Section: Research Program

Reconfigurable Architecture Design

Over the last two decades, there has been a strong push of the research community to evolve static programmable processors into run-time dynamic and partial reconfigurable (DPR) architectures. Several research groups around the world have hence proposed reconfigurable hardware systems operating at various levels of granularity. For example, functional-level reconfiguration has been proposed to increase the efficiency of programmable processors without having to pay for the fpga penalties. These coarse-grained reconfigurable architectures (CGRAs) provide operator-level configurable functional blocks and word-level datapaths. The main goal of this class of architectures is to provide flexibility while minimizing reconfiguration overhead (there exists several recent surveys on this topic [101] , [86] , [71] , [102] ). Compared to fine-grained architectures, CGRAs benefit from a massive reduction in configuration memory and configuration delay, as well as a considerable reduction in routing and placement complexity. This, in turns, results in an improvement in the computation volume over energy cost ratio, even if it comes at the price of a loss of flexibility compared to bit-level operations. Such constraints have been taken into account in the design of DART [83] [11] , CRIP [73] , Adres [93] or others [104] . These works have led to commercial products such as the Extreme Processor Platform (XPP) [74] from PACT or Montium (http://www.recoresystems.com/ ) from Recore systems.

Another strong trend is the design of hybrid architectures which combine standard gpp or dsp cores with arrays of configurable elements such as the Lx [85] , or of field-configurable elements such as the Xirisc processor [91] and more recently by commercial platforms such as the Xilinx Zynq. Some of their benefits are the following: functionality on demand (set-top boxes for digital TV equipped with decoding hardware on demand), acceleration on demand (coprocessors that accelerate computationally demanding applications in multimedia or communications applications), and shorter time-to-market (products that target asic platforms can be released earlier using reconfigurable hardware).

Dynamic reconfiguration enables an architecture to adapt itself to various incoming tasks. This requires complex resource management and control which can be provided as services by a real-time operating system (RTOS) [92] : communication, memory management, task scheduling [82] , [77] [1] and task placement. Such an Operating System (OS) based approach has many advantages: it provides a complete design framework, that is independent of the technology and of the underlying hardware architecture, helping to drastically reduce the full platform design time. Due to the unpredictable execution of tasks, the OS must be able to allocate resource to tasks at run-time along with mechanisms to support inter-task communication. An efficient way to support such communications is to resort to a network-on-chip [99] . The role of the communication infrastructure is then to support transactions between different components of the platform, either between macro-components – main processor, dedicated modules, dynamically reconfigurable component – or within the elements of the reconfigurable components themselves.

In Cairn we mainly target reconfigurable system-on-chip (RSoC) defined as a set of computing and storing resources organized around a flexible interconnection network and integrated within a single silicon chip (or programmable chip such as FPGAs). The architecture is customized for an application domain, and the flexibility is provided by both hardware reconfiguration and software programmability. Computing resources are therefore highly heterogeneous and raise many issues that we discuss in the following:

• Reconfigurable hardware blocks with a dynamic behavior where reconfigurability can be achieved at the bit- or operator-level. Our research aims at defining new reconfigurable architectures including computing and memory resources. Since reconfiguration must happen as fast as possible (typically within a few cycles), reducing the configuration time overhead is also a key issue.

• When performance and power consumption are major constraints, it is acknowledged that optimized specialized hardware blocks (often called IPs for Intellectual Properties) are the best (and often the only) solution. Therefore, we also study architecture and tools for specialized hardware accelerators and for multi-mode components.

• Customized processors with a specialized instruction-set also offer a viable solution to trade between energy efficiency and flexibility. They are particularly relevant for modern FPGA platforms where many processor cores can be embedded. For this topic, we focus on the automatic generation of heterogeneous (sequential or parallel) reconfigurable processor extensions that are tightly coupled to processor cores.