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.