Rounding mechanisms in processors

Abstract

An arithmetic unit, for example a multiply and accumulate (MAC) unitfor a processing engine includes a partial product reduction treeThe partial product reduction tree will generate carry results and provides a final output to a final adderconnected to the partial production reduction tree. Unbiased rounding logicis provided. A carry propagation tree is responsive to the carry results for anticipating a zero on each of N least significant bits of the final adder. When zero is anticipated on each of N least significant bits of the final adder, the carry propagation tree is operable to generate an output signalwhich is used by the unbiased rounding stage to force the (N+1)th least significant bit of the final adder to zero. Through the use of a carry propagation tree to predict, or anticipate zeros on the N least significant bits, unbiased rounding can be effected without a time penalty in that a carry propagation tree can be configured to be at least a rapid as the carry propagation of the final adder. Where a zero anticipation function is provided, this can also be mapped onto the carry propagation tree, thus providing an efficient hardware implementation through sharing of that hardware between functions.