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117 lines
1.2 KiB
117 lines
1.2 KiB
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Exercise 3.30: Figure 3.27 shows a
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ripple-carry adder formed by stringing together
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n
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full-adders.
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This is the simplest form of parallel adder for adding two
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n
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-bit binary
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numbers. The inputs
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A
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1
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,
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A
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2
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,
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A
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3
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, …,
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A
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n
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and
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B
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1
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,
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B
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2
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,
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B
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3
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,
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…,
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B
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n
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are the two binary numbers to be added (each
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A
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k
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and
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B
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k
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is a 0 or a 1). The circuit generates
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S
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1
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,
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S
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2
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,
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S
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3
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, …,
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S
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n
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,
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the
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n
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bits of the sum, and
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C
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, the carry from the addition. Write a
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procedure ripple-carry-adder that generates this circuit. The procedure
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should take as arguments three lists of
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n
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wires each—the
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A
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k
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, the
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B
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k
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, and the
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S
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k
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—and also another wire
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C
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. The major drawback of the
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ripple-carry adder is the need to wait for the carry signals to propagate.
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What is the delay needed to obtain the complete output from an
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n
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-bit
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ripple-carry adder, expressed in terms of the delays for and-gates, or-gates,
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and inverters?
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