Arithmetic logic unit with merged circuitry for comparison, minimum/maximum selection and saturation for signed and unsigned numbers

A microprocessor comprising:

• an input control circuit for receiving a first data word and generating a second data word;
• and
• an arithmetic logic unit (ALU) capable of performing a saturation operation and including a comparator circuit, the comparator circuit comprising:
• a min/max comparator for comparing the second data word and a third data word and generating a comparator output signal;
• and
• an output control circuit for providing either the second data word or the third data word as an output data word based on the comparator output signal and an output control signal;
• wherein the arithmetic logic unit (ALU) is capable of using the comparator circuit in operations other than the saturation operation,
• wherein the ALU is configured to perform unsigned saturation operations, wherein during unsigned saturation operations, the output control circuit provides the second data word as the output data word when the third data word is greater than the second data word, and provides the third data word as the output data word when the third data word is less than the second data.

A method for performing a saturation operation on an input word in the arithmetic logic unit (ALU) of a microprocessor such that circuit area requirements of the microprocessor can be minimized without substantially degrading the performance of the microprocessor, the ALU including a comparator circuit comprising a min/max comparator, the arithmetic logic unit (ALU) being capable of using the comparator circuit in operations other than the saturation operation, the method comprising:

• providing the input word to the comparator circuit;
• providing a saturation threshold value to the comparator circuit, wherein the saturation threshold value comprises an unsigned threshold value when the saturation operation comprises unsigned saturation, wherein the saturation threshold value comprises an upper signed threshold value when the saturation operation comprises signed saturation and the input word is positive, and wherein the saturation threshold value comprises the inverse of the upper signed threshold value when the saturation operation comprises signed saturation and the input word is negative;
• comparing the input word to the saturation threshold value using the min/max comparator;
• and
• replacing the input word with the saturation threshold value when the input word exceeds the saturation threshold value,
• wherein comparing the input word to the saturation threshold value using the min/max comparator comprises:
• inverting the most significant bit (MSB) of the input word before providing the input word to the min/max comparator during signed saturation;
• and
• inverting the MSB of the saturation threshold value before providing the saturation threshold value to the min/max comparator during signed saturation.

A method for performing a signed inverse limiting operation on a first input word in an arithmetic logic unit (ALU) of a microprocessor such that circuit area requirements of the microprocessor can be minimized without substantially degrading the performance of the microprocessor, the ALU including a general purpose comparator, the method comprising:

• setting a second input word equal to an upper threshold value when the first input word is positive;
• setting the second input word equal to the inverse of the upper threshold value when the first input word is negative;
• inverting the most significant bit (MSB) of the first input word to generate a third input word;
• inverting the MSB of the second input word to generate a fourth input word;
• providing the third input word and the fourth input word to the general-purpose comparator;
• replacing the first input word with the second input word when the first input word is positive and the general purpose comparator indicates that the third input word is not greater than the fourth input word;
• and
• replacing the first input word with the second input word when the first input word is negative and the general-purpose comparator indicates that the third input word is greater than the fourth input word.