A new silicon civilization

Sun Journal

February 16, 1997|By Steve Auerweck | Steve Auerweck,SUN STAFF

There is a new, seemingly pervasive architecture in our lives. It is the architecture of computer chips -- the ultimate planned community, where nothing is present by accident.

Chips have interstate highways and secondary roads, warehouses and industrial zones. A chip is a wholly planned metropolis of transistors and silicon.

Pentium is the brand of chip, the silicon city, that is most in demand. Designed and manufactured by Intel Corp., it is the standard for personal computers. You've seen the ads: "Intel Inside," they say. But in the ads, all you see is a small black box, while the soul of the machine is the sliver of processed silicon inside. It's there that words are processed, spreadsheets are calculated, aliens are vaporized.

Look closer, and there are the neighborhoods and highways - an urban landscape in a half-inch square.

1. On and off

Digital computers manipulate information by turning electricity on and off. Transistors handle the switching. On the Pentium chip, there are 3.2 million of them.

Their switching on and off is a physical representation of a binary number system, in which the only digits are 0 and 1. A computer can represent numbers, even very large numbers, by turning a very small voltage on - to represent a 1 - and off - to represent a 0.

2. Hair-thin highways

The microscopic equivalent of streets are the circuit paths connecting transistors and other parts.

On the Pentium-166, the smallest feature - a certain part of one transistor - is 0.3 micron across (a micron being 0.001 millimeter). A human hair typically measures 100 microns across.

Circuits are formed from polished sheets of the purest possible silicon; the manufacturing process involves chemical treatment, photographic etching and depositing vaporized materials. The part you see is only the surface: The chip is actually four layers deep.

Why so small? The speed of computers is largely constrained by the speed of electrons through the circuits. The shorter the path, the faster the computer.

3. Caches

The Pentium has two separate chunks of memory to hold the most recently used instructions and data. Each "cache" holds 8,192 bytes. (A byte is eight bits; most numbers are held in four bytes.)

Any time an instruction or piece of data can be found in the cache, rather than having to be retrieved from memory elsewhere in the computer, things go much faster.

4. Execution units

The Pentium understands hundreds of instructions that break functions into the most basic parts - pick up a number here, put it there. Most work is handled by two "integer execution units."

The chip can take in two instructions at once and process them at the same time, as long as the second doesn't depend on the results of the first.

Each execution unit is designed like an assembly line. The chip moves each instruction along through five steps:

* The instruction is retrieved from memory and put in line to be handled.

* It's then decoded.

* The chip generates the "address," or location in memory, of the data the instruction needs.

* The instruction is executed.

* The results are stored in memory.

While one instruction is in the execution phase, another is in address-generation - called "pipelining," which dramatically increases the chip's processing speed.

5. Floating-Point Unit (FPU)

The FPU manipulates numbers that have a fractional component. You can handle accounting with integers, or whole numbers, but for scientific formulas, three-dimensional graphics and the like, you usually need floating-point.

They are much more complicated to handle because each is stored in two parts - a main portion and an exponent that tells how large the number is.

6. Control logic

Control logic is the chip's administrative center. It handles a host of decisions, including which processing unit will handle which instruction, and whether two instructions can be processed simultaneously.

Pub Date: 2/16/97

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