Cell (processor)

The Cell is a microprocessor design being developed by IBM in cooperation with Toshiba and Sony. The Cell chip is intended to be scalable from handheld devices to mainframe computers by utilizing parallel processing. Sony plans to use the chip in their PlayStation 3 game console.

While the Cell chip can have a number of different configurations, the workstation and PlayStation 3 version of Cell consists of one "Processing Element" ("PE"), and eight "Attached Processing Units" ("APU"). The PE is based on the POWER Architecture, basis of their existing POWER line and related to the PowerPC used by Apple Computer and others. The PE is not the primary processor for the system, but acts as a contoller for the other eight APU's, which handle most of the computational workload.

Each APU is a VLIW 128-bit vector processor with a 1024-bit external bus. The bus is attached to an 8MB high speed memory, one for each APU, which is also visible to the PE to be loaded with data and programs as needed. The APU's memory is also connected to the next APU in line, allowing data to be processed by one APU and then handed off to the next at very high speed. In general use the system will load the APUs with small programs, known as apulettes, chaining the APU's together to handle each step in a complex operation. For instance, a set top box could load up apulettes for reading a DVD, video and audio decoding, and display, and the data would be passed off from APU to APU until finally ending up on the TV. Each APU is expected to give 32 GFLOPS of performance, thereby giving the entire Processing Unit 256 GFLOPS of performance.

In some ways the Cell system resembles early Seymore Cray designs in reverse. The famed CDC 6600 used a single very fast processor to handle the math, while a series of ten slower systems were given smaller programs to keep the main memory fed with data. In the Cell the problem has been reversed, reading the data is no longer the difficult problem due to the complex encodings used in industry; today the problem is efficiently decoding that data into an ever-less-compressed version as quickly as possible.

In other ways the Cell resembles a modern desktop computer on a single chip. Modern graphics cards have multiple elements very similar to the APU's, known as vertex shader units, with an attached high speed memory. Programs, known as shaders, are downloaded onto the units to process the basic geometry fed from the computer's CPU, apply styles and display it. The main differences are that the Cell's APUs appear to be much more general purpose than the average graphics card shader units, and the ability to chain the APUs under program control offers considerably more flexibility, allowing the Cell to handle graphics, sound, or anything else. Given that the Cell is intended to be used in the PlayStation, the idea of a CPU+graphics card combination that is the fastest in the world is not entirely surprising.

Cell allows for multiple processing units to be put onto one die, and the patent showed four on one die, called the "Broadband Engine", potentially giving over 1 Teraflops of performance. It is unclear how many processing units will be incorporated into either the PlayStation 3 or workstations.

Early versions of Cell may clock around 4.6 GHz, according to paper abstracts presented for the International Solid-State Circuits Conference (ISSCC) to be held in February 2005. According to Sony, the chips are in early production for workstations, using IBM's 90 nanometre process, with full production using Sony's 65-nm process, with 45-nm process a distinct possibility for PlayStation 3, at their Nagasaki fabrication plant. Sony currently is using its 90-nm process to produce the integrated GS/EE for the PSX, the Japan-only combination PlayStation2/DVR unit.

There will be several versions of the Cell chip with varying number of processing units depending on the device where the chip is used. The companies designing the chip have claimed that by scaling the number of units in the chip, as well as the number of PEs on a single die, or by linking multiple chips to each other via network or memory bus, supercomputer-like performance can be made available in consumer devices.

Efforts to create similar multiple-core processors by Sun Microsystems, including MAJC (pronounced "magic"), a very similar effort, have missed their mark. The first MAJC chip was originally designed, similar to IBM's Cell, for multimedia processing, but instead of selling the chip to set-top box and game machine manufacturers, Sun repositioned the MAJC chip as a high-end graphics processor for workstations.

In addition, Stanford University's Imagine Stream Processor seems to share some similar concepts.

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