Flash memory#NOR memories

NOR Flash refers to a type of flash memory, which consists of NOR gates. NOR memory has been developed as a replacement for flexible EPROM, PROM or ROM. NOR flash is addressed via a classical address and data bus interface and can be connected without additional glue logic to the bus system of a controller.

Construction

As can be randomly accessed during read access to all bytes must be considered for write access, the bytes are grouped into blocks. Although the individual bytes can be written independently of each other (however, only a transfer of the bits from 1 to 0 is possible), but on the other hand, an erase operation is always only one complete block applicable. All bits in the bytes are reset to 1. Depending on the block type used is possible during the erase operation can also access other blocks. The granularity of the block size is usually 64 KB - however, it varies from device to device, and sometimes within a block: Some flash ROMs have to reduce the memory requirements for applications with boot code in the lower or upper memory blocks of smaller size.

NOR Flash's own technological reasons for the same structure size smaller storage capacity than NAND flashes because its memory cells require more space on the die. NOR flash memory are usually completely free of errors when delivered, unlike NAND flash memory where defective memory blocks up to a certain number per chip are common. NOR flash cells are generally designed for more than 100,000 write and erase cycles.

Areas of application

NOR Flashes are used mainly for storing boot code or the firmware of a computer system.

Benefits

• Linearly addressable memory, allow code to run (execute in place)
• High write speed for small amounts of data
• Easy connection to controller systems due to the SRAM -like bus system

Disadvantages

• Relatively high power
• Slow writing and deleting large amounts of data
• Only for relatively small storage capacities (up to 1 gigabit ( Sept. 2009) )