Microprocessor and Memory Basics

Other types of memory

RAM Random Access Memory. This is the memory that we have been describing up until now. It is readable and writeable, but normally "forgets" when the power is switched off. There are two main types of RAM in use, SRAM and DRAM.

SRAM is Static RAM, which has a simple interface, good storage density, speedy access and low power consumption (when not in active use). SRAM is used for fast cache memories on PC motherboards and mobile phone memories (because of its low power consumption).

DRAM has a complex interface because it needs to have its contents refreshed continuously (as it "forgets" in milliseconds). It consumes power even when not in use and is slower than SRAM. However it provides very dense storage and is frequently used as PC main memory.

ROM Read Only Memory. The contents of this memory is set when the chip is manufactured and can never be changed by the microprocessor.

EPROM Erasable Programmable Read Only Memory. This form of memory can be programmed using a special device that uses higher voltages than in a normal microprocessor circuit. EPROMs cannot be changed by the microprocessor. However, they can be erased using ultraviolet light through a small window on the top, or sometimes by applying a higher voltage again (in which case they are known as EEPROMs for Electrically Erasable PROMS).

Most ROMs today are programmable - the bootstrap memory of a PC, "memory sticks" and the SIM cards of mobile phones are examples of memory.

More long-term storage is achieved through the use of magnetic disks (the hard drive of personal computers), CDROMs, and DVDs.