A method and system are disclosed for processing instructions within a data processing system including a processor having a plurality of execution units. According to the method of the present invention, a number of instructions stored within a memory within the data processing system are retrieved from memory. A selected instruction among the number of instructions is decoded to determine if the selected instruction would be noneffective if executed by the processor. In a preferred embodiment of the present invention, noneffective instructions include instructions with invalid opcodes and instructions that would not change the value of any data register within the processor. In response to determining that the selected instruction would be noneffective if executed by the processor, the selected instruction is recoded into a specified instruction format prior to dispatching the selected instruction to one of the number of execution units. Detecting noneffective instructions prior to dispatch reduces the decode logic required within the dispatcher and enhances processor performance.