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Group Carry Generator

IP.com Disclosure Number: IPCOM000096479D
Original Publication Date: 1963-Jun-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 60K

Publishing Venue

IBM

Related People

Schmookler, MS: AUTHOR [+2]

Abstract

Fast addition is realized using only one type of element. Fast addition of binary numbers is achieved by the use of a carry speed-up technique known as carry look-ahead. Such is described, e. g., in a paper by O. L. McSorley entitled "High-Speed Arithmetic a in Binary Computers" published in the January 1961 Proceedings of the IRE on pages 68-70. Large binary adders, of the type described, are broken up into a number of groups, carry look-ahead circuits generating both inter-group carries, called group-carries, and carries within the groups, called bit-carries.

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Group Carry Generator

Fast addition is realized using only one type of element. Fast addition of binary numbers is achieved by the use of a carry speed-up technique known as carry look-ahead. Such is described, e. g., in a paper by O. L. McSorley entitled "High-Speed Arithmetic a in Binary Computers" published in the January 1961 Proceedings of the IRE on pages 68-70. Large binary adders, of the type described, are broken up into a number of groups, carry look-ahead circuits generating both inter-group carries, called group-carries, and carries within the groups, called bit-carries.

This group-carry generator uses And-Or-Invert logic. Each standard And-Or- Invert circuit can, depending upon which inputs are used, perform And-Invert (AE), Or-Invert (OI) or And (&) and Or-Invert (together designated AOI) operations. For example, the output or an AI circuit is 0, when both its inputs are 1's.

Traditional group propagate and group generate functions, used in look- ahead adders, are not particularly amenable to the use of And-Or-Invert circuits. Therefore, these traditional functions are replaced by two new group functions UG and VG. These are derived from the bit-propagate P and bit-generate G functions. The functions UG and VG for each group are then used to generate a group carry CG for each group.

Bit function logic, comprising AI and OI circuits for each pair or incoming bits A and B (in complement form), is used to supply the generate G and propagate P correspon...