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COUnit6.pdf


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COMPUTER ORGANIZATION

10CS46

FIG-2: 4 - Bit parallel Adder.
6.2 DESIGN OF FAST ADDERS:
In an n-bit parallel adder (ripple-carry adder), there is too much delay in
developing the outputs, so through sn-1 and cn. On many occasions this delay is not
acceptable; in comparison with the speed of other processor components and speed of the
data transfer between registers and cache memories. The delay through a network
depends on the integrated circuit technology used in fabricating the network and on the
number of gates in the paths from inputs to outputs (propagation delay). The delay
through any combinational logic network constructed from gates in a particular
technology is determined by adding up the number of logic-gate delays along the longest
signal propagation path through the network. In the case of the n-bit ripple-carry adder,
the longest path is from inputs x0, y0, and c0 at the least-significant-bit (LSB) position to
outputs cn and sn-1 at the most-significant-bit (MSB) position.
Using the logic implementation indicated in Figure-1, cn-1 is available in 2(n—1)
gate delays, and sn-1 is one XOR gate delay later. The final carry-out, cn is available after
2n gate delays. Therefore, if a ripple-carry adder is used to implement the
addition/subtraction unit shown in Figure-3, all sum bits are available in 2n gate delays,
including the delay through the XOR gates on the Y input. Using the implementation
cn



cn-1 for overflow, this indicator is available after 2n+2 gate delays. In summary, in a
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