The design of the Inferno virtual machine

Virtual machines are an important component of modern portable environments such as Inferno and Java because they provide an architecture-independent representation of executable code. Their performance is critical to the success of such environments, but they are difficult to design well because they are subject to conflicting goals. On the one hand, they offer a way to hide the differences between instruction architectures; on the other, they must be implemented efficiently on a variety of underlying machines. A comparison of the engineering and evolution of the Inferno and Java virtual machines provides insight into the tradeoffs in their design and implementation. We argue that the design of virtual machines should be rooted in the nature of modern processors, not language interpreters, with an eye towards on-the-fly compilation rather than interpretation or specialpurpose silicon. Dis, the Inferno Virtual Machine In early 1995, we set out to apply the ideas of the Plan 9 operating system [1] to a wider range of devices and networks. The resulting system, Inferno [2], is a small operating system and execution environment that supports application portability across a wide variety of processors and operating systems. Unaware of the contemporary work to establish Java [3] from the technology of the Oak project, we independently concluded that a virtual machine (VM) was a necessary component of such a system [4]. Because of improvements in processor speed and the feasibility of on-the-fly compilers, a VM can execute quickly enough to be economically viable. The Inferno virtual machine, called Dis, has several unusual aspects to its design: the instruction set, the module system, and the garbage collector. The Dis instruction set provides a close match to the architecture of existing processors. Instructions are of the form