lecture 15

9.5 Read only memories:
These are memories in which data can only be read; in fact, once data are stored in the memory, they are saved permanently and are not supposed to be often modified. However, there are some types of read only memories in which data can be written and read several times. There are many types of read only memories:
MROM: Read Only Memory programmable by Mask. Data are written during the process of fabrication of the memory using a mask, according to the specifications of the client (program given by the client).
PROM: Programmable Read Only Memory. This type of memory is programmed by the user himself (not by the manufacturer as for the MROM). However, once a PROM is programmed, it can no more be modified.
EPROM: Erasable Programmable Read Only Memory. This memory can be erased several times by the user and reprogrammed. The inconvenient of this type of memory is that not part but the entire program has to be cancelled anytime the used want to modify the program.
EEPROM: Electrically Erasable PROM. This type of memory can be erased several times by the user. Its major advantage is that; it allows the programmer to modify the program without be obliged to cancel the entire program as it is the case for the EPROM.
9.6 Random Access Memory:
Data can be written and read in random access memories as long as they are supply by an electrical voltage. There are two principal types of RAM: Static RAM and DRAM. A static RAM save data as long as it is supplied by an electrical voltage, without any need of rewrite the information periodically. SRAM should be periodically refreshed in order to save data stored within it.