Selective state retention design using symbolic simulation

Addressing both standby and active power is a major challenge in developing system-on-chip designs for battery-powered products. Powering off sections of logic or memories loses internal register and RAM states so designers have to weigh up the benefits and costs of implementing state retention on some or all of the power gated subsystems where state recovery has significant real-time or energy cost, compared to resetting the subsystem and re-acquiring state from scratch. Library IP and EDA tools can support state retention in hardware synthesized from standard RTL, but due to the silicon area costs there is strong interest in only retaining certain selective state for example the ldquoarchitectural staterdquo of a CPU to implement sleep modes. Currently there is no known rigourous technique for checking the integrity of selective state retention, and this is due to the complexity of checking that the correctness of the design is not compromised in any way. The complexity is exacerbated due to the interaction between the retained and the non-retained state, and exhaustive simulation rapidly becomes infeasible. This paper presents a case study based on symbolic simulation for assisting the designers to design and implement selective retention correctly. The main finding of our study is that the programmer visible state or the architectural state of the CPU needs to be implemented using retention registers whilst other micro-architectural enhancements such as pipeline registers, TLBs and caches can be implemented using normal registers without retention. This has a profound impact on power and area savings for chip design. By selectively retaining the state of the programmer's ldquoarchitecturalrdquo model and not the increasing proportion of extra state, one can incorporate energy-efficient sleep modes. To the best of our knowledge this is the first study in the area of rigourous design and implementation of selective state retention.