ROM and RAM
Read Only Memory
Read only memory or "ROM" can be written to once, and then only read from thereafter. The benefit to this type of memory is that information can be accessed from it very quickly since there is no need to refresh the data since it's permanently stored, but the information cannot be changed or it is very difficult to change once it is written to the device. ROM memory will retain its information even when the power is turned off.
ROM memory is well suited for storing information that doesn't change, but is accessed frequently. In early computers, the BIOS (Basic Input/Output System) were stored on ROM chips, thus giving them the name "ROM Bios". Some machines even fit their entire operating systems on ROM chips (such as ROM Basic). The BIOS (Basic Input/Output System) is critical to the operation of a computer, and it is important that the BIOS remain safe and readable.
The drawback to the ROM BIOS was that it could not be upgraded very easily, and you couldn't store dynamic information. To allow for the storage of user set variables, such as hard drive parameters, and the date and time, a standard RAM chip would be used in conjunction with the ROM BIOS chip.
This CMOS (Complementary Metal Oxide Semiconductor) chip was just a basic RAM chip that was used to store user settings and keep clock data. A small battery plugged into the system main board powered the CMOS when the system was off so that it would correctly store the user settings and date/time information.
Programmable Read Only Memory
Only one major difference between PROMs and ROMs exist. Where ROMs were produced with information already encoded on the chips, a PROM, begins its life as a blank slate, with no information encoded onto it (actually, it's all 1s in the beginning). These blank chips were then later "burned" with the desired information using a specially designed ROM programming tool (also called a ROM Burner).
PROMs represented a great leap over standard ROMs, but this was still not enough. While PROM technology greatly eased chip program design changes and corrections, you still needed to replace the old chip if you wanted to upgrade the PROM. This required a special chip extraction tool, and a steady hand to safely remove the old chip without damaging the socket.
The next major improvement came in the form of the EPROM (Erasable/Programmable Read Only Memory). EPROMs were produced exactly like a regular PROM, starting life as a blank chip, and that was later programmed with a special programming tool. Unlike regular PROMs however, exposing the chip to ultraviolet light could erase an EPROM so that it could be rewritten.
There was also still the need for the physical removal of the chip to replace, so the danger of causing physical damage to the socket was still a major possibility.