The Missing Memristor
11 May, 2010 § 3 Comments
Hewlett Packard recently announced that they have fabricated memristor chips and plan to bring them to market by 2013 . This paper is an evaluation of the memristor and focuses on the potential impact of its introduction.
The memristor is an electrical circuit element that is similar to a resistor but has the potential to maintain state between turning power on and off. The memristor was first described by Leon Chua in 1971 as the fourth fundamental element, based on a relationship between charge and flux . These memristors are about half the size of the transistors found in current flash storage technology such as iPods and portable USB drives, allowing capacity of these devices to double. Further, Hewlett Packard has claimed the ability to improve the data-write cycle limit 10-fold, from flash technology’s 100,000 to the memristor’s 1,000,000 data-write cycles .
A Computer Architecture Perspective
The memristor hopes to replace the memory hierarchy that is seen in almost all computers today. Because the memristor’s state is nonvolatile and is planned to reach about four nanometers in width, large amounts of memory can be stored directly on the processing chip. The memory hierarchy seen in today’s computers may have three or four levels of volatile cache, another level of volatile DRAM, and finally a level of nonvolatile memory. Putting the nonvolatile memory close to the logical unit on the chip has the potential to allow memory accesses of 1,000 times faster .
Memristor caches may be able to remember state and resume said state even as the power to the devices comes and goes. During current computer start up routines, the bootloader needs to populate the translation lookaside buffer (TLB) at each startup. With memristor technology, the TLB can remember state between power cycles, allowing a much quicker start up process. As the technology matures, this same thinking could be extended all the way to resuming running processes when the machine starts back up.
At its initial implementation, Hewlett Packard’s goal is place memristors between DRAM and disk technology, eventually spreading in both directions to replace disk and DRAM . Due to the memristors speed and data integrity guarantees, this goal is likely to bring hard drive-like reliability with speeds faster than DRAM. Consumers are likely to see these speed increases when writing to their iPods and USB drives, since the higher write-cycle limits will allow the same sector to be written to instead of current flash implementations that try to write to the same sector less often.
Another use for memristors is for computing logic. Using material implication, a group of three memristors can be used to compute any boolean logic equation that is requested . The motivation of using memristors for logic computation comes from the ability to run the computations on the nanoscale. The use of material implication complicates the boolean logic but is not enough to deter the gains in size reduction. The gains are enough to have one memristor replace 15 transistors .
Combining the use of memristors for storage and logic may also present a new application for the device. Hypothetically, a crossbar of memristors will have the ability to re-purpose groups of memristors for data-intensive or compute-intensive operations. Memristors are also very efficient power users . Embedding memristors within sensor networks has the possibility to allow sensor networks to collect more data and have much longer lifetimes.
The power walls and memory walls can effectively be written off if the memristor proves its claims.
Impact for the General User
As previously mentioned, the memristor technology will allow much faster write-times compared to flash technology. General computer users will be able to sync up their portable devices in 1/1000th of the time that it takes them today. Not only will devices be able to sync faster, but they will be able to store more data than could have ever been concieved of before. HP expects to reach a storage density of about 20 gigabytes per square centimeter by 2013. As a comparison, the surface area of an Apple iPod Nano is about 64 cm2 which could translate to over 1.25 terabytes of data on the surface of the device alone.
General users can expect to see devices that use this extra storage space to continuously collect data. The vast amounts of data that can be collected are in line with Intel’s “Era of Tera” forecast . Devices will be able to use the data for facial recognition of past acquaintances, weather forecasting through crowd-sourced data collection, and even more interesting applications that have yet to be thought of.
Critiques of the Memristor
To implement the memristor, titanium dioxide is used as the semiconductor instead of the traditional silicone. There is still much to be understood about titanium dioxide, as a team from the National Institute of Standards and Technology (NIST) said . Silicone has had many years to mature as a semiconductor element in use with electrical circuits, whereas researchers are just beginning to understand how to use titanium dioxide in electrical circuits. “The fundamentals of why these metal oxides switch the way they do are not well understood,” said NIST researcher Curt Richter . Further time and research may resolve the unknowns, but at this point there might not be enough known information about it to be sure of its promise.
Hewlett Packard and Intel have had a long history of working together to introduce new technology, such as their work with the EPIC Itanium architecture. Intel has been approached to work on the memristor technology with Hewlett Packard but has decided not to take Hewlett Packard up on their offer. Instead, Intel is looking to focus their energy on phase-change memory . Intel has already shipped phase-change memory samples in 2008 and plans to start shipping mass quantities in 2010 , as opposed to Hewlett Packard’s expected release date of 2013 for the memristor.
The memristor presents an amazing opportunity for change in the computer memory hierarchy, storage capacity, and nonvolatile state. Lab results for the research have shown that we are just at the cusp of all the capabilities of memristors. With that being said, there are still some unknowns about the technology that will challenge its adoption. The material used as the semiconductor is relatively new to electronic circuits, and research from competitors is appearing to be quicker to market than the memristor.
Much of the publications for the memristor are from it’s main researcher, R. Stanley Williams of Hewlett Packard. Many of the claims have not been validated by a third-party and could very easily be exaggerated. If given the opportunity to invest in memristor technology, I would advise taking a deeper look in to phase-change memory and it’s other competitors that look to be closer to reaching the market. Computing technology changes very quickly and there is a low probability that the computing environment three years from now is unchanged from today.
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