Wednesday, June 21, 2006
NOR, Where Art Thou?
Posted by Jon Westfall in "DEVELOPER" @ 07:00 PM
"I've been asked why OEMs would use NOR flash in a persistent storage world. (If words like "NOR flash" or "XIP" are confusing, read this.) I'm going to give you some history about where NOR flash came from, explain how we got from there to here, and prognosticate a bit on the future. The first PocketPC to have any sort of embedded flash memory was the original iPaq (it was released in 2000). Before that, all our devices shipped on what was known as "Mask ROMs." A Mask ROM is created at the factory and can't ever be erased or changed. Obviously, we never stored user data in these (there was no way to write them), and the only way to ever "upgrade" a device with a Mask ROM was to physically remove the chip and put a new one in. (Yes, I'm ignoring EEPROMs. They were using during development, but few, if any, devices were shipped to customers with them.) It's important to note that code in Mask ROMs could be executed in place. That is to say, they XIPed. Now, NAND flash had been around for a long time. Those old Mask ROM PDAs had CF slots, and the CF cards were made of NAND-based Flash memory (frequently called Flash ROM even though that's not technically correct). NAND, however, can't XIP, so it couldn't be used as a direct replacement for Mask ROMs. "
For those of you chip junkies out there that wondered why NOR hasn't been used much in our devices, check out this excellent post on the Windows Mobile Team Blog!
For those of you chip junkies out there that wondered why NOR hasn't been used much in our devices, check out this excellent post on the Windows Mobile Team Blog!
