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# Efficient Method of Creating Greater-Than Or Less-Than Functions

IP.com Disclosure Number: IPCOM000099664D
Original Publication Date: 1990-Feb-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 80K

IBM

## Related People

Butler, ND: AUTHOR [+3]

## Abstract

Described is a method of using Ripple Carry Generators (RCGs) in place of conventional combinations of logic to reduce the hardware required for greater-than and less-than functions.

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Efficient Method of Creating Greater-Than Or Less-Than Functions

Described is a method of using Ripple Carry Generators
(RCGs) in place of conventional combinations of logic to reduce the
hardware required for greater-than and less-than functions.

The normal method of creating the greater-than or the less-than
function is to use a large amount of combinatorial logic which can
give a result very quickly, but uses a large number of cells.  An
alternative is to use a 'Ripple Carry Generator'.  Fig. 1 shows the
function of a Ripple Carry Generator.

Many cell libraries for Gate Array or Standard Cell VLSI
devices provide Ripple Carry Generator macros.  These are usually
much more cell efficient than the equivalent fast combinatorial
circuit.  A Ripple Carry Generator is equivalent in function to an
AND-OR gate with three two-input AND gates into a three-input OR gate
connected, as in Fig. 2.  The AND-OR gate usually uses more cells
than the Ripple Carry Generator and, obviously, has more connections
and wiring associated with it.  Using AND-OR gates as a replacement
for Ripple Carry Generators would still yield a lower cell count than
the fast combinatorial method.

Fig. 3 (also refer to Tables 2 and 3) shows Ripple Carry
Generator blocks being used to compare two numbers A and B, with the
carry input at 0 and at 1.  The carry input can be used to select
the type of comparison as shown.  In this example A and B are 2 bit
number...