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# Logic Circuit to Enhance the Function of an Arithmetic Logic Unit

IP.com Disclosure Number: IPCOM000034467D
Original Publication Date: 1989-Feb-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 2 page(s) / 37K

IBM

## Related People

Kocis, TJ: AUTHOR

## Abstract

Disclosed is a method to enhance the functionality of a common industry or VLSI macro Arithmetic Logic Unit (ALU) using simple external logic. Common industry and VLSI macro ALUs are designed with a specific set of functions. A designer may want to enhance this function set to increase the overall processor function or performance. This may be accomplished by inserting logic before the ALU as shown in the figure. In this way, the designer may conditionally modify the operation ALU dependent on the "New Carry In" or the "Conditional Inputs". For example, consider an ALU whose set of functions include both "Outputs = A Inputs" and "Outputs = the 2's complement of the A Inputs".

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Logic Circuit to Enhance the Function of an Arithmetic Logic Unit

Disclosed is a method to enhance the functionality of a common industry or VLSI macro Arithmetic Logic Unit (ALU) using simple external logic. Common industry and VLSI macro ALUs are designed with a specific set of functions. A designer may want to enhance this function set to increase the overall processor function or performance. This may be accomplished by inserting logic before the ALU as shown in the figure. In this way, the designer may conditionally modify the operation ALU dependent on the "New Carry In" or the "Conditional Inputs". For example, consider an ALU whose set of functions include both "Outputs = A Inputs" and "Outputs = the 2's complement of the A Inputs". For a specific value of "New Control Inputs", the designer wants the ALU to perform the function "Outputs = A Inputs", if the most significant bit (msb) of the "A Inputs" is a logical zero, or "Outputs = the 2's complement of the A Inputs" if the msb of the "A Inputs" is a logical one. To accomplish this, the msb of the "A Inputs" becomes the "Conditional Inputs". The Enhancement Logic is then designed to generate the "ALU Carry in" and "ALU Control" inputs to perform the desired operation based on the specific "New control Inputs" and the msb of the "A Inputs".

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