A few months ago, I talked about computer memory and its various types. One of the important types is Read Only Memory (ROM). This contains the basic instructions (including the instruction that is executed first when power is applied to the device) to bring up all of the supporting programs (Operating System, etc.) that allow you to do what you want to do.
What happens if you have a device (such as a cellular phone or a game system) that is "updatable"? This device also has ROM but, somehow, the basic system that is in ROM is still able to be changed.
This is possible because there are different types of ROM. In particular, there is a type called "Electrically Erasable Programmable Read-Only Memory" (EEPROM). Like general ROM, this memory is non-volatile -- it will retain its contents even when there is no power. However, by applying a higher-than-normal power through the device, the contents can be erased and then new contents can be written. Thus, a device which is meant to be upgradable can split its base program memory into two parts -- one in ROM which still contains the initial program upon powering up the device and one in EEPROM which should not normally be altered either during use or when powered down.
The program in ROM is enhanced to include the program(s) that allow updating the EEPROM. Then, when an update is desired, it stores the new system program to be written to the EEPROM in some type of RAM, erases the original contents of the EEPROM and then copies over the new system program into the EEPROM.
There are variations on this, of course. When a device is updated via a host computer, there is the option to back up the data and current contents before doing the update. This reduces the danger involved in case power is interrupted, or an error occurs, before the update can complete. The time when an update is in progress is still a critical period of time and, if at all possible, the device should remain powered up until updating is complete.
Once the update is complete, the device should operate the same as before (with whatever improvements exist within the new system program).
