Roland Lezuo
ABSTRACT
We present the CASM language, an abstract state machine (ASM) based modeling language originally designed for verifying compiler backends. We demonstrate the expressiveness by describing an instruction set simulator (ISS) for MIPS in approximately 700 lines of code. Further we present a refinement of the models to cycle-accurately describe two implementations of the classic 5-stage MIPS pipeline. Utilizing symbolic execution allows us to prove semantic equivalence of the pipeline implementations and the instruction set description. Finally we compile the models to C++ and provide a small runtime to create a system simulator achieving a performance of approx. 1 MHz in MiBench and SPECInt2000 benchmarks.
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Index Terms
Using the CASM language for simulator synthesis and model verification
Reviews
Edel M Sherratt
CASM is a modeling language based on the abstract state machine formalism introduced by Yuri Gurevich under the name “evolving algebras” in the early 1990s and developed by Blass, Gurevich, Börger, and many others. An abstract state machine has a signature, like the signature of an abstract data type. The signature defines a set of variables and functions, and these form the basis from which terms are recursively built up. Each term in an abstract state machine defines a location, and each abstract state associates values from an underlying base set with locations. An abstract state machine has a rule or program, which, given a state, generates a set of changes to be made to the association between locations and values. This set of changes is called an update set, and a state transition occurs when the updates in the update set are applied. An abstract state machine applies its rule repeatedly until an empty or inconsistent set of updates is obtained. This paper presents an application of CASM to model the MIPS pipeline, a five-stage architecture that allows fetch, decode, access memory, and writeback operations to overlap in time. A further model adds no-operation “bubbles” to be inserted into the pipeline. The CASM models were verified by proving the equivalence of pipelined and simpler, high-level models. They were then compiled to C++ to form simulators whose performance was measured using the MiBench and SpecInt2000 benchmarks. Overall, the paper provides a useful overview of abstract state machines and an introduction to the modeling and simulation of pipelined architecture. Online Computing Reviews Service
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