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One Step Carry Adder

IP.com Disclosure Number: IPCOM000008303D
Publication Date: 2002-Jun-04
Document File: 4 page(s) / 69K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method that performs parallel additions with adder tables, then uses results from the carry table to determine the correct overall results. Benefits include faster look up capability.

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One Step Carry Adder

Disclosed is a method that performs parallel additions with adder tables, then uses results from the carry table to determine the correct overall results. Benefits include faster look up capability.

General Description

The adder breaks the operands into n bit blocks.  Each pair of blocks is sent to an adder table as a 2n bit address. The table returns 2 (n+1) bit results, one with a carry of 0 and one with a carry of 1. The first n bits of each sum are placed behind a selection line. The last bit, n+1, from each of the sums (except the last pair) is placed on part of the address line of the carry table (see Figures 1 and 2).

The carry table has an address line of 2 bits for every block minus one.  The carry table address is as follows: 

§         The first bit is the carry in to the first block. This carry in is used primarily to support 2’s  complement subtraction; instead of negating the number before addition, it is inverted and the adder adds one as it performs the arithmetic.

§         The second bit is the carry in if the first bit is a zero.

§         The third bit is the carry in if the first bit is a one.

§         The fourth or fifth is the carry value, depending on whether the second or third bit was selected.

The address maps to the actual carries, one bit for each block. The carries are then used as a control to the selection lines, so that for each block either the carry in 0 or 1 sum is used. The carry out of the total answer is the last bit of one of the last sums (i.e. the n+1 bits that weren’t sent to the carry table).

The preferred arrangement of a 32 bit adder—which selects table sizes so that the overall size is red...