Overview
The Cell Broadband Engine—or Cell as it is more commonly known—is a microprocessor designed to bridge the gap between conventional desktop processors (such as the Athlon 64, and Core 2 families) and more specialized high-performance processors, such as the NVIDIA and ATI graphics-processors (GPUs). The longer name indicates its intended use, namely as a component in current and future digital distribution systems; as such it may be utilized in high-definition displays and recording equipment, as well as computer entertainment systems for the HDTV era. Additionally the processor may be suited to digital imaging systems (medical, scientific, etc.) as well as physical simulation (e.g., scientific and structural engineering modeling).
In a simple analysis, the Cell processor can be split into four components: external input and output structures, the main processor called the Power Processing Element (PPE) (a two-way simultaneous multithreaded Power ISA v.2.03 compliant core), eight fully functional co-processors called the Synergistic Processing Elements, or SPEs, and a specialized high-bandwidth circular data bus connecting the PPE, input/output elements and the SPEs, called the Element Interconnect Bus or EIB.
To achieve the high performance needed for mathematically intensive tasks, such as decoding/encoding MPEG streams, generating or transforming three-dimensional data, or undertaking Fourier analysis of data, the Cell processor marries the SPEs and the PPE via EIB to give access, via fully cache coherent DMA (direct memory access), to both main memory and to other external data storage. To make the best of EIB, and to overlap computation and data transfer, each of the nine processing elements (PPE and SPEs) is equipped with a DMA engine. Since the SPE's load/store instructions can only access its own local memory, each SPE entirely depends on DMAs to transfer data to and from the main memory and other SPEs' local memories. A DMA operation can transfer either a single block area of size up to 16KB, or a list of 2 to 2048 such blocks. One of the major design decisions in the architecture of Cell is the use of DMAs as a central means of intra-chip data transfer, with a view to enabling maximal asynchrony and concurrency in data processing inside a chip.
The PPE, which is capable of running a conventional operating system, has control over the SPEs and can start, stop, interrupt, and schedule processes running on the SPEs. To this end the PPE has additional instructions relating to control of the SPEs. Unlike SPEs, the PPE can read and write the main memory and the local memories of SPEs through the standard load/store instructions. Despite having Turing complete architectures, the SPEs are not fully autonomous and require the PPE to prime them before they can do any useful work. Though most of the "horsepower" of the system comes from the synergistic processing elements, the use of DMA as a method of data transfer and the limited local memory footprint of each SPE pose a major challenge to software developers who wish to make the most of this horsepower, demanding careful hand-tuning of programs to extract maximal performance from this CPU.
The PPE and bus architecture includes various modes of operation giving different levels of memory protection, allowing areas of memory to be protected from access by specific processes running on the SPEs or the PPE.
Both the PPE and SPE are RISC architectures with a fixed-width 32-bit instruction format. The PPE contains a 64-bit general purpose register set (GPR), a 64-bit floating point register set (FPR), and a 128-bit Altivec register set. The SPE contains 128-bit registers only. These can be used for scalar data types ranging from 8-bits to 128-bits in size or for SIMD computations on a variety of integer and floating point formats. System memory addresses for both the PPE and SPE are expressed as 64-bit values for a theoretic address range of 264 bytes (16 exabytes or 16,777,216 terabytes). In practice, not all of these bits are implemented in hardware. Local store addresses internal to the SPU processor are expressed as a 32-bit word. In documentation relating to Cell a word is always taken to mean 32 bits, a doubleword means 64 bits, and a quadword means 128 bits.
Read more about this topic: Cell (microprocessor)