As pito and me wrote: external EEPROM. They are small, can store many data and are cheap. But you have to use a function to store data and retrieve data. They are not in an array in SRAM.If your project is like my projects, you keep on adding things. So you need to store more and more data.For example a AT24C64, 8kbyte EEPROM costs 3 dollars on Ebay.A 24C512, 64kbyte EEPROM costs 3.50 dollars. You can use 4 of these on the same I2C bus.A 24CM01, 128kbyte EEPROM costs 15 dollars on Ebay. That is weird, two 24C512 are cheaper, and the manufacturer price is less than 2 dollars.