits will you need to represent each character if you want to use the minimum number of bits? 40 characters need to be represented by binary coded Zebronian (BCZ), so you will need 6 bits. 5 bits would only give you 32 (2 5 ) unique characters. Note that 6 bits would allow you to represent 64 characters. 11. Perform the following binary multiplications: a b c a b c Perform the following binary multiplications: a b c a b c Perform the following binary divisions: a b c a b. c Perform the following binary divisions: a b c a b. 10 c Use the double-dabble method to convert directly to decimal. (Hint: you have to change the multiplier.) converted to decimal is 104 as shown below: x3 x3 x3 x Page 5

6 16. Using signed-magnitude representation, complete the following operations: (-0)= (-0) + 0 = = (-0) + (-0) = (assuming 4 bit representation) +0 + (-0) = = 1000 (-0) (-0) + 0 = = 1000 (-0) = = 0000 (+0) (-0) + (-0) = = 0000 (+0) 17. Suppose a computer uses 4-bit one s complement numbers. What value will be stored in the variable j after the following pseudocode routine terminates? 0 j // Store 0 in j. -3 k // Store -3 in k. while k = 0 j = j + 1 k = k - 1 end while J (Binary) K (Binary) ( ) (where last carry is added to sum doing 1's complement addition) ( ) ( ) ( ) ( ) (This is overflow -- but you can ignore) If the floating-point number storage on a certain system has a sign bit, a 3-bit exponent and a 4-bit significand: a. What is the largest positive and the smallest positive number that can be stored on this system if the storage is normalized? (Assume no bits are implied, there is no biasing, exponents use two's complement notation, and exponents of all zeros and all ones are allowed.) b. What bias should be used in the exponent if we prefer all exponents to be non-negative? Why would you choose this bias? a. Largest Positive: x 2 3 = = 7.5 Smallest Positive: x 2-4 = = 1/32 = Page 6

7 b. For all non-negative exponents, we would need a bias of Using the model in the previous question, including your chosen bias, add the following floating-point numbers and express your answer using the same notation as the addend and augend: Calculate the relative error, if any, in your answer to the previous question Error = 2.4% 20. Assume we are using the simple model for floating-point representation as given in this book (the representation uses a 14-bit format, 5 bits for the exponent with a bias of 16, a normalized mantissa of 8 bits, and a single sign bit for the number): a. Show how the computer would represent the numbers and 0.25 using this floating-point format. b. Show how the computer would add the two floating-point numbers in part a by changing one of the numbers so they are both expressed using the same power of 2. c. Show how the computer would represent the sum in part b using the given floatingpoint representation. What decimal value for the sum is the computer actually storing? Explain. a = = b. Adding in part(a) we get: x 2 7 = x x 2-1 = x 2 7 = x 2 7 c. The sum from part b in the given floating point notation is: which is x 2 7 = = What causes divide underflow and what can be done about it? Divide underflow is happens when the divisor is much smaller than the dividend. The computer may not be able to reconcile the two numbers of greatly different magnitudes, and thus the smaller gets represented by zero. The result of the division then becomes the equivalent of a division by zero error. Compare this to underflow, which is a condition that can occur when the result of a floating point operation would be smaller in magnitude Page 7

8 (closer to zero, either positive or negative) than the smallest representable quantity. Division underflow could be avoided by using repeated subtraction. 22. Why do we usually store floating-point numbers in normalized form? What is the advantage of using a bias as opposed to adding a sign bit to the exponent? There are two reasons to use normalized form. First, normalized form creates a standard so each floating point value has a unique binary representation. Second, with normalized numbers, we can "imply" the high-order 1 in the significand, giving us an extra bit of accuracy for "free". Using a bias allows an extra bit position to be used for the magnitude of the exponent, as no sign bit is required. 23. Let a = , b = and c = Using the floating-point model described in the text (the representation uses a 14-bit format, 5 bits for the exponent with a bias of 16, a normalized mantissa of 8 bits, and a single sign bit for the number), perform the following calculations, paying close attention to the order of operations. What can you say about the algebraic properties of floating-point arithmetic in our finite model? Do you think this algebraic anomaly holds under multiplication as well as addition? b + (a + c) = (b + a) + c = b + (a + c) = b + [1.0 x x 2 1 ] = b + [1.0 x x 2 9 ) = b x 2 9 = b x 2 10 (but we can only have 8 bits in the mantissa) = b x 2 10 = b x 2 9 = (-1.0 x 2 9 ) + (1.0 x 2 9 ) = 0 (b + a) + c = [(-1.0 x 2 9 ) + (1.0 x 2 9 )] + c = 0 + c = c = 1.0 x 2 1 = 10 When one number is significantly larger than the other, round off error occurs due to the limited number of bits in the mantissa. Multiplication does not require the values to be expressed with the same powers of 2, and does not suffer from this problem. 24. a. Given that the ASCII code for A is , what is the ASCII code for J? b. Given that the EBCDIC code for A is , what is the EBCDIC code for J? a. If A = , then J = b. If A = , then J = 0001 Page 8

9 25. Assume a 24-bit word on a computer. In these 24 bits, we wish to represent the value 295. a. If our computer uses even parity, how would the computer represent the string 295? b. If our computer uses 8-bit ASCII and even parity, how would the computer represent the string 295? c. If our computer uses packed BCD, how would the computer represent the number +295? Binary Value ASCII Packed BCD Decode the following ASCII message, assuming 7-bit ASCII characters and no parity: (Blue indicates a correction was made from what is in the textbook.) = J = O = H = N = space = D = O = E 27. Why would a system designer wish to make Unicode the default character set for their new system? What reason(s) could you give for not using Unicode as a default? (Hint: Think about language compatibility versus storage space.) Although Unicode would make systems compatible (with each other and with other ASCII systems(), it requires 16 bytes for storage compared to the 7 or 8 required by ASCII or EBCDIC. 28. Write the 7-bit ASCII code for the character 4 using the following encoding: a. Non-return-to-zero b. Non-return-to-zero-invert c. Manchester Code d. Frequency modulation e. Modified frequency modulation f. Run length limited (Assume 1 is high, and 0 is low. ) Page 9

10 29. Why is NRZ coding seldom used for recording data on magnetic media? NRZ coding is seldom used for recording data on magnetic media because it has unacceptably high error rates and not contain sufficient transitions to keep read/write heads synchronized. 30. Assume we wish to create a code using 3 information bits, 1 parity bit (appended to the end of the information), and odd parity. List all legal code words in this code. What is the Hamming distance of your code? The legal code words are: and the Hamming distance of these code words is Are the error-correcting Hamming codes systematic? Explain. Error-correcting Hamming codes interleave additional error-checking (parity) bits into the actual information bits, but do not append these bits. In a systematic code, the first k bist of the codeword must be the same as the corresponding message bits (the parity bits must be appended), so by definition, Hamming codes are not systematic. Page 10

11 32. Compute the Hamming distance of the following code: Compute the Hamming distance of the following code: Suppose we want an error-correcting code that will allow all single-bit errors to be corrected for memory words of length 10. a. How many parity bits are necessary? b. Assuming we are using the Hamming algorithm presented in this chapter to design our error-correcting code, find the code word to represent the 10-bit information word: a. m + r + 1 <= 2 r 10+ r + 1 <= 2 r 11 + r <= 2 r which implies that r is 4 b. The code word for is found as follows: Parity bit 1 checks 1,3,5,7,9,11,13, so Bit 1 must be 0 (assuming even parity) Parity bit 2 checks 2,3,6,7,10,11,14, so Bit 2 must be 0 Parity bit 4 checks 4,5,6,7,12,14, so Bit 4 must be 1 Parity bit 8 checks 8,9,10,11,12,13,14, so Bit 8 must be Suppose we are working with an error-correcting code that will allow all single-bit errors to be corrected for memory words of length 7. We have already calculated that we need 4 check bits, and the length of all code words will be 11. Code words are created according to the Hamming Algorithm presented in the text. We now receive the following code word: Page 11

12 Assuming even parity, is this a legal code word? If not, according to our error-correcting code, where is the error? The error is in bit Repeat exercise 35 using the following code word: Bit 1 checks 1,3,5,7,9 and 11, but this is an odd number of 1's error Bit 2 checks 2,3,6,7,10, and 11, which is an odd number of 1's error Bit 4 checks 4,5,6, and 7, which is an even number of 1's ok Bit 8 checks 8,9,10, and 11, which is an odd number of 1's error Since errors occur in bit positions 1, 2, and 8, the error is in bit number 1+2+8= Name two ways in which Reed-Soloman coding differs from Hamming coding. Reed-Soloman codes are formed by polynomial division operating on whole characters (symbols), and are thus block-level codes. Hamming codes are bit-parity operations over certain defined bits of a symbol (so these are bit-level codes). Reed-Soloman codes are good at detecting and correcting burst errors, whereas Hamming codes are not. 38. When would you choose a CRC code over a Hamming code? A Hamming code over a CRC? CRCs are useful for checking data sent over telecommunication lines. If a CRC error occurs, a retransmission is requested. You would choose a CRC when you can ask for retransmission and do not want to sustain the (space and time) overhead of Hamming codes. Hamming codes are good at forward error correction: they can correct errors when retransmission is not possible, such as when data is stored on a disk. CRC is better than Hamming in terms of speed, but Hamming is better than CRC in terms of complexity (Hamming codes do not require complex circuits). 39. Find the quotients and remainders for the following division problems modulo 2. a b c d a. Remainder 110 b. 111 Remainder 1100 c Remainder 110 d Remainder 1000 Page 12

13 40. Find the quotients and remainders for the following division problems modulo 2. a b c d a. Remainder 101 b Remainder 101 c. 11 Remainder1100 d. 0 Remainder Using the CRC polynomial 1011, compute the CRC code word for the information word, Check the division performed at the receiver. Codeword: Using the CRC polynomial, compute the CRC code word for the information word, Check the division performed at the receiver. The codeword is Dividing this by modulo 2 should yield a zero remainder. Append three 0s to the end of the information word and divide: > remainder The information word (with appended zeros) + remainder = codeword so we have: = To check the division: > remainder Page 13

14 *43. Pick an architecture (such as 80486, Pentium, Pentium IV, SPARC, Alpha, or MIPS). Do research to find out how your architecture approaches the concepts introduced in this chapter. For example, what representation does it use for negative values? What character codes does it support? No answer. Sample Exam Questions 1. Fill in the following addition table for base Row 1: 0, 1, 2 Row 2: 1, 2, 0 Row 3: 2, 0, 1 2. Perform the following base conversions using subtraction or division-remainder: a = 3 b = 5 c = 7 d = 9 a b c d Show the representation of -16 (assuming 8-bit registers) using: a. signed-magnitude representation b. signed-1's complement c. signed-2's complement a b c Given the 8-bit binary number: What decimal number does this represent if the computer uses: a. signed-magnitude representation Page 14

15 b. signed-1's complement c. signed-2's complement a b c Assuming 2 s complement 8-bit representation, consider the following: +70 = = Is this correct? Why or why not? No. The result is a different sign than the two numbers we are adding so overflow has occurred. 6. If a computer uses signed-2's complement representation and 8 bit registers, what range of integers can this computer represent? What range of integers can the computer represent if it is using signed magnitude representation? Range (signed 2's complement): to Range (signed magnitude): to A 15-bit floating point number has 1 bit for the sign of the number, 5 bits for the exponent and 9 bits for the mantissa (which is normalized). Numbers in the exponent are in signed magnitude representation. No bias is used and there are no implied bits. Show the representation for the smallest positive number this machine can represent SIGN EXPONENT MANTISSA Value is x Assume we are using the simple model for floating-point representation as given in the book (the representation uses a 14-bit format, 5 bits for the exponent with a bias of 16, a normalized mantissa of 8 bits, and a single sign bit for the number). Show how the computer would represent the numbers 96.5 and using this floating-point format = = Page 15

16 9. Find the Hamming distance d for the following code: {01010,10101,11111,00000,00110} The distance is a. To detect e single-bit errors, we need a Hamming distance d =. b. To correct e single-bit errors, we need a Hamming distance d =. a. e+1 b. 2e a. Suppose we want an error correcting code that will allow all single-bit errors to be corrected for memory words of length 11. How many check bits are necessary? b. Suppose we are now working with memory words of length 8. We have already calculated that we need 4 check bits, and the length of all codewords will be 12. We now receive the following code word: Is this a legal codeword, assuming odd parity? If not, where is the error? a. 11+r+1<=2 r implies an r of 4. b Bit 1 checks 1,3,5,7,9 and 11, but this is an even number of 1's error Bit 2 checks 2,3,6,7,10, and 11, which is an odd number of 1's ok Bit 4 checks 4,5,6, 7, and 12, which is an odd number of 1's ok Bit 8 checks 8,9,10, 11 and 12, which is an odd number of 1's ok Since an error occurs in bit position 1, this is where the error is. 12. Given that the ASCII code for the character "A" is , the ASCII code for "H" would be: a b. 101 c d e. none of these The correct answer is D. 13. How many base-3 digits does it take to obtain as many combinations as can be done with 5 binary digits? a. eleven b. three c. four d.five e. this can't be done The correct answer is C. Page 16

17 14. ASCII, EBCDIC, and Unicode are translated into other codes before they are transmitted or recorded. These data encoding methods include all but which of the following: a. non-return-to-zero encoding b. Manchester coding c. non-return-to-zero-invert encoding d. run-length-limited coding e. all of these are encoding methods for recorded or transmitted data The correct answer is E. TRUE OR FALSE. 1. BCD stands for Binary Coded Decimal and encodes each digit of a decimal number to an 8-bit binary form. 2. Unicode is a 16-bit code, occupying twice the disk space for text as ASCII or EBCDIC would require. 3. Hamming codes, used for error detection and correction, are useful for burst errors (where we could reasonably expect multiple adjacent bits to be incorrect); Reed- Soloman coding is more useful for random errors (where one can reasonably expect errors to be rare events). 4. A signed-magnitude integer representation includes more negative numbers than it does positive ones. 5. CRCs are useful when you can ask for retransmission; Hamming codes are good when retransmission is not possible, such as when data is stored on a disk. 1. True 2. True 3. False 4. True 5. False Page 17

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