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# Increment Decrement Logic

IP.com Disclosure Number: IPCOM000092046D
Original Publication Date: 1968-Aug-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 47K

IBM

## Related People

Elliott, JE: AUTHOR

## Abstract

The logic shown can be used to increment or decrement a binary number by one. The binary number is applied to the left side of the logic. The incremented or decremented number appears at the right side of the logic. The two Exclusive-Or's at the bottom of the drawing form part of an odd-parity bit generator.

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Increment Decrement Logic

The logic shown can be used to increment or decrement a binary number by one. The binary number is applied to the left side of the logic. The incremented or decremented number appears at the right side of the logic. The two Exclusive-Or's at the bottom of the drawing form part of an odd-parity bit generator.

The increment-decrement logic operation is based upon the following changes in a binary number when it is incremented or decremented by one. Whether incrementing or decrementing, the lowest order bit is always changed to its opposite state. When incrementing, the lowest order bit having a 0 state, and all lower order bits, must be changed to their opposite state. When decrementing, the lowest order bit in the 1 state, and all lower order bits, must be changed to their opposite state.

The increment-decrement logic is divided into five portions according to function. First, Exclusive-Or 10 operates to change the state of the lowest order bit, whether the binary number is being incremented or decremented. Second, Exclusive-Or's 21, 22, and 23 operate to compare a bit with the next lower order bit. If adjacent bits are in the same state, their Exclusive-Or has an output. Third, Exclusive-Or 20 compares the lowest order bit with the decrement signal. If the lowest order bit is in the 1 state, Exclusive-Or 20 has an output only if there is no decrement signal, i.e., there is an increment signal. If the lowest order bit is a 0 Exclusive-Or...