1. A machine readable medium having stored thereon a plurality of instructions, said plurality of instructions including packed floating point instructions, followed by a single transition instruction, followed by scalar floating point instructions, said plurality of instructions, when executed by a processor, causing said processor to, execute said packed floating point instructions and said scalar floating point instructions on the contents of a shared architecturally visible register file, wherein said shared architecturally visible register file is operated as a flat register file while executing said packed floating point instructions, wherein said shared architecturally visible register file is operated as a stack while executing said scalar floating point instructions; and alter data to indicate the stack is empty responsive to executing said single transition instruction between execution of said packed floating point instructions and said scalar floating point instructions.