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Chapter 2: Custom single-purpose processors - page 8 / 29

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Embedded Systems Design: A Unified Hardware/Software Introduction, (c) 2000 Vahid/Givargis

Combinational components

With enable input e all O’s are 0 if e=0

With carry-in input Ci

sum = A + B + Ci

May have status outputs carry, zero, etc.

O =

I0 if S=0..00

I1 if S=0..01

I(m-1) if S=1..11

O0 =1 if I=0..00

O1 =1 if I=0..01

O(n-1) =1 if I=1..11

sum = A+B

  (first n bits)

carry = (n+1)’th

  bit of A+B

less    = 1 if A<B

equal  =1 if A=B

greater=1 if A>B

O = A  op  B

op determined

by S.

n-bit, m x 1

Multiplexor

O

S0

S(log m)

n

n

I(m-1)

I1

I0

log n x n

Decoder

O1

O0

O(n-1)

I0

I(log n  -1)

n-bit

Adder

n

A

B

n

sum

carry

n-bit

Comparator

n

n

A

B

less

equal

greater

n bit,

m function

ALU

n

n

A

B

S0

S(log m)

n

O

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