Parameter variation in scaled technologies beyond 90nm will pose a major challenge for design of future high performance microprocessors. In this paper, we discuss process, voltage and temperature variations; and their impact on circuit and microarchitecture. Possible solutions to reduce the impact of parameter variations and to achieve higher frequency bins are also presented.

/pdf/parameter-variations-and-impact-on-circuits-and-k3j22hqcec.pdf

Parameter variations and impact on circuits and microarchitecture

Using nonvolatile memories in memory hierarchy has been investigated to reduce its energy consumption because nonvolatile memories consume zero leakage power in memory cells One of the difficulties is, however, that the endurance of most nonvolatile memory technologies is much shorter than the conventional SRAM and DRAM technology This has limited its usage to only the low levels of a memory hierarchy, eg, disks, that is far from the CPUIn this paper, we study the use of a new type of nonvolatile memories -- the Phase Change Memory (PCM) as the main memory for a 3D stacked chip The main challenges we face are the limited PCM endurance, longer access latencies, and higher dynamic power compared to the conventional DRAM technology We propose techniques to extend the endurance of the PCM to an average of 13 (for MLC PCM cell) to 22 (for SLC PCM) years We also study the design choices of implementing PCM to achieve the best tradeoff between energy and performance Our design reduced the total energy of an already low-power DRAM main memory of the same capacity by 65%, and energy-delay2 product by 60% These results indicate that it is feasible to use PCM technology in place of DRAM in the main memory for better energy efficiency

/pdf/a-durable-and-energy-efficient-main-memory-using-phase-1ulke5et76.pdf

A durable and energy efficient main memory using phase change memory technology

This article provides a practical introduction to the design trade-offs of the currently available 3D IC technology options. It begins with an overview of techniques, such as wire bonding, microbumps, through vias, and contactless interconnection, comparing them in terms of vertical density and practical limits to their use. We then present a high-level discussion of the pros and cons of 3D technologies, with an analysis relating the number of transistors on a chip to the vertical interconnect density using estimates based on Rent's rule. Next, we provide a more detailed design example of inductively coupled interconnects, with measured results of a system fabricated in a 0.35-/spl mu/m technology and an analysis of misalignment and crosstalk tolerances. Lastly, we present a case study of a fast Fourier transform (FFT) placed and routed in a 0.18-/spl mu/m through-via silicon-on-insulator (SOI) technology, comparing the 3D design to a traditional 2D approach in terms of wire length and critical-path delay.

Demystifying 3D ICs: the pros and cons of going vertical

A significant part of future microprocessor real estate will be dedicated to L2 or L3 caches. These on-chip caches will heavily impact processor perfor- mance, power dissipation, and thermal management strategies. There are a number of interconnect design considerations that influence power/performance/area characteristics of large caches, such as wire mod- els (width/spacing/repeaters), signaling strategy (RC/differential/transmission), router design, etc. Yet, to date, there exists no analytical tool that takes all of these parameters into account to carry out a design space exploration for large caches and estimate an optimal organization. In this work, we implement two major extensions to the CACTI cache modeling tool that focus on interconnect design for a large cache. First, we add the ability to model different types of wires, such as RC-based wires with different power/delay characteristics and differential low-swing buses. Second, we add the ability to model Non-uniform Cache Access (NUCA). We not only adopt state-of-the-art design space exploration strategies for NUCA, we also enhance this exploration by considering on-chip network contention and a wider spectrum of wiring and routing choices. We present a validation analysis of the new tool (to be released as CACTI 6.0) and present a case study to showcase how the tool can improve architecture research methodologies. Keywords: cache models, non-uniform cache archi- tectures (NUCA), memory hierarchies, on-chip intercon- nects.

/pdf/optimizing-nuca-organizations-and-wiring-alternatives-for-3rrqqn2dbv.pdf

Optimizing NUCA Organizations and Wiring Alternatives for Large Caches with CACTI 6.0

Three-dimensional integration enables stacking memory directly on top of a microprocessor, thereby significantly reducing wire delay between the two. Previous studies have examined the performance benefits of such an approach, but all of these works only consider commodity 2D DRAM organizations. In this work, we explore more aggressive 3D DRAM organizations that make better use of the additional die-to-die bandwidth provided by 3D stacking, as well as the additional transistor count. Our simulation results show that with a few simple changes to the 3D-DRAM organization, we can achieve a 1.75x speedup over previously proposed 3D-DRAM approaches on our memory-intensive multi-programmed workloads on a quad-core processor. The significant increase in memory system performance makes the L2 miss handling architecture (MHA) a new bottleneck, which we address by combining a novel data structure called the Vector Bloom Filter with dynamic MSHR capacity tuning. Our scalable L2 MHA yields an additional 17.8% performance improvement over our 3D-stacked memory architecture.

/pdf/3d-stacked-memory-architectures-for-multi-core-processors-1ufiq9rsba.pdf

3D-Stacked Memory Architectures for Multi-core Processors

3D die stacking is an exciting new technology that increases transistor density by vertically integrating two or more die with a dense, high-speed interface. The result of 3D die stacking is a significant reduction of interconnect both within a die and across dies in a system. For instance, blocks within a microprocessor can be placed vertically on multiple die to reduce block to block wire distance, latency, and power. Disparate Si technologies can also be combined in a 3D die stack, such as DRAM stacked on a CPU, resulting in lower power higher BW and lower latency interfaces, without concern for technology integration into a single process flow. 3D has the potential to change processor design constraints by providing substantial power and performance benefits. Despite the promising advantages of 3D, there is significant concern for thermal impact. In this research, we study the performance advantages and thermal challenges of two forms of die stacking: Stacking a large DRAM or SRAM cache on a microprocessor and dividing a traditional microarchitecture between two die in a stack Results: It is shown that a 32MB 3D stacked DRAM cache can reduce the cycles per memory access of a twothreaded RMS benchmark on average by 13% and as much as 55% while increasing the peak temperature by a negligible 0.08?C. Off-die BW and power are also reduced by 66% on average. It is also shown that a 3D floorplan of a high performance microprocessor can simultaneously reduce power 15% and increase performance 15% with a small 14?C increase in peak temperature. Voltage scaling can reach neutral thermals with a simultaneous 34% power reduction and 8% performance improvement.

Die Stacking (3D) Microarchitecture

This short paper explores an implementation of a new technology called 3D die stacking and describes research activity at Intel. 3D die stacking is the bonding of two die either face-to-face or face-to-back in order to construct the 3D structure. In this work, a face-to-face bonding is utilized because it yields a higher density die-to-die inter-connect than is possible with face-to-back. With sufficiently dense die-to-die interconnect devices as complex as an iA32 microprocessor can be repartitioned or split between two die in order to simultaneously improve performance and power. The 3D structure of this emerging technology is examined and applied in this paper to a real x86 deeply pipelined high performance microprocessor. In this initial study, it is shown that a 3D implementation can potentially improve the performance by 15% while improving power by 15%.

/pdf/3d-processing-technology-and-its-impact-on-ia32-j3nm7pk4pg.pdf

3D processing technology and its impact on iA32 microprocessors

The trends in layout rules and the affect these rules have upon design are described The slow improvement in resolution has created a need for litho-friendly design The problems caused by LFD and the increase in number and complexity of rules are shown Issues in implementing LFD are discussed

Layout rule trends and effect upon CPU design

Pulsed wordline (PWL) & pulsed bitline (PBL) techniques to improve SRAM cell stabilities in single-Vcc microprocessor designs are evaluated in 65nm CMOS. At 0.7V Vcc, PWL improves cell failure rate by 15times while incurring <1% area overhead. Both PBL & PWL with read-modify-write (PWL-RMW) provide the best improvements (26times) in cell stability, with significant area overheads (4-8%)

Wordline & Bitline Pulsing Schemes for Improving SRAM Cell Stability in Low-Vcc 65nm CMOS Designs

An integrated circuit implements a cache static random access memory (SRAM) storage element which includes a central processor unit (CPU) bus interface incorporating multiplexers and buffers circuits for optimizing burst read and write operations across the CPU bus. Theses circuits allow a full cache line to be read/written in a single access of the SRAM array. Control logic is utilized within the CPU bus interface for controlling CPU bursts in the order defined by the CPU. The memory bus interface includes internal buffers used in performing memory bus reads, write-throughs, write-backs and snoops. Tracking logic is employed for determining the appropriate internal buffer to be utilized for a particular memory bus cycle. Additionally, a data path is included for transparently passing data between the CPU and memory bus interfaces without disturbance of the SRAM array.

Cache memory integrated circuit for use with a synchronous central processor bus and an asynchronous memory bus

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