Computer ArchitectureFall 2007 © September 5, 2007 Karem Sakallah CS 447 – Computer Architecture Lecture 3 Computer Arithmetic (2)

Computer ArchitectureFall 2007 © Last Time °Representation of finite-width unsigned and signed integers °Addition and subtraction of integers °Multiplication of unsigned integers

Computer ArchitectureFall 2007 © Today °Review of multiplication of unsigned integers °Multiplication of signed integers °Division of unsigned integers °Representation of real numbers °Addition, subtraction, multiplication, and division of real numbers

Computer ArchitectureFall 2007 © Multiplication °Complex °Work out partial product for each digit °Take care with place value (column) °Add partial products

Computer ArchitectureFall 2007 © 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 ° Product (143 dec) ° Note: need double length result

Computer ArchitectureFall 2007 © Unsigned Binary Multiplication

Computer ArchitectureFall 2007 © Execution of Example

Computer ArchitectureFall 2007 © Flowchart for Unsigned Binary Multiplication

Computer ArchitectureFall 2007 © 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

Computer ArchitectureFall 2007 © Observation °Which of these two multiplications is more difficult? 98,765 x 10,001 98,765 x 9,999 °Note: 98,765 x 10,001 = 98,765 x (10, ) 98,765 x 9,999 = 98,765 x (10,000 – 1)

Computer ArchitectureFall 2007 © In Binary Let Q = d d d d d QL = d d d d d QR =

Computer ArchitectureFall 2007 © In Binary Let Q = d d d d d QL = d d d d d QR = Then Q = QL – QR And M x Q = M x QL – M x QR

Computer ArchitectureFall 2007 © A bit more explanation P = M x Q where M and Q are n-bit two’s complement integers e.g., Q = -2 3 q q q q 0 which can be re-written as follows: Q = 2 0 (q -1 –q 0 )+2 1 (q 0 -q 1 )+2 2 (q 1 -q 2 )+2 3 (q 2 -q 3 ) leading to P =M x 2 0 x (q -1 - q 0 ) + M x 2 1 x (q 0 - q 1 ) + M x 2 2 x (q 1 - q 2 ) + M x 2 3 x (q 2 - q 3 )

Computer ArchitectureFall 2007 © Finally! P =M x 2 0 x (q -1 - q 0 ) + M x 2 1 x (q 0 - q 1 ) + M x 2 2 x (q 1 - q 2 ) + M x 2 3 x (q 2 - q 3 ) qiqi q i-1 q i-1 – q i Action 000Noop 011Add M 10Subtract M 110Noop

Computer ArchitectureFall 2007 © Example of Booth’s Algorithm

Computer ArchitectureFall 2007 © Booth’s Algorithm

Computer ArchitectureFall 2007 © Division °More complex than multiplication °Negative numbers are really bad! °Based on long division

Computer ArchitectureFall 2007 © Division of Unsigned Binary Integers Quotient Dividend Remainder Partial Remainders Divisor

Computer ArchitectureFall 2007 © Flowchart for Unsigned Binary Division

Computer ArchitectureFall 2007 © Real Numbers °Numbers with fractions °Could be done in pure binary = =9.625 °Where is the binary point? °Fixed? Very limited °Moving? How do you show where it is?

Computer ArchitectureFall 2007 © Floating Point °+/-.significand x 2 exponent °Misnomer °Point is actually fixed between sign bit and body of mantissa °Exponent indicates place value (point position) Sign bit Biased Exponent Significand or Mantissa

Computer ArchitectureFall 2007 © Floating Point Examples

Computer ArchitectureFall 2007 © Signs for Floating Point °Mantissa is stored in two’s complement °Exponent is in excess or biased notation e.g. Excess (bias) 128 means 8 bit exponent field Pure value range Subtract 128 to get correct value Range -128 to +127

Computer ArchitectureFall 2007 © 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 x 10 3 )

Computer ArchitectureFall 2007 © FP Ranges °For a 32 bit number 8 bit exponent +/  1.5 x °Accuracy The effect of changing lsb of mantissa 23 bit mantissa  1.2 x About 6 decimal places

Computer ArchitectureFall 2007 © Expressible Numbers

Computer ArchitectureFall 2007 © Density of Floating Point Numbers

Computer ArchitectureFall 2007 © IEEE 754 Formats

Computer ArchitectureFall 2007 © FP Arithmetic +/- °Check for zeros °Align significands (adjusting exponents) °Add or subtract significands °Normalize result

Computer ArchitectureFall 2007 © FP Addition & Subtraction Flowchart

Computer ArchitectureFall 2007 © 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

Computer ArchitectureFall 2007 © Floating Point Multiplication

Computer ArchitectureFall 2007 © Floating Point Division