Advanced Dividers Lecture 11. Required Reading Chapter 15 Variation in Dividers 15.3, Combinational...

Advanced Dividers

Lecture 11

Required Reading

Chapter 15 Variation in Dividers15.3, Combinational and Array Dividers

Chapter 16, Division by Convergence

Behrooz Parhami, Computer Arithmetic: Algorithms and Hardware Design

May 2012 Computer Arithmetic, Division Slide 3

Division versus MultiplicationDivision is more complex than multiplication: Need for quotient digit selection or estimation

Overflow possibility: the high-order k bits of z must be strictly less than d; this overflow check also detects the divide-by-zero condition.

Pentium III latenciesInstruction Latency Cycles/IssueLoad / Store 3 1Integer Multiply 4 1Integer Divide 36 36Double/Single FP Multiply 5 2Double/Single FP Add 3 1Double/Single FP Divide 38 38

The ratios haven’t changed much in later Pentiums, Atom, or AMD products*

*Source: T. Granlund, “Instruction Latencies and Throughput for AMD and Intel x86 Processors,” Feb. 2012

Classification of Dividers

Sequential

Radix-2 High-radix

RestoringNon-restoring

• regular• SRT• using carry save adders• SRT using carry save adders

ArrayDividers

Dividersby Convergence

Fractional Division

Unsigned Fractional Division

zfrac Dividend .z-1z-2 . . . z-(2k-1)z-2k

dfrac Divisor .d-1d-2 . . . d-(k-1) d-k

qfrac Quotient .q-1q-2 . . . q-(k-1) q-k

sfrac Remainder .000…0s-(k+1) . . . s-(2k-1) s-2kk bits

Integer vs. Fractional Division

For Integers:

z = q d + s 2-2k

z 2-2k = (q 2-k) (d 2-k) + s (2-2k)

zfrac = qfrac dfrac + sfrac

For Fractions:

wherezfrac = z 2-2k

dfrac = d 2-k

qfrac = q 2-k

sfrac = s 2-2k

Unsigned Fractional Division Overflow

Condition for no overflow:

zfrac < dfrac

Sequential Fractional DivisionBasic Equations

s(0) = zfrac

s(j) = 2 s(j-1) - q-j dfrac for j=1..k

2k · sfrac = s(k)

sfrac = 2-k · s(k)

Fig. 13.2 Examples of sequential division with integer and fractional operands.

ArrayDividers

Sequential Fractional DivisionBasic Equations

sfrac(0) = zfrac

s(j) = 2 s(j-1) - q-j dfrac

s(k)frac

= 2k sfrac

Restoring Unsigned Fractional Division

s(0) = z

for j = 1 to k if 2 s(j-1) - d > 0 q-j = 1 s(j) = 2 s(j-1) - d else q-j = 0 s(j) = 2 s(j-1)

Restoring Array Divider

s s s –4 –5 –6

z z z z–1 –2 –3 –4

d d d –1 –2 –3

–1 –2 –3 –4 –5 –6 –1 –2 –3 –1 –2 –3 –4 –5 –6

Dividend z = .z z z z z z Divisor d = .d d d Quotient q = .q q q Remainder s = .0 0 0 s s s

Non-Restoring Unsigned Fractional Division

s(-1) = z-dfor j = 0 to k-1 if s(j-1) > 0 q-j = 1 s(j) = 2 s(j-1) - d else q-j = 0 s(j) = 2 s(j-1) + dend forif s(k-1) > 0 q-k = 1else

q-k = 0

Nonrestoring Array Divider

Dividend z = z .z z z z z z Divisor d = d .d d d Quotient q = q .q q q Remainder s = 0 .0 0 s s s s

0 –1 –2 –3 –4 –5 –6 0 –1 –2 –3 0 –1 –2 –3 –3 –4 –5 –6

s s s s–3 –4 –5 –6

q –3

d d d d0 –1 –2 –3z z z z0 –1 –2 –3

Similarity to array multiplier is deceiving

Critical path

Division by Convergence

Chapter Goals

Show how by using multiplication as thebasic operation in each division step,the number of iterations can be reduced

Chapter Highlights

Digit-recurrence as convergence methodConvergence by Newton-Raphson iterationComputing the reciprocal of a numberHardware implementation and fine tuning

16.1 General Convergence MethodsSequential digit-at-a-time (binary or high-radix) division can be viewed as a convergence scheme

As each new digit of q = z / d is determined, the quotient value is refined, until it reaches the final correct value

0.101101

1Meanwhile, the remainders = z – q d approaches 0; the scaled remainder is kept in a certain range, such as [– d, d)

Convergence is from below in restoring division and oscillating in nonrestoring division

Elaboration on Scaled Remainder in Division

Quotient digit selection keeps the scaled remainder bounded (say, in the range –d to d) to ensure the convergence of the true remainder to 0

The partial remainder s(j) in division recurrence isn’t the true remainder but a version scaled by 2j

Division with left shifts

s(j) = 2s(j–1) – qk–j (2k d) with s(0) = z and

|–shift–| s(k) = 2ks|––– subtract –––|

0.101101

Recurrence Formulas for Convergence Methods

u (i+1) = f(u

(i), v (i), w

v (i+1) = g(u

(i), v (i), w

w (i+1) = h(u

(i), v (i), w

u (i+1) = f(u

(i), v (i))

v (i+1) = g(u

(i), v (i))

The complexity of this method depends on two factors:

a. Ease of evaluating f and g (and h) b. Rate of convergence (number of iterations needed)

Constant

Desiredfunction

Guide the iteration such that one of the values converges to a constant (usually 0 or 1)

The other value then converges to the desired function

16.2 Division by Repeated Multiplications

Remainder often not needed, but can be obtained by another multiplication if desired: s = z – qd

Motivation: Suppose add takes 1 clock and multiply 3 clocks64-bit divide takes 64 clocks in radix 2, 32 in radix 4

Divide faster via multiplications faster if 10 or fewer needed

)1()1()0(

xxzxdz

Force to 1

Converges to q

To turn the identity into a division algorithm, we face three questions:

1. How to select the multipliers x(i) ?

2. How many iterations (pairs of multiplications)? 3. How to implement in hardware?

Formulation as a Convergence Computation

)1()1()0(

xxzxdz

Force to 1

Converges to q

d (i+1) = d

(i) x (i) Set d

(0) = d; make d (m) converge to 1

z (i+1) = z

(i) x (i) Set z

(0) = z; obtain z/d = q z (m)

Question 1: How to select the multipliers x (i)

? x (i) = 2 – d

This choice transforms the recurrence equations into:

d (i+1) = d

(i) (2 d

(i)) Set d (0) = d; iterate until d

(m) 1 z

(i+1) = z (i)

(2 d (i)) Set z

u (i+1) = f(u

(i), v (i))

v (i+1) = g(u

(i), v (i))

Fits the general form

Determining the Rate of Convergence

d (i+1) = d

(i) x (i) Set d

(0) = d; make d (m) converge to 1

z (i+1) = z

(i) x (i) Set z

Question 2: How quickly does d (i)

converge to 1?

We can relate the error in step i + 1 to the error in step i:

d (i+1) = d

(i) (2 d

(i)) = 1 – (1 – d (i))2

1 – d (i+1) = (1 – d

For 1 – d (i) , we get 1 – d

(i+1) 2: Quadratic convergence

In general, for k-bit operands, we need

2m – 1 multiplications and m 2’s complementations

where m = log2 k

Quadratic Convergence

Table 16.1 Quadratic convergence in computing z/d by repeated multiplications, where 1/2 d = 1 – y < 1

––––––––––––––––––––––––––––––––––––––––––––––––––––––– i d

(i) = d (i–1)

x (i–1), with d

(0) = d x (i) = 2 – d

(i) ––––––––––––––––––––––––––––––––––––––––––––––––––––––– 0 1 – y = (.1xxx xxxx xxxx xxxx)two 1/2 1 + y 1 1 – y

2 = (.11xx xxxx xxxx xxxx)two 3/4 1 + y 2

2 1 – y 4 = (.1111 xxxx xxxx xxxx)two 15/16 1 + y

4 3 1 – y

8 = (.1111 1111 xxxx xxxx)two 255/256 1 + y 8

4 1 – y 16 = (.1111 1111 1111 1111)two = 1 – ulp

–––––––––––––––––––––––––––––––––––––––––––––––––––––––Each iteration doubles the number of guaranteed leading 1s (convergence to 1 is from below)

Beginning with a single 1 (d ½), after log2 k iterations we get as close to 1 as is possible in a fractional representation

Graphical Depiction of Convergence to q

Fig. 16.1 Graphical representation of convergence in division by repeated multiplications.

1 1 – ulp

Iteration i

0 1 2 3 4 5 6

16.5 Hardware ImplementationRepeated multiplications: Each pair of ops involves the same multiplier

d (i+1) = d

(i) (2 d

(i)) Set d (0) = d; iterate until d

(m) 1 z

(i+1) = z (i)

(2 d (i)) Set z

Fig. 16.6 Two multiplications fully overlapped in a 2-stage pipelined multiplier.

z x(i)(i)

d x(i)(i)

x(i)z(i)d(i+1)

d(i+1)

x(i+1)

z x(i)(i)

d x(i+1)(i+1)

z(i+1)

2's Complz(i+1) x(i+1)

z x(i+1)(i+1)

d(i+2)

d x(i+1)(i+1)

16.3 Division by Reciprocation

Fig. 16.2 Convergence to a root of f(x) = 0 in the Newton-Raphson method.

The Newton-Raphson method can be used for finding a root of f (x) = 0

xx(i+1)x

Tangent at x(i)

Root x(i)(i+2)

Start with an initial estimate x(0) for the root

Iteratively refine the estimate via the recurrence

x(i+1) = x(i) – f (x(i)) / f (x(i))

Justification:

tan (i) = f (x(i)) = f (x(i)) / (x(i) – x(i+1))

Computing 1/d by Convergence

1/d is the root of f (x) = 1/x – d

f (x) = –1/x2

Substitute in the Newton-Raphson recurrence x(i+1) = x(i) – f (x(i)) / f (x(i)) to get:

x (i+1) = x

(i) (2 x

One iteration = Two multiplications + One 2’s complementation

Error analysis: Let (i) = 1/d – x(i) be the error at the ith iteration

(i+1) = 1/d – x

(i+1) = 1/d – x (i)

(2 – x (i)

d) = d (1/d – x (i))2 = d (

Because d < 1, we have (i+1) < (

Choosing the Initial Approximation to 1/d

With x(0) in the range 0 < x(0) < 2/d, convergence is guaranteed

Justification: | (0) | = | x(0) – 1/d | < 1/d

(1) = | x(1) – 1/d | = d ((0))2 = (d (0)) (0) < (0)

For d in [1/2, 1):

Simple choice x(0) = 1.5

Max error = 0.5 < 1/d

Better approx. x(0) = 4(3 – 1) – 2d = 2.9282 – 2d

Max error 0.1

16.4 Speedup of Convergence Division

Division can be performed via 2 log2 k – 1 multiplications

This is not yet very impressive

64-bit numbers, 3-ns multiplier 33-ns division

Three types of speedup are possible:

Fewer multiplications (reduce m) Narrower multiplications (reduce the width of some x(i)s) Faster multiplications

)1()1()0(

xxzxdz

q Compute y = 1/d

Do the multiplication yz

Initial Approximation via Table Lookup

Convergence is slow in the beginning: it takes 6 multiplications to get 8 bits of convergence and another 5 to go from 8 bits to 64 bits

d x(0) x(1) x(2) = (0.1111 1111 . . . )two

Approx to 1/d

Better approx

Read this value, x(0+), directly from a table, thereby reducing 6 multiplications to 2

A 2ww lookup table is necessary and sufficient for w bits of convergence after 2 multiplications

Example with 4-bit lookup: d = 0.1011 xxxx . . . (11/16 d < 12/16)Inverses of the two extremes are 16/11 1.0111 and 16/12 1.0101 So, 1.0110 is a good estimate for 1/d1.0110 0.1011 = (11/8) (11/16) = 121/128 = 0.1111001 1.0110 0.1100 = (11/8) (3/4) = 33/32 = 1.000010

Visualizing the Convergence with Table Lookup

Fig. 16.3 Convergence in division by repeated multiplications with initial table lookup.

1 1 – ulp

Iterations

After table lookup and 1st pair of multiplications, replacing several iterations

After the 2nd pair of multiplications

Convergence Does Not Have to Be from Below

Fig. 16.4 Convergence in division by repeated multiplications with initial table lookup and the use of truncated multiplicative factors.

1 1 ± ulp

Iterations

SequentialDividers

with Carry-Save Adders

Block diagram of a radix-2 SRT divider with partialremainder in stored-carry form

Pentium bug (1)October 1994

Thomas Nicely, Lynchburg Collage, Virginiafinds an error in his computer calculations, and tracesit back to the Pentium processor

Tim Coe, Vitesse Semiconductorpresents an example with the worst-case error

c = 4 195 835/3 145 727

Pentium = 1.333 739 06...Correct result = 1.333 820 44...

November 7, 1994

Late 1994

First press announcement, Electronic Engineering Times

Pentium bug (2)

Intel admits “subtle flaw”

Intel’s white paper about the bug and its possible consequences

Intel - average spreadsheet user affected once in 27,000 yearsIBM - average spreadsheet user affected once every 24 days

Replacements based on customer needs

Announcement of no-question-asked replacements

November 30, 1994

December 20, 1994

Pentium bug (3)

Error traced back to the look-up table used bythe radix-4 SRT division algorithm

2048 cells, 1066 non-zero values {-2, -1, 1, 2}

5 non-zero values not downloaded correctly to the lookup table due to an error in the C script

Follow-upCourses

DIGITAL SYSTEMS DESIGN

1. ECE 681 VLSI Design for ASICs (Fall semesters) H. Homayoun, project/lab, front-end and back-end ASIC design with Synopsys tools

2. ECE 699 Digital Signal Processing Hardware Architectures (Spring semesters) A. Cohen, project, FPGA design for DSP

3. ECE 682 VLSI Test Concepts (Spring semesters)

T. Storey, homework

NETWORK AND SYSTEM SECURITY

1. ECE 646 Cryptography and Computer Network Security (Fall semesters) K.Gaj, hardware, software, or analytical project

2. ECE 746 Advanced Applied Cryptography (Spring semesters)

J.-P. Kaps, hardware, software, or analytical project

3. ECE 899 Cryptographic Engineering (Spring semesters)

J.-P. Kaps, research-oriented project

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