Apparatus and method for subtracting significand values of floating-point operands

A method of operating an apparatus to subtract a first significand value of a first floating-point operand and a second significand value of a second floating-point operand, comprising:

• asserting a shift signal when a difference is detected between at least one low order bit of a first exponent value of the first floating-point operand and a corresponding at least one low order bit of a second exponent value of the second floating-point operand;
• employing first processing circuitry to produce a first difference value by performing a first subtraction operation to subtract the second significand value from the first significand value when the shift signal is unasserted, and to subtract a right-shifted version of the second significand value from the first significand value when the shift signal is asserted;
• employing second processing circuitry to produce a second difference value by performing a second subtraction operation to subtract the first significand value from the second significand value when the shift signal is unasserted, and to subtract a right-shifted version of the first significand value from the second significand value when the shift signal is asserted;
• determining, from the significand values subjected to the first subtraction operation, a first estimated left shift amount;
• determining, from the significand values subjected to the second subtraction operation, a second estimated left shift amount;
• and
• producing, as a shifted difference value, the first difference value left shifted by the first estimated left shift amount when the first difference value is a non-negative value, and the second difference value left shifted by the second estimated left shift amount when the second difference value is a non-negative value.

An apparatus comprising:

• input means for providing a first significand value of a first floating-point operand and a second significand value of a second floating-point operand;
• significand shift control means for asserting a shift signal when a difference is detected between at least one low order bit of a first exponent value of the first floating-point operand and a corresponding at least one low order bit of a second exponent value of the second floating-point operand;
• first processing means for producing a first difference value by performing a first subtraction operation to subtract the second significand value from the first significand value when the shift signal is unasserted, and to subtract a right-shifted version of the second significand value from the first significand value when the shift signal is asserted;
• second processing means for producing a second difference value by performing a second subtraction operation to subtract the first significand value from the second significand value when the shift signal is unasserted, and to subtract a right-shifted version of the first significand value from the second significand value when the shift signal is asserted;
• first shift estimation means for determining, from the significand values subjected to the first subtraction operation, a first estimated left shift amount;
• second shift estimation means for determining, from the significand values subjected to the second subtraction operation, a second estimated left shift amount;
• and
• shifted difference value generation means for producing, as a shifted difference value, the first difference value left shifted by the first estimated left shift amount when the first difference value is a non-negative value, and the second difference value left shifted by the second estimated left shift amount when the second difference value is a non-negative value.