Serial EEPROM's come in a range of capacities from a few bytes to over 128 kilobytes and are typically used to store configuration parameters, and in modern microcontrollers they replace the hitherto nonvolatile memory. For example, in personal computers these chips are used to store the BIOS code and related system settings. Unlike EPROM's, EEPROM's can be programmed and erased electrically. This is the main advantage. EEPROM's are still limited in that repeated write and erase cycles eventually damage the thin insulating layer. Some early EEPROMs could only perform about 1000 erase-write cycles.