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Multi-Processor Systems-on-Chips

Multi-Processor Systems-on-Chips (MPSoCs) are the most efficient form of computing.
MPSoCs will soon become available at the retail level, and will become affordable and widely used.

Note: This article was written in 2005, and MPSoCs have since become available and widely used.

Originally, personal computers were isolated machines, each with a single processor. Such a processor was called a CPU (central processing unit), and was on a computer chip attached to the computer's main board. The main board was called a motherboard, and had other chips attached to it. The other chips were controlled by the CPU chip, which sent signals to the other chips via the motherboard's wiring system.

Before long, an advance called networking was accomplished (with NetBios extensions to MS-DOS) whereby different computers could be attached to each other with cables. You can think of that as external networking. The CPU of one computer could communicate with the CPU of another computer, with external wires.

Later, another advance was made whereby a single computer could have more than one CPU chip. The different CPU chips would be on the same motherboard, and communicate with each other over the motherboard “bus” wiring. This is computationally more efficient than external networking.

Even more efficient is to have more than one CPU on a chip, with direct on-chip wiring between those CPUs instead of communicating through the slower and more expensive main board (bus) wiring. This concept is shown in the group set diagram at right.

The most computationally efficient is to also have other components on the chip (not just CPUs).

A system-on-chip is a complete electronic system on a chip, including memory and one or more processors which are called CPUs or “cores.” If there is more than one processor (core) on the chip, it is said to be a multiprocessor system-on-chip (MPSoC).

In a SoC, different components are on the same chip, providing much higher efficiency at lower cost. For example:

“Although a 64-bit bus might be prohibitively expensive for a board-level design, 128- and 256-bit connections running at hundreds of megahertz are easy to design into SoCs.” [ 1 ]

The MPSoC cores can be heterogeneous (not all the same), each sized for specific tasks, and do not need to be fast to perform specific tasks as fast as much larger and “faster” conventional processors, doing so at a fraction of the cost with much lower power consumption and less silicon space. [ 2 ]

The MPSoC cores use RISC optimization at the compile level instead of CISC hardware optimization that wastes energy and silicon space.

“The extra [silicon space] that would be spent on register renaming, instruction wake-up, speculation/prediction, and register bypass on a complex single processor can be spent on providing higher bandwidth on an on-chip multiprocessor.” [ 3 ]

Footnotes:

1.	Chris Rowen, “Performance and Flexibility for MPSoC Design,” in Jerraya & Wolf (Eds.) Multiprocessor Systems-On-Chips (MPSoCs) (Morgan Kaufmann / Elsevier, 2005), p. 120.

2.	Rowen, p. 147.

3.	Mahmut Kandemir and Nikil Dutt, “Memory Systems and Compiler Support for MPSoC Architectures,” in MPSoCs, p. 251.

Memory

On-chip memory can be SRAM or DRAM.

Static Random Access Memory (SRAM) uses multiple transistors per bit of data stored, and does not lose the data while it is supplied with electricity.

Dynamic RAM (DRAM) does lose its data, in less than a second, and must be regularly refreshed. DRAM is slower, but uses less silicon space because it only uses one transistor (and a capacitor) per bit of data. Therefore, DRAM usually costs less than SRAM.

Historically, DRAM was too slow to be tightly coupled with high performance processors. But that is no longer true. Advances in DRAM technology now make the DRAM clock rate fast enough for DRAM to be used on-chip. Advances in DRAM technology include SDRAM and DDR SDRAM (synchronous and double data rate). [ 1 ]

“Advances in embedded DRAM technology have made it possible to combine DRAM and logic on the same chip.” [ 2 ]

However, the SRAM manufacturing process is more similar to the manufacturing process of the rest of a system-on-chip, [ 3 ] so depending on the manufacturing it may be necessary to use SRAM instead of DRAM for on-chip memory.

“On-chip memory's manufacturing needs must be balanced against the requirements of the logic circuits on the chip.” [ 4 ]

1.	Patterson and Hennessy, Computer Organization and Design (Morgan Kaufmann / Elsevier, 2005), p. 490, B.9

2.	Kandemir and Dutt, p. 254.

3.	Silc, Robic and Ungerer, Processor Architecture (Springer, 1999), p. 302.

4.	Wayne Wolf, “Embedded Systems-on-Chips,” in Dorf (Ed.) The Engineering Handbook 2nd Ed., (CRC, 2005), p. 123-4.

Articles

The following articles provide an overview of MPSoC technology and general computing:

Introduction
Hardware
Architecture
Software
Threads
Messaging
2008 Addendum

Sunday, 06-Jul-2008 15:40:51 GMT