Re: Carry Select Adder
Posted by cadet1620 on Dec 27, 2012; 12:58am
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Carry-Select-Adder-tp4025925p4025928.html
Another interesting adder is the Carry Bypass Adder. This is a combination of ripple carry and carry look-ahead adder. The 8-bit version would use two 4-bit ripple carry adders two 4-bit look-ahead generators that only need to supply the P (propagate) signal. Based on the P signal, a multiplexer selects whether to select the carry in or the carry out of the 4-bit adder to pass on to the next adder. Less hardware, but slower than a full Carry Look-ahead Adder.
Multiple levels of carry bypass can be used in longer adders.
--Mark
[Edited to correct schematics.]
URL: http://nand2tetris-questions-and-answers-forum.52.s1.nabble.com/Carry-Select-Adder-tp4025925p4025928.html
Another interesting adder is the Carry Bypass Adder. This is a combination of ripple carry and carry look-ahead adder. The 8-bit version would use two 4-bit ripple carry adders two 4-bit look-ahead generators that only need to supply the P (propagate) signal. Based on the P signal, a multiplexer selects whether to select the carry in or the carry out of the 4-bit adder to pass on to the next adder. Less hardware, but slower than a full Carry Look-ahead Adder.
Multiple levels of carry bypass can be used in longer adders.
--Mark
[Edited to correct schematics.]
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