Introduction Today's virtual machines use a layer of software that allows programs compiled in one instruction set to be executed on a processor executing a (different) native instruction set. Virtual machines have become popular in recent years for providing platform independence; however, virtual machines also open many new opportunities for enhancing performance. The co-design of virtual machine software and the underlying hardware microarchitecture will enable enhanced instruction level parallelism and more adaptable performance mechanisms than are possible when hardware and application software are separated by instruction set architectures as is traditionally done. In future high performance computers, a virtual instruction set architecture (V-ISA) will be the level for maintaining architectural compatibility. The V-ISA will be implemented with a virtual machine that blends software and hardware in a symbiotic manner via co-design. The hardware will support an implementationdep