| From: Alvin Starr via talk <talk@gtalug.org> | Years ago I worked on an avionics project where we were storing data in an | NVRAM. | We had to write code to manage the wear-out in the cells and if I remember | correctly the wear-out was in the hundreds of thousands to millions of writes | before problems would occur. | | I could not see normal usage causing wear-out unless someone were to have some | kind of bug causing a high write rate. The following is based on knowledge plus a fair bit of superstitious extrapolation and interpolation. NAND flash and NOR flash are different. You are surely talking about a NOR flash Lennart is talking about an unknown system. I suspect it used NAND flash, if the description is right. NOR flash: - allows update of each byte separately (most are 1 byte wide) - each update only wears the byte updated - are small (eg. 8M is large). Older consumer routers used NOR flash and that is why their flash sizes were so limited. - it is reasonable to run programs directly out of NOR flash (it looks like RAM) NAND flash - allows updates in pages, but only to previously erased pages - erases can only be done in large blocks. The size of these blocks is generally secret but I think that they are in the range of 128KiB to 1MiB. - are so alien to deal with that they have a complicated controller between them and your system. The controller emulates a disk drive. This does wear levelling. There is some work at moving this into the OS for performance reasons, but it is not common and only for data-centres. - density is everything in NAND flash systems. The original ones used a single bit per cell (SLC) but now 3 bits per cell is common. To store 3 bits per cell, 8 charge levels must be distinguished. This makes them much slower and less reliable (shorter-lived). - each cell can only be erased a small number of times (perhaps 1000) but the wear-levelling usually prevents this being a big problem. It is not part of published specifications. - consumer routers now use NAND flash for firmware and allow orders of magnitude larger firmware. For reasons I haven't investigated, it took OpenWRT years to support NAND flash routers. - it is unreasonable to run programs directly out of NAND flash. They can, however, be paged in, just as one would do from a HDD. This wrecks real-time performance, so routers would not do this. They copy the contents of flash to RAM as part of booting. Normal PCs: - have used a battery-backed-up CMOS RAM module for firmware settings. Everything is CMOS now, I think, but when this dedicated memory was added to the PC, CMOS was notable. Especially for its low standby power requirement. - use flash memories with sequential access for firmware (SPI). The power-on sequence fetches the firmware and dumps it into RAM so that it can be accessed randomly. Dedicated programs are needed to update the flash through the SPI interface. - uses RAM, just as we are used to - uses SSD, with its own controller to emulate HDD but more quickly.