1. A data processor comprising: a plurality of registers; and instruction execution circuitry having an associated instruction set, wherein said instruction set comprises an instruction specifying at least a mask operand, a register operand and an immediate value operand, and said instruction execution circuitry, based on an instance of said instruction, determines whether a condition code is set to a value indicated in said mask operand and sets a respective one of a plurality of registers specified by said register operand of said instance to a value of said immediate value operand based on said condition code being set to the value indicated in said mask operand, and sets said respective one of said plurality of registers specified by said register operand of said instance to zero based on said condition code not being set to the value indicated in said mask operand.