Rectangular array signed digit multiplier

An arithmetic circuit, comprising:

• a first register for storing a first operand;
• a second register for storing a second operand;
• a multiplier circuit having first and second inputs respectively coupled to said first and second registers, said multiplier circuit operable to generate and output a result comprising a product of said first and second operands in signed digit format, said product always being exact;
• a data path coupling said output of said multiplier to said second input of said multiplier;
• a multiplexer circuit disposed in said data path, said multiplexer having a first input coupled to said second register, a second input coupled to said output of said multiplier circuit and an output coupled to said second input of said multiplier circuit,
• wherein said multiplexer circuit is operable to select one of said signed digit result and said second operand to be input into said multiplier circuit, and said multiplier circuit is operable to use said signed digit result as an operand of a multiplication operation without converting said signed digit result to a non-redundant format;
• and
• conversion circuitry coupled to said multiplier circuit for extracting a plurality of the most significant bits from a signed digit value output by said multiplier circuit, said conversion circuitry further operable to selectively change the value of an overflow bit associated with said signed digit value responsive to the sign of the most significant digit of the signed digit value not extracted such that the accuracy of the value remaining in said signed digit value is not affected by said extraction of said plurality of most significant bits.

A method of processing data in an arithmetic circuit comprising the steps of:

• storing a first operand in a first register;
• storing a second operand in a second register;
• multiplying the first operand by the second operand in a rectangular aspect ratio multiplier circuit to generate a result in signed digit format comprising the product of the first and second operands, the product always being exact;
• adding a third operand to the product of the first and second operands simultaneously with said step of multiplying such that the result comprises a sum of the product and the third operand;
• shifting the result left a number of bit positions substantially equal to the number of bit positions of said first operand;
• and
• using the result in place of the second operand in a subsequent pass through the multiplier circuit without converting the result to non-redundant format prior to the subsequent pass.