Computer Organization - Arithmetic & Logic Unit.pptx
- 1. Arithmetic & Logic Unit • Does the calculations • Everything else in the computer is there to service this unit • Handles integers • May handle floating point (real) numbers • May be separate FPU (maths co- processor) • May be on chip separate FPU (486DX +)
- 2. ALU Inputs and Outputs
- 3. Integer Representation • Only have 0 & 1 to represent everything • Positive numbers stored in binary —e.g. 41=00101001 • No minus sign • No period • Sign-Magnitude • Two’s compliment
- 4. Sign-Magnitude • Left most bit is sign bit • 0 means positive • 1 means negative • +18 = 00010010 • -18 = 10010010 • Problems —Need to consider both sign and magnitude in arithmetic —Two representations of zero (+0 and -0)
- 5. Two’s Complment • +3 = 00000011 • +2 = 00000010 • +1 = 00000001 • +0 = 00000000 • -1 = 11111111 • -2 = 11111110 • -3 = 11111101
- 6. Benefits • One representation of zero • Arithmetic works easily (see later) • Negating is fairly easy —3 = 00000011 —Boolean complement gives 11111100 —Add 1 to LSB 11111101
- 7. Geometric Depiction of Twos Complement Integers
- 8. Negation Special Case 1 • 0 = 00000000 • Bitwise not 11111111 • Add 1 to LSB +1 • Result 1 00000000 • Overflow is ignored, so: • - 0 = 0 √
- 9. Negation Special Case 2 • -128 = 10000000 • bitwise not 01111111 • Add 1 to LSB +1 • Result 10000000 • So: • -(-128) = -128 X • Monitor MSB (sign bit) • It should change during negation
- 10. Range of Numbers • 8 bit 2s compliment —+127 = 01111111 = 27 -1 — -128 = 10000000 = -27 • 16 bit 2s compliment —+32767 = 011111111 11111111 = 215 - 1 — -32768 = 100000000 00000000 = -215
- 11. Conversion Between Lengths • Positive number pack with leading zeros • +18 = 00010010 • +18 = 00000000 00010010 • Negative numbers pack with leading ones • -18 = 10010010 • -18 = 11111111 10010010 • i.e. pack with MSB (sign bit)
- 12. Addition and Subtraction • Normal binary addition • Monitor sign bit for overflow • Take twos compliment of substahend and add to minuend —i.e. a - b = a + (-b) • So we only need addition and complement circuits
- 13. Hardware for Addition and Subtraction
- 14. Multiplication • Complex • Work out partial product for each digit • Take care with place value (column) • Add partial products
- 15. Multiplication Example • 1011 Multiplicand (11 dec) • x 1101 Multiplier (13 dec) • 1011 Partial products • 0000 Note: if multiplier bit is 1 copy • 1011 multiplicand (place value) • 1011 otherwise zero • 10001111 Product (143 dec) • Note: need double length result
- 18. Flowchart for Unsigned Binary Multiplication
- 19. Multiplying Negative Numbers • This does not work! • Solution 1 —Convert to positive if required —Multiply as above —If signs were different, negate answer • Solution 2 —Booth’s algorithm
- 21. Example of Booth’s Algorithm
- 22. Division • More complex than multiplication • Negative numbers are really bad! • Based on long division
- 23. 00111 1 Division of Unsigned Binary Integers 1011 00001101 10010011 1011 00111 0 101 1 101 1100 Quotien t Dividen d Remainde r Partial Remainder s Diviso r
- 24. Flowchart for Unsigned Binary Division
- 25. Real Numbers • Numbers with fractions • Could be done in pure binary —1001.1010 = 23 + 20 +2-1 + 2-3 =9.625 • Where is the binary point? • Fixed? —Very limited • Moving? —How do you show where it is?
- 26. Floating Point • +/- .significand x 2exponent • Misnomer • Point is actually fixed between sign bit and body of mantissa • Exponent indicates place value (point position) Misnomer:เรียกชื่อ
- 28. Signs for Floating Point • Mantissa is stored in 2s compliment • Exponent is in excess or biased notation —e.g. Excess (bias) 128 means —8 bit exponent field —Pure value range 0-255 —Subtract 128 to get correct value —Range -128 to +127
- 29. Normalization • FP numbers are usually normalized • i.e. exponent is adjusted so that leading bit (MSB) of mantissa is 1 • Since it is always 1 there is no need to store it • (c.f. Scientific notation where numbers are normalized to give a single digit before the decimal point • e.g. 3.123 x 103 )
- 30. FP Ranges • For a 32 bit number —8 bit exponent —+/- 2256 ≈ 1.5 x 1077 • Accuracy —The effect of changing lsb of mantissa —23 bit mantissa 2-23 ≈ 1.2 x 10-7 —About 6 decimal places
- 32. Density of Floating Point Numbers
- 33. IEEE 754 • Standard for floating point storage • 32 and 64 bit standards • 8 and 11 bit exponent respectively • Extended formats (both mantissa and exponent) for intermediate results
- 34. IEEE 754 Formats
- 35. FP Arithmetic +/- • Check for zeros • Align significands (adjusting exponents) • Add or subtract significands • Normalize result
- 36. FP Addition & Subtraction Flowchart
- 37. FP Arithmetic x/÷ • Check for zero • Add/subtract exponents • Multiply/divide significands (watch sign) • Normalize • Round • All intermediate results should be in double length storage
- 40. Required Reading • Stallings Chapter 9 • IEEE 754 on IEEE Web site