all-flash array (AFA) M.2 SSD

EEPROM vs. flash memory: What's the difference?

Even though there might be a 10 million-to-one price-per-gigabyte difference between EEPROM and NAND flash, each memory type makes sense in its own role.

Electrically erasable programmable read-only memory and flash are both non-volatile -- or persistent -- technologies. However, don't confuse EEPROM, which is often called E2PROM or simply E2, with flash memory.

A major difference in comparing EEPROM vs. flash memory is how admins manage them.

EEPROM uses two transistors per bit, and flash uses only one. The extra transistor enables a program to change the contents of a memory location 1 byte at a time. Since flash doesn't have this extra transistor, it is erased in blocks that range from 32 KB to 128 MB or larger. Block erase is clumsy to use, but it makes a flash chip half as large as an EEPROM chip, and that translates into a disproportionally large cost difference.

EEPROM generally sells for about 10,000 times the price of 1 byte of NOR flash and 10 million times that of NAND flash. Each memory type has a market that makes sense, despite the huge price differences.

NOR vs. NAND flash

The inventors of NAND noticed that there was great interest in using a chip that was a lot cheaper than EEPROM, so they invented NOR flash. NOR must be erased before being written to, and the erases must be performed in large blocks, both of which saved considerable die area on the chip to make it much cheaper than EEPROM. Although these two attributes made NOR much clumsier to use than EEPROM, its low cost led to its rapid adoption, and it was designed into a large number of high-volume systems.

A major difference in comparing EEPROM vs. flash memory is how admins manage them.

These same inventors decided that they should find a way to make the chip even cheaper, even if it was clumsier. This was an excellent strategy. NAND flash revenues are about 50 times higher than revenues in the NOR flash market, which is well over twice the size of the EEPROM market.

A matter of size and price

Memory chip people speak a different language than storage admins. Where storage devices are specified in bytes, memory chips are expressed in bits. It's a habit that has been difficult to change. Memory chip folks don't speak of capacities the way storage people do; they say that the size of a memory chip is its density.

EEPROM chips sell in densities from 128 bits to 32 Mb. While that lowest number -- 16 bytes -- may seem unrealistically small to a storage admin, it's a good size to store device settings, serial numbers, IP addresses or small data logs in applications with tiny needs, such as power meters and automobile odometers. EEPROM plays the role of "the chip that costs a dime." Few EEPROM chips command even a $1 price.

NOR densities range from 256 bits up to 2 Gb, which is also small. NOR is good at random read accesses, so it has a longstanding foothold in microcontroller applications, where it's used for code storage. Microcontroller firmware tends to be small. NOR chip prices range from below $1 to about $10.

NAND provides mass storage in semiconductor form, which means it must have a much higher density than EEPROM or NOR flash. NAND flash chips today range from 1 Gb to 1 Tb. Packages with stacked chips -- up to 16 high -- can store 16 Tb, or 2 TB. Chips generally sell for $1 and up. That 16 Tb chip would command a price north of $40.

Chart comparing EEPROM to NOR and NAND flash
A rough comparison of EEPROM, NOR flash and NAND flash from the perspectives of price per gigabyte, density and chip price

EEPROM vs. flash memory uses

Where will the storage administrator run into these technologies?

  • NAND flash is the storage that is used in all SSDs, USB flash drives and smartphones. It's the number an admin gives to describe the amount of storage.
  • NOR flash is invisible to anyone other than the folks who design firmware. If a server, PC BIOS, bridge or router needs to be updated, then that will be NOR. But admins are probably unlikely to know that it's also often in HDDs and numerous items that they never thought might contain a processor, such as a TV or a USB keyboard, or just about any of the "things" connected to the internet of things.
  • EEPROM stores serial numbers and other unique information in almost anything connected to Ethernet, as well as the chip in a charge card, the inkjets in a printer and even the calibration tag for a blood glucose monitor.

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