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CPE 626 CPU Resources: Adders & Multipliers Aleksandar Milenkovic E-mail: milenka@ece.uah.edumilenka@ece.uah.edu Web:http://www.ece.uah.edu/~milenkahttp://www.ece.uah.edu/~milenka

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2 Outline Full Adder Ripple Carry Adder Carry-Look-Ahead Adder Manchester Adders Carry Select Adder Carry Skip Adder Conditional Sum Adder Hybrid Designs

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3 Full Adder Inputs data inputs A, B carry in C in Outputs sum S carry out C out

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4 Full Adder

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6 Transmission-Gate Adder (1) A = 1 => -A = 0 => TG is open => out = -B A = 0 => -A = 1 => TG is closed => out = B A = 1 => -A = 0 => TG is closed => out = B A = 0 => -A = 1 => TG is open => out = -B TG XOR TG XNOR

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7 Transmission-Gate Adder (2)

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8 Ripple Carry Adder - RCA (1) Method 1 G[i] = A[i]*B[i] P[i] = A[i] B[i] C[i] = G[i] + P[i]*C[i-1] S[i] = P[i] C[i-1] Method 2 G[i] = A[i]*B[i] P[i] = A[i] + B[i] C[i] = G[i] + P[i]*C[i-1] S[i] = A[i] B[i] C[i-1]

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9 Ripple Carry Adder - RCA (2) Replace AND-OR pair with fast 2-inputs NAND gates RCA delay is proportional to n and is limited by the propagation of the carry signal through all of the stages

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10 Ripple Carry Adder - RCA (3) Used in odd stages! Used in even stages!

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11 Ripple Carry Adder - RCA (4) Carry equations C[i+1] = A[i]*B[i] + P[i]*C[i] or C[i+1] = (A[i] + B[i])*(P[i]’ + C[i]) P[i]’ = NOT(P[i]) Even stages C1[i+1]’ = P[i]*C3[i]*C4[i] C2[i+1] = A[i] + B[i] C[i+1] = C1[i]*C2[i] Odd stages C3[i+1]’ = P[i]*C1[i]*C2[i] C4[i+1]’ = A[i]*B[i] C[i+1] = C3[i]’ + C4[i]’ Inputs to stage zero: C3[0] = C4[0] = ‘0’

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12 Carry-Look-Ahead Adder – CLA (1) Idea: speed up carry computation – C i+1 = G i + P i* C i Propagate: P i = A i + B i if P i = 1, then carry from (i-1)th stage is propagated Generate: G i = A i *B i if G i = 1 there is carry out

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13 Carry-Look-Ahead Adder – CLA (2)

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14 Carry-Look-Ahead Adder – CLA (3) Domino implementation (Dynamic Carry Gates)

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15 Carry-Look-Ahead Adder – CLA (4)

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16 Carry-Look-Ahead Adder – CLA (5)

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17 Brent-Kung CLA a) lookahead terms b) CLG cell c) cells can be rearranged into tree d) simplified representations for part a) e) simplified representation for part c) f) lookahead logic for 8-bit adder g) Brent-Kung adder Reduces delay, increases the regularity, reduces the number of unnecessary switching events (power)

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18 Manchester Adder Circuits (1)

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19 Manchester Adder Circuits (2) Dynamic Stage Static Stage MUX stage

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20 4-bit Manchester Adder

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21 Carry Bypass

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22 Carry Select Adder (1) Compute 2 versions of the addition with different carry-ins, one assuming that the carry-in is 0 and another assuming that it is 1

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23 Carry Select Adder (2)

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24 Carry Skip Adder: Motivation Computing P 3-0 is much simpler than computing G 0-3 Let’s compute only P 3-0 !

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25 Carry Skip Adder Carries begin rippling simultaneously through each block; If any block generates a carry, then the carry out will be true, even the carry-in may not be not true yet. If at the start of each add operation the carry-in to each block is 0, then correct carry-outs will be generated – carry-out can be thought of as if it is the G signal Practical only if the carry-in signals can be easily cleared at the start of each operation – e.g. precharging CMOS

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26 Carry Skip Adder: Analysis Assume it takes 1 time unit for signal to propagate through two logic level n bits wide adder blocks of size k It will take k units for a carry to ripple through a block of size k Critical path k units for the first block n/k – 2 units to skip the blocks k units to ripple through the last block Increase the efficiency by varying the blocks size 20 bits (4, 4, 4, 4, 4,): Delay = 4 + 3 + 4 = 11 20 bits (2, 5, 6, 5, 2): Delay = 9

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27 Conditional Sum Adder (1)

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28 Conditional Sum Adder (2)

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29 Conditional Sum Adder (3) A00101101 B10110110 0Si 0 100110110 Ci 0 00100100 Si 1 011001001 Ci 1 10111111 1Si 0 100100110 Ci 0 0110 Si 1 111001001 Ci 1 0111 2Si 0 110100110 Ci 0 01 Si 1 111001001 Ci 1 011 3S0S0 111000110 0 S1S1 111001001 0

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30 Hybrid Designs: An Example Combine CLA (Carry Look-Ahead) with RCA

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Digital Integrated Circuits© Prentice Hall 1995 Combinational Logic COMBINATIONAL LOGIC.

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