Design Of Energy Efficient Application Specific Instruction Set Processors

After a brief introduction to low-power VLSI design, the design space of ASIP instruction set architectures (ISAs) is introduced with a special focus on important features for digital signal processing. Based on the degrees of freedom offered by this design space, a consistent ASIP design flow is proposed: this design flow starts with a given application and uses incremental optimization of the ASIP hardware, of ASIP coprocessors and of the ASIP software by using a top-down approach and by applying application-specific modifications on all levels of design hierarchy. A broad range of real-world signal processing applications serves as vehicle to illustrate each design decision and provides a hands-on approach to ASIP design. Finally, two complete case studies demonstrate the feasibility and the efficiency of the proposed methodology and quantitatively evaluate the benefits of ASIPs in an industrial context.

Modern consumers carry many electronic devices, like a mobile phone, digital camera, GPS, PDA and an MP3 player. The functionality of each of these devices has gone through an important evolution over recent years, with a steep increase in both the number of features as in the quality of the services that they provide. However, providing the required compute power to support (an uncompromised combination of) all this functionality is highly non-trivial. Designing processors that meet the demanding requirements of future mobile devices requires the optimization of the embedded system in general and of the embedded processors in particular, as they should strike the correct balance between flexibility, energy efficiency and performance. In general, a designer will try to minimize the energy consumption (as far as needed) for a given performance, with a sufficient flexibility. However, achieving this goal is already complex when looking at the processor in isolation, but, in reality, the processor is a single component in a more complex system. In order to design such complex system successfully, critical decisions during the design of each individual component should take into account effect on the other parts, with a clear goal to move to a global Pareto optimum in the complete multi-dimensional exploration space. In the complex, global design of battery-operated embedded systems, the focus of Ultra-Low Energy Domain-Specific Instruction-Set Processors is on the energy-aware architecture exploration of domain-specific instruction-set processors and the co-optimization of the datapath architecture, foreground memory, and instruction memory organisation with a link to the required mapping techniques or compiler steps at the early stages of the design. By performing an extensive energy breakdown experiment for a complete embedded platform, both energy and performance bottlenecks have been identified, together with the important relations between the different components. Based on this knowledge, architecture extensions are proposed for all the bottlenecks.

Customizable processors have been described as the next natural step in the evolution of the microprocessor business: a step in the life of a new technology where top performance alone is no longer sufficient to guarantee market success. Other factors become fundamental, such as time to market, convenience, energy efficiency, and ease of customization. This book is the first to explore comprehensively one of the most fundamental trends which emerged in the last decade: to treat processors not as rigid, fixed entities, which designers include “as is in their products; but rather, to build sound methodologies to tailor-fit processors to the specific needs of such products. This book addresses the goal of maintaining a very large family of processors, with a wide range of features, at a cost comparable to that of maintaining a single processor. First book to present comprehensively the major ASIP design methodologies and tools without any particular bias Written by most of the pioneers and top international experts of this young domain Unique mix of management perspective, technical detail, research outlook, and practical implementation

This book describes a new design approach for energy-efficient, Domain-Specific Instruction set Processor (DSIP) architectures for the wireless baseband domain. The innovative techniques presented enable co-design of algorithms, architectures and technology, for efficient implementation of the most advanced technologies. To demonstrate the feasibility of the author’s design approach, case studies are included for crucial functionality of advanced wireless systems with increased computational performance, flexibility and reusability. Designers using this approach will benefit from reduced development/product costs and greater scalability to future process technology nodes.

The current embedded processors often do not satisfy increasingly demanding computation requirements of embedded applications within acceptable energy efficiency, whereas application-specific integrated circuits require excessive design costs. In the Stanford Elm project, it was identified that instruction and data delivery, not computation, dominate the energy consumption of embedded processors. Consequently, the energy efficiency of delivering instructions and data must be sufficiently improved to close the efficiency gap between application-specific integrated circuits and programmable embedded processors. This dissertation demonstrates that the compiler and run-time system can play a crucial role in improving the energy efficiency of delivering instructions and data. Regarding instruction delivery, I present a compiler algorithm that manages L0 instruction scratch-pad memories that reside between processor cores and L1 caches. Despite the lack of tags, the scratch-pad memories with our algorithm can achieve lower miss rates than caches with the same capacities, saving significant instruction delivery energy. Regarding data delivery, I present methods that minimize memory-space requirements for parallelizing stream applications, applications that are commonly found in the embedded domain. When stream applications are parallelized in pipelining, large enough buffers are required between pipeline stages to sustain the throughput (e.g., double buffering). For static stream applications where production and consumption rates of stages are close to compile-time constants, a compiler analysis is presented, which computes the minimum buffer capacity that maximizes the throughput. Based on this analysis, a new static streamscheduling algorithm is developed, which yields considerable speed-up and data delivery energy saving compared to a previous algorithm. For dynamic stream applications, I present a dynamically-sized array-based queue design that achieves speed-up and data delivery energy saving compared to a linked-list based queue design.

To the hard-pressed systems designer this book will come as a godsend. It is a hands-on guide to the many ways in which processor-based systems are designed to allow low power devices. Covering a huge range of topics, and co-authored by some of the field’s top practitioners, the book provides a good starting point for engineers in the area, and to research students embarking upon work on embedded systems and architectures.

Modern signal processing systems require more and more processing capacity as times goes on. Previously, large increases in speed and power efficiency have come from process technology improvements. However, lately the gain from process improvements have been greatly reduced. Currently, the way forward for high-performance systems is to use specialized hardware and/or parallel designs. Application Specific Integrated Circuits (ASICs) have long been used to accelerate the processing of tasks that are too computationally heavy for more general processors. The problem with ASICs is that they are costly to develop and verify, and the product life time can be limited with newer standards. Since they are very specific the applicable domain is very narrow. More general processors are more flexible and can easily adapt to perform the functions of ASIC based designs. However, the generality comes with a performance cost that renders general designs unusable for some tasks. The question then becomes, how general can a processor be while still being power efficient and fast enough for some particular domain? Application Specific Instruction set Processors (ASIPs) are processors that target a specific application domain, and can offer enough performance with power efficiency and silicon cost that is comparable to ASICs. The flexibility allows for the same hardware design to be used over several system designs, and also for multiple functions in the same system, if some functions are not used simultaneously. One problem with ASIPs is that they are more difficult to program than a general purpose processor, given that we want efficient software. Utilizing all of the features that give an ASIP its performance advantage can be difficult at times, and new tools and methods for programming them are needed. This thesis will present ePUMA (embedded Parallel DSP platform with Unique Memory Access), an ASIP architecture that targets algorithms with predictable data access. These kinds of algorithms are very common in e.g. baseband processing or multimedia applications. The primary focus will be on the specific features of ePUMA that are utilized to achieve high performance, and how it is possible to automatically utilize them using tools. The most significant features include data permutation for conflict-free data access, and utilization of address generation features for overhead free code execution. This sometimes requires specific information; for example the exact sequences of addresses in memory that are accessed, or that some operations may be performed in parallel. This is not always available when writing code using the traditional way with traditional languages, e.g. C, as extracting this information is still a very active research topic. In the near future at least, the way that software is written needs to change to exploit all hardware features, but in many cases in a positive way. Often the problem with current methods is that code is overly specific, and that a more general abstractions are actually easier to generate code from.

This book introduces a novel design methodology which can significantly reduce the ASIP development effort through high degrees of design automation. The key elements of this new design methodology are a powerful application profiler and an automated instruction-set customization tool which considerably lighten the burden of mapping a target application to an ASIP architecture in the initial design stages. The book includes several design case studies with real life embedded applications to demonstrate how the methodology and the tools can be used in practice for accelerating the overall ASIP design process.