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CAPTURE AND USE OF DON'T CARES IN COMPUTER DESIGN LANGUAGES

IP.com Disclosure Number: IPCOM000039610D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Brayton, RK: AUTHOR

Abstract

Described are several methods of defining and using don't care information during the processing of digital computer description languages. The methods provide a means of reducing the number of logic gates required to implement both control and data-flow logic. Don't cares as used in digital computer architectural design can be used by a designer to reduce the amount of logic gate hardware required to implement a digital system. This is done by selectively assigning the don't care points, in either the on-set or off-set of a logic function, and combining them with existing computer algorithms. Typically, hardware description specifications do not allow the designer to specify a don't care set and to make use of it during logic design.

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CAPTURE AND USE OF DON'T CARES IN COMPUTER DESIGN LANGUAGES

Described are several methods of defining and using don't care information during the processing of digital computer description languages. The methods provide a means of reducing the number of logic gates required to implement both control and data-flow logic. Don't cares as used in digital computer architectural design can be used by a designer to reduce the amount of logic gate hardware required to implement a digital system. This is done by selectively assigning the don't care points, in either the on-set or off-set of a logic function, and combining them with existing computer algorithms. Typically, hardware description specifications do not allow the designer to specify a don't care set and to make use of it during logic design. If the designer wants to make use of don't care information, the assignment must be made for each don't care point to the on-set or off-set when writing the hardware specification. However, the use of don't care information to minimize logic is generally considered to be inferior to available computer programs. Therefore, the concept described herein outlines the advantages of using don't care points as a means of reducing overall design logic. Don't cares occur in logic design in a number of ways: 1) Operational codes typically do not occupy all the points in the n-dimensional Boolean space of the n logic variables used to encode the operations. The code points not used are illegal or are code points which will never be implemented in a properly operating computer. These illegal code points can be used as don't care points for most of the control signals if there is another signal which will interrupt operation when an illegal operation occurs. 2) A digital system typically has many pieces of hardware which do different operations, such as a microprocessor which may have facilities, such as an arithmetic logic unit (ALU) and a rotator, with an instruction to direct the use of one or the other. If either one of the facilities are not being used during an operation, a don't care situation exists, since the user does not care what computation the microprocessor does as the result is not used. Therefore, that particular operation is a don't care point for that facility. 3) In general, any signal which takes over and suppresses the re...