1. An all-digital grade calculating circuit for use in a fuzzy inference system, comprising: a) input means for receiving a digital data input value representing an unfuzzified input; b) a first memory having a plurality of storage locations for storing a plurality of digital membership functions, each of the plurality of storage locations contains a one of the plurality of digital membership functions, each digital membership function comprises: i) a first bit field representing a bending point of the digital membership function; ii) a second bit field representing a multiplication rule of the digital membership function; and iii) a third bit field representing a type code of the digital membership function; c) first accessing means for accessing the first memory to retrieve a one of the plurality of digital membership functions; d) digital subtraction circuit means having a first input coupled to receive the digital data input value from the input means and a second input coupled to receive the first bit field of the one of the plurality of digital membership functions retrieved from the first accessing means, the digital subtraction circuit means outputting a digital distance value output that is equal to the difference between data values presented to the first and second inputs of the subtraction circuit means; e) first digital shifter circuit means having a first input coupled to receive the digital distance value from the digital subtraction circuit means and having a second input coupled to receive the second bit field of the one of the plurality of digital membership functions retrieved from the first accessing means, the first digital shifter circuit means outputting a multiplied digital distance value output that is equal to the digital distance value shifted by an amount specified by the second bit field of the retrieved digital membership function; and f) selection means coupled to receive the multiplied digital distance value and having an input for receiving the third bit field of the one of the plurality of digital membership functions retrieved from the first accessing means, the selection means selecting a, digital grade output that is equal to one of the multiplied digital distance value, a predetermined maximum digital grade and a predetermined minimum digital grade under control of the third bit field.