From GEAR Software Knowledgebase
The read-only mode of NOR memories is similar to reading from a common memory, provided address and data bus is mapped correctly, so NOR flash memory is much like any address-mapped memory. NOR flash memories can be used as execute-in-place memory, meaning it behaves as a ROM memory mapped to a certain address. NOR flash memories have no intrinsic bad block management, so when a flash block is worn out, either the software using it has to handle this, or the device breaks.
When unlocking, erasing or writing NOR memories, special commands are written to the first page of the mapped memory. These commands are defined as the common flash interface (defined by Intel) and the flash circuit will provide a list of all available commands to the physical driver.
Apart from being used as a ROM, the NOR memories can of course also be partitioned with a file system and used as any storage device.