1. A computer system comprising in combination: a combination shift-rotate/merge functional unit having input operands comprised of a left half and right half parts, an instruction decode and issue means for decoding and issuing instructions to the functional unit, said functional unit comprising means responsive to said instructions to carry out a shift function to shift the bits of one or more of said operands to more or less significant bit positions, and to carry out a merge function to merge said operands, a status mask generator responsive to said instructions to generate a status mask, and a status generator responsive to said status mask and said operands to predict the status of the result to be produced by said functional unit in response to said instructions, said status including a status condition which depends upon said shift function and a status condition which depends upon said merge function.