ROM - Read Only Memory
Does this term mean the memory which should only be read and not written to , or the memory which can only be read ?
"ROM in PC's and laptops are usually seperate chips in the motherboard where BIOS is present (Please correct me if I am wrong) . Can they be overwritten (I guess we can) ?
When we consider an android phone , it gets really confusing . What is the difference between ROM and "internal memory" in an Android phone ? It seems the same .
ROM - Read Only Memory
True ROM is extremely rare nowadays. Most everything that used to be ROM is actually non-volatile RAM, like flash memory. However, people occasionally use the term to refer to data that could be put into ROM if we were so inclined. Usually the operating system uses the MMU to prevent any writes.
The Android operating system allows providers to install software that the user is unable to erase without rooting the device. Thus, from the user's point of view that memory is effectively read only, even though it might physically reside on the exact same chip as memory the user thinks of as read/write.
It really depends on the type of ROM which is used (type of hardware):
- Can only be read - PROM (Programmable ROM) - it is is programmed only once and can only be read.
- Usually can only be read - EPROM (Erasable Programmable ROM) - can be erased by strong UV light, but this is not a typical/easy process so it is only read usually.
- Can be read and written - EEPROM (Electrically Erasable Programmable ROM) - can be read/written like RAM but is volatile.
As Karl explained true ROM memory is very rare (and used in pretty specialized uses). Most memory that's referred to as ROM is actually re-writable these days, that does include the BIOS of pretty much every modern PC.
Also, the term "ROM" has been used to refer to the content of that ROM memory. For example, distributions of the Android operating system are often referred to as "ROMs" (ROM Manager is a popular application for managing the OS on a rooted Android device). This particular case shows that even when the system was never stored on real ROM memory, the fact that for the average user the memory is considered "read-only" leads to it being called ROM.