NOR flash memory is one of two types of nonvolatile storage technologies. NAND is the other.
Nonvolatile memory does not require power to retain data.
NOR and NAND use different logic gates -- the fundamental building block of digital circuits -- in each memory cell to map data. Both types of flash memory were invented by Toshiba, but commercial NOR flash memory was first introduced by Intel in 1988. NAND flash was introduced by Toshiba in 1989.
NOR flash vs. NAND flash
NOR flash is faster to read than NAND flash, but it's also more expensive, and it takes longer to erase and write new data. NAND has a higher storage capacity than NOR.
NAND devices are accessed serially, using the same eight pins to transmit control, addressing and data. NAND can write to a single memory address, doing so at eight bits -- one byte -- at a time.
In contrast, older, parallel NOR flash memory supports one-byte random access, which enables machine instructions to be retrieved and run directly from the chip, in the same way a traditional computer retrieves instructions directly from main memory. However, NOR has to write in larger chunks of data at a time than NAND. Parallel NOR flash has a static random access memory (SRAM) interface that includes enough address pins to map the entire chip, enabling access to every byte stored within it.
NOR flash is also more expensive to produce than NAND. That, and its random access function, mean NOR is mostly used for code execution, while NAND is mostly used for data storage.
NOR flash is most often used in mobile phones, scientific instruments and medical devices. NAND has found a market in devices to which large files are frequently uploaded and replaced, such as MP3 players, digital cameras and USB flash drives.
Some devices use both NAND and NOR flash. A smartphone or tablet, for instance, may use embedded NOR to boot up the operating system and a removable NAND card for all its other memory or storage requirements.
Major vendors and products
According to a June 2016 report by the research firm Technavio, the top five NOR flash memory vendors are Cypress Semiconductor Corp., GigaDevice Semiconductor, Macronix International Co. Ltd., Micron Technology Inc. and Winbond Electronics Corp.
Among those, Cypress Semiconductor also supplies embedded system components that use NOR flash memory for industries like automotive and the internet of things. In comparison, Micron Technologies is mainly known as a supplier of flash chips -- both NOR and NAND -- as well as DRAM, to component manufacturers.
The future of NOR flash
While NOR flash memory has traditionally been based on a parallel interface, the latest innovation is SPI NOR flash, which uses a serial peripheral interface (SPI) bus. The bus communicates data synchronously like a parallel interface, but is configured as a serial connection. It uses a clock signal to keep the incoming and outgoing data streams in sync.
SPI NOR flash, also called serial NOR flash, has become the standard because it is a much smaller form factor with fewer connector pins, but it retains the same memory density and speed as parallel NOR flash memory.