Method and apparatus for performing carry look-ahead addition in a data processor

摘要

A carry look-ahead (CLA) adder accommodates a late carry-in from a low- order external 32-bit adder to enable a 96-bit addition to be performed in the same time in which the CLA adder (60) performs a 64-bit addition. Within each adder slice, intermediate group propagate and group generate terms are generated for each bit location in the adder (60), while the adder simultaneously generates an n-bit group propagate and group generate term. The intermediate group propagate and group generate terms are combined with carry-in terms to generate, in parallel, local carry- out terms within each adder slice. The local carry-out terms and intermediate group propagate and group generate terms are used to form a carry chain path which allows the adder to delay the carry-in of an external carry term.