CIS 351

Advanced Adders

Winter 2022

Remember: Designs like the carry-select and carry-lookahead adders are not "all-or-nothing". For example, you can place carry-lookahead logic on only some of the full adders, thereby limiting the size in exchange for time. (See Figure 1.) Similarly, you can apply the carry-select pattern one time only. (See Figure 2.)

Figure 1: Adder using only one carry lookahead input
Figure 2: Carry-Select applied at one level only

Suppose you begin with a 16-bit ripple-carry adder. You want to make it faster, but can't afford a "full" carry-lookahead or carry-select adder, so, you decide to do "one step" only.

  1. Which design above will be faster (Figure 1 or Figure 2)?
  2. Which design above will be smaller (Figure 1 or Figure 2)?

Updated Thursday, 6 January 2022, 3:47 PM

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