This paper provides an introductory overview of the Cell multiprocessor. Cell represents a revolutionary extension of conventional microprocessor architecture and organization. The paper discusses the history of the project, the program objectives and challenges, the design concept, the architecture and programming models, and the implementation. Introduction: History of the project Initial discussion on the collaborative effort to develop Cell began with support from CEOs from the Sony and IBM companies: Sony as a content provider and IBM as a leading-edge technology and server company. Collaboration was initiated among SCEI (Sony Computer Entertainment Incorporated), IBM, for microprocessor development, and Toshiba, as a development and high-volume manufacturing technology partner. This led to high-level architectural discussions among the three companies during the summer of 2000. During a critical meeting in Tokyo, it was determined that traditional architectural organizations would not deliver the computational power that SCEI sought for their future interactive needs. SCEI brought to the discussions a vision to achieve 1,000 times the performance of PlayStation2** [1, 2]. The Cell objectives were to achieve 100 times the PlayStation2 performance and lead the way for the future. At this stage of the interaction, the IBM Research Division became involved for the purpose of exploring new organizational approaches to the design. IBM process technology was also involved, contributing state-of-the-art 90-nm process with silicon-on-insulator (SOI), low-k dielectrics, and copper interconnects [3]. The new organization would make possible a digital entertainment center that would bring together aspects from broadband interconnect, entertainment systems, and supercomputer structures. During this interaction, a wide variety of multi-core proposals were discussed, ranging from conventional chip multiprocessors (CMPs) to dataflow-oriented multiprocessors. By the end of 2000 an architectural concept had been agreed on that combined the 64-bit Power Architecture* [4] with memory flow control and ''synergistic'' processors in order to provide the required computational density and power efficiency. After several months of architectural discussion and contract negotiations, the STI (SCEI-Toshiba-IBM) Design Center was formally opened in Austin, Texas, on March 9, 2001. The STI Design Center represented a joint investment in design of about $400,000,000. Separate joint collaborations were also set in place for process technology development. A number of key elements were employed to drive the success of the Cell multiprocessor design. First, a holistic design approach was used, encompassing processor architecture, hardware implementation, system structures, and software programming models. Second, the design center staffed key leadership positions from various IBM sites. Third, the design incorporated many flexible elements ranging from reprogrammable synergistic processors to reconfigurable I/O interfaces in order to suppor...

The implementation of a first-generation CELL processor that supports multiple operating systems including Linux consists of a 64b power processor element (PPE) and its L2 cache, multiple synergistic processor elements (SPE) [1] that each has its own local memory (LS) [2], a high-bandwidth internal element interconnect bus (EIB), two configurable non-coherent I/O interfaces, a memory interface controller (MIC), and a pervasive unit that supports extensive test, monitoring, and debug functions. The high level chip diagram is shown in Fig. 10.2.1. The key attributes include hardware content protection, virtualization and realtime support combined with extensive single-precision floatingpoint capability. By extending the Power architecture with SPE having coherent DMA access to system storage and with multioperating-system resource-management, CELL supports concurrent real-time and conventional computing. With a dual-threaded PPE and 8 SPEs this implementation is capable of handling 10 simultaneous threads and over 128 outstanding memory requests. Figure 10.2.7 shows the die micrograph with roughly 234M transistors from 17 physical entities and 580k repeaters and 1.4M nets implemented in 90nm SOI technology with 8 levels of copper interconnects and one local interconnect layer. At the center of the chip is the EIB composed of four 128b data rings plus a 64b tag operated at half the processor clock rate. The wires are arranged in groups of four, interleaved with GND and VDD shields twisted at the center to reduce coupling noise on the two unshielded wires. To ensure signal integrity, over 50% of global nets are engineered with 32k repeaters. The SoC uses 2965 C4s with four regions of different row-column pitches attached to a low-cost organic package. This structure supports 15 separate power domains on the chip, many of which overlap physically on the die. The processor element design, power and clock grids, global routing, and chip assembly support a modular design in a building-block-like construction.The chip contains 3 distinct clock-distribution systems, each sourced by an independent PLL, to support processor, bus interface, and memory-interface requirements. The main high-frequency clock grid covers over 85% of the chip, delivering the clock signal to processors and miscellaneous circuits. Second and third clock grids, each operating at fractions of the main clock signal, are interleaved with the main clock-grid structure, creating multiple clock frequency islands within the chip. All clock grids are constructed on the lowest impedance final two layers of metal, and are supported by a matrix of over 850 individually tuned buffers. This enables control of the clock-arrival times and skews, especially on the main clock grid that supports regions of widely varying clock-load densities. High-frequency clock-signal distribution optimization and verification rely on wire simulation models that includes frequency-sensitive inductance and resistance phenomena. As shown in Fig. 10.2.2, final worst-case clock skew ac...

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This paper reviews the design challenges that current and future processors must face with stringent power limits and high frequency targets, and the design methods required to address the continuing system integration trends. This paper then describes the implementation of a first-generation CELL processor and the design methods used to overcome the above challenges. A CELL Processor consists of a 64 bit Power Architecture processor coupled with multiple synergistic processors, a flexible IO interface, and a memory interface controller that supports multiple operating systems including Linux. This multi-core SoC, implemented in 90nm SOI technology, achieved a high clock rate by maximizing custom circuit design while maintaining reasonable complexity through design modularity and reuse.

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