How eFuses Work and Why They're Not As Bad as You Think

Will all the confusion surrounding Motorola's Droid X, it seems that some are still unaware of what exactly eFuses do. According to some, the minuscule circuitry can render your Droid inoperable on a whim, an unfortunate side effect of daring to modify your Android device. Others claim a Motorola CEO will personally destroy your phone should you attempt to remove MotoBlur for stock, Eclair goodness. But whatever the case may be, it's clear that people have it all wrong.

Originally conceived by IBM in the early 2000's, eFuses were intended as a simple way to alter the function and performance of a chip in real time. In other words, the company had designed a chip that could "reroute chip logic, much the way highway traffic patterns can be altered by opening and closing new lanes," according to Bernard Meyerson, vice president and chief technologist of IBM at the time. The idea was that a chip could regulate speed or power consumption issues by simply tripping a fuse, or more impressively, "repair unexpected, potentially costly flaws."

However, it wasn't until the release of the Xbox 360 that users would begin to look at eFuses in a different light — and a far more sinister one at that.

Microsoft's current console would be one of the first commercial implementations of eFuse technology, thanks to IBM's Xenon CPU. The technology was first discovered during a teardown of the console, but hackers weren't entirely sure how it was being used. And while it wouldn't take long to discover the answer, hackers and homebrew developers were less than enthused. As it turned out, Microsoft was "blowing" eFuses after significant updates to the Xbox 360 kernel and system software. Unlike the company's previous console, this would prevent hackers from downgrading to a previous version of the Xbox OS and exploiting potential bugs. The console's security measures relied on the status of these eFuses; attempt to run an older software revision, and those checks would fail.

Clearly, Microsoft was employing eFuses in a very different and inventive manner than IBM had originally intended. But it raised a number of important questions with regards to how eFuses worked. Because these fuses were a hardware component of the CPU, many wondered just how many Microsoft had to play with. Would they eventually run out, allowing hackers to modify the system? But perhaps more alarmingly, a reverse engineer by the name of Speedy22 theorized that Microsoft could actually blow the CPU's eFuses in such a way that "it may be possible to shut power off to the CPU permanently or increase the core supply to the point of self-destruct."

This is what makes the discovery of eFuses in the Motorola Droid all the more interesting. Some users feared that Motorola could include a similar implementation to that of the Xbox 360, going so far as to brick any Droid X that had been tampered with. The problem is, Microsoft never bricked any 360's using an eFuse system. In fact, they've never intentionally bricked *any* Xbox 360 console, and it's hard to believe that Motorola would act any differently with their own device.

A more likely scenario is that Motorola will follow Microsoft's update path. Most Android roots occur via exploits in older version firmwares that a user can still flash to his or her device. By blowing an eFuse each time an update is applied, Motorola could remove the ability to downgrade to a previous OS revision, and thus reducing the chance of root or custom ROMs. This seems far more likely considering recent statements from the company, explaining that eFuses ensure "that the device only runs on updated and tested versions of software." Of course, that's still not a good thing for developers and ROM hacking enthusiasts, but it's still better than a complete brick as some would like to assume.

The sad thing is, controversies like this shed a bad light on what is otherwise an impressive piece of technology. In its original incarnation, eFuses appeared to be an inventive way to reduce hardware faults, and make chips more adaptable to the ever-changing conditions within a machine. And while Microsoft and Motorola's interests may lie in maintaining security, a brick-inducing kill-switch it is most definitely not.     
 
Images via Flickr user steeljam, Wikipedia.