When a moth flew into one of the first computers, at the US Naval Surface Warfare Centre in 1947, Admiral Grace Hopper coined the term "bug". Today's users are more familiar with the idea of bugs in the software on their PCs: programmers' errors which cause "unwarranted or unintended behaviour", to quote the Hacker's Dictionary.

With a word processor or spreadsheet it is a simple matter to install an amended version of the same package. When the error is in the design of a microprocessor chip, however, there is only one solution - to open the box and replace the defective piece of silicon. So when a flaw was found in the Pentium, the latest and fastest microprocessor produced by Intel, all hell broke loose.

Intel, the world's leading microprocessor maker, has staked hundreds of millions of dollars on the success of this new chip. Intel shares were suspended as panic spread among traders.

Although the problem had been diagnosed weeks earlier, panic was triggered by IBM's costly decision last week to halt shipment of PCs containing the chip. Intel had already fixed the bug, but IBM's action has cast doubt on the fate of the thousands of machines already in the distribution chain that contain defective Pentiums.

Why the panic? With rapid obsolescence, innovators such as Intel need to recoup their R&D investment by keeping prices high until rival "clone" manufacturers catch up. This has been particularly important for the Pentium, which Intel is trying to sell tohome users hungry for a powerful chip that can make short work of CD-roms.

Last week's war of words focused on how much the bug matters. Intel claimed that it is likely to cause a mistake only once every 27,000 years. IBM disagreed. "Intel has made the assumption that the user makes 1,000 floating point calculations each day," said IBM spokesman Kevin Pedmutter. "But one typical spreadsheet recalculation alone makes 5,000 such divisions each second. For only 15 minutes a day, that's 4.5 million divisions." Intel's chief executive officer, Andy Grove, responded with fury: "If you know where a meteor will land, you can go there and get hit."

Bugs in modern processors are nothing new. It's just that the problem has got worse, because modern wafer-scale integration has crammed more and more transistors on to the tiny space available. In the past, engineers have even turned these flaws to theiradvantage. Microsoft and IBM originally used a bug in Intel's 286 processor to allow early versions of their OS/2 operating system to run DOS applications.

Semiconductor manufacturers are now more likely to be plagued by low "yield levels" - the proportion of chips leaving the factory that are usable - than by design problems. However, the Pentium has been the most troublesome of Intel's offspring, beset b y overheating, low yield levels and now the floating point bug.

Many in the industry see the Pentium's problems as poetic justice. Intel has been accused of confusing the market, promising upgrades which never appeared and preferring to fight competitors in the courts rather than the marketplace. It's a rough game, and one that Intel's customers also know how to play. IBM has its own brand of high-end personal computer chips, the PowerPC, which will benefit from the problems of the Pentium.

Admiral Hopper's original moth is preserved at the History of American Technology museum, but it is unlikely the Pentium bug will be joining it. With 3 million transistors, a simple circuit diagram would take up yards. The museum wouldn't be able to findthe space.