VLSI Arithmetic-Lect-5

VLSI ArithmeticLecture 5

Prof. Vojin G. Oklobdzija

University of California

http://www.ece.ucdavis.edu/acsel

Review

Lecture 4

Ling’s Adder

Huey Ling, “High-Speed Binary Adder”

IBM Journal of Research and Development, Vol.5, No.3, 1981.

Used in: IBM 3033, IBM S370/168, Amdahl V6, HP etc.

Oklobdzija 2004 Computer Arithmetic 4

Ling’s Derivations

ai bi pi gi ti

0 0 0 0 0

0 1 1 0 1

1 0 1 0 1

1 1 0 1 1

iii CCH 11

iii bag

ai bi

ci

si

ci+1

gi implies Ci+1 which implies Hi+1 , thus: gi= gi Hi+1

iiii CpgC 1define:

11

iiiiii

iiiiiiiii

HpCpCp

CppgpCpCp

1 iiii HpCp

111

11

iiiiii

iiiiiiii

HtHpHg

CpHgCpgC

11 iii HtC


Ling’s Derivations

iii CCH 11 iiii CpgC 1From: and

iiiiiiiii CgCCpgCCH 11

iiii HtgH 11 11 iii HtCbecause:

fundamental expansion

Now we need to derive Sum equation


Ling Adder

Variation of CLA:

Ling, IBM J. Res. Dev, 5/81

iiii CpgC 1

iii CpS

iii bap

iii bag

iiii HtgH 11

iiiiii HtgHtS 11

iii bat

iii bag

Ling’s equations:


Ling Adder

iiii

iiiiii

Cpgg

CpCggC

1

iiii CtgC 1 iiii HtgH 11

Ling’s equation:

see: Doran, IEEE Trans on Comp. Vol 37, No.9 Sept. 1988.

Ling uses different transfer function.Four of those functions have desiredproperties (Ling’s is one of them)

Variation of CLA:

ai bi

ci

si

ci+1

ai-1 bi-1

ci-1

si-1

gi, ti gi-1, ti-1

Hi+1 Hi


Ling Adder

inCttttgtttgttgtgC 012301231232334

in

in

CtttgttgtggH

CttttgtttgttgtgH

01201212234

101200121122234

Conventional:

Ling:

Fan-in of 5

Fan-in of 4


Advantages of Ling’s Adder• Uniform loading in fan-in and fan-out

• H16 contains 8 terms as compared to G16 that contains 15.

• H16 can be implemented with one level of logic (in ECL), while G16 can not (with 8-way wire-OR).

(Ling’s adder takes full advantage of wired-OR, of special importance when ECL technology is used - his IBM limitation was fan-in of 4 and wire-OR of 8)


Ling: Weinberger Notes






Advantage of Ling’s Adder

• 32-bit adder used in: IBM 3033, IBM S370/ Model168, Amdahl V6.

• Implements 32-bit addition in 3 levels of logic

• Implements 32-bit AGEN: B+Index+Disp in 4 levels of logic (rather than 6)

• 5 levels of logic for 64-bit adder used in HP processor


Implementation of Ling’s Adder in CMOS

(S. Naffziger, “A Subnanosecond 64-b Adder”, ISSCC ‘ 96)


S. Naffziger, ISSCC’96

01212234 gttgtggH

11 iii HtC



01212234 gttgtggH



01212234 gttgtggH











)( 0711711111515161516 gttgtggpHpC








Ling Adder Critical Path


Ling Adder: Circuits

A0

B0

A1 B1A1

B1

A2

B2

A2 B2

CKG3

G4

CK

A3

B3P4

A2 B2

B3A3B1

A0 B0

A1

CK

CK

P

LCH LCL

C1H C0LC1L C0H

SumH

CK

K

G

SumL LCH LCL

C1H C0LC1L C0H

CK

P2

P1

G0

CKLC

G2G1


LCS4 – Critical G Path

4b

in1

G3

12b

P4(k,p) or (g,p) G4

C15

32b

C47 C15C31

S63 S48S62

16b


LCS4 – Logical Effort Delay

Prefix-4 Ling/Conditional-Sum (Dynamic - Long Carry Path)

Stages Branch LE ParasiticTotal

Branch Total LEPath Effort fo, opt

Effort Delay

(ps)

Parasitic Delay

(ps)

Total Delay

(ps)

Total Delay (FO4)

dg3# (dg3) 4.0 0.98 2.97g4 (NAND2) 2.0 1.11 1.84C15# (GG4) 1.0 1.01 1.80C15 (INV) 1.0 1.00 1.00C47# (LC) 3.0 1.03 3.32C47 (INV) 1.0 1.00 1.00C47#b (INV) 1.0 1.00 1.00C47b (INV) 1.0 1.00 1.00S63# (SUM) 16.0 0.86 1.36S63 (INV) 1.0 1.00 1.00

3.74E+023.84E+02 9.73E-01 7.2701.81 13666


Results:

• 0.5u Technology

• Speed: 0.930 nS

• Nominal process, 80C, V=3.3V

See: S. Naffziger, “A Subnanosecond 64-b Adder”, ISSCC ‘ 96

Prefix Addersand

Parallel Prefix Adders


from: Ercegovac-Lang


Prefix Adders

(g0, p0)

Following recurrence operation is defined:

(g, p)o(g’,p’)=(g+pg’, pp’)

such that:

Gi, Pi =

(gi, pi)o(Gi-1, Pi-1 )

i=0

1 ≤ i ≤ n

ci+1 = Gifor i=0, 1, ….. n

c1 = g0+ p0 cin (g-1, p-1)=(cin,cin)

This operation is associative, but not commutativeIt can also span a range of bits (overlapping and adjacent)


from: Ercegovac-Lang


Parallel Prefix Adders: variety of possibilitiesfrom: Ercegovac-Lang


Pyramid Adder:M. Lehman, “A Comparative Study of Propagation Speed-up Circuits in Binary Arithmetic

Units”, IFIP Congress, Munich, Germany, 1962.






Hybrid BK-KS Adder


Parallel Prefix Adders: S. Knowles 1999

operation is associative: h>i≥j≥k

operation is idempotent: h>i≥j≥k

produces carry: cin=0


Parallel Prefix Adders: Ladner-Fisher

Exploits associativity, but not idempotency. Produces minimal logical depth


Two wires at each level. Uniform, fan-in of two.Large fan-out (of 16; n/2); Large capacitive loading combined with the long wires (in the last stages)

Parallel Prefix Adders: Ladner-Fisher(16,8,4,2,1)


Parallel Prefix Adders: Kogge-Stone

Exploits idempotency to limit the fan-out to 1. Dramatic increase in wires. The wire span remains the same as in Ladner-Fisher.

Buffers needed in both cases: K-S, L-F


Kogge-Stone Adder


Parallel Prefix Adders: Brent-Kung

• Set the fan-out to one

• Avoids explosion of wires (as in K-S)

• Makes no sense in CMOS:– fan-out = 1 limit is arbitrary and extreme– much of the capacitive load is due to wire

(anyway)

• It is more efficient to insert buffers in L-F than to use B-K scheme


Brent-Kung Adder


Parallel Prefix Adders: Han-Carlson

• Is a hybrid synthesis of L-F and K-S

• Trades increase in logic depth for a reduction in fan-out:– effectively a higher-radix variant of K-S.– others do it similarly by serializing the prefix

computation at the higher fan-out nodes.

• Others, similarly trade the logical depth for reduction of fan-out and wire.


Parallel Prefix Adders: variety of possibilitiesfrom: Knowles

bounded by L-F and K-S at ends


Parallel Prefix Adders: variety of possibilitiesKnowles 1999

Following rules are used:

• Lateral wires at the jth level span 2j bits

• Lateral fan-out at jth level is power of 2 up to 2j

• Lateral fan-out at the jth level cannot exceed that a the (j+1)th level.



• The number of minimal depth graphs of this type is given in:

• at 4-bits there is only K-S and L-F, afterwards there are several new possibilities.


Parallel Prefix Adders: variety of possibilities

example of a new 32-bit adder [4,4,2,2,1]

Knowles 1999


Parallel Prefix Adders: variety of possibilities

Example of a new 32-bit adder [4,4,2,2,1]

Knowles 1999



• Delay is given in terms of FO4 inverter delay: w.c.(nominal case is 40-50% faster)

• K-S is the fastest• K-S adders are wire limited (requiring 80% more area)• The difference is less than 15% between examined schemes



Conclusion• Irregular, hybrid schmes

are possible• The speed-up of 15% is

achieved at the cost of large wiring, hence area and power

• Circuits close in speed to K-S are available at significantly lower wiring cost

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