Random Access Memory (RAM)
Random memory means that data can be read at any given moment regardless of its address or the hardware chip that contains it. RAM can also access one piece of data regardless of what has preceded or followed it. The randomness at which RAM allows software to set and retrieve data is what characterizes RAM from other types of memory.
Dynamic Random Access Memory (DRAM)
RAM comes in two varieties: static RAM (SRAM) and dynamic RAM. Dynamic RAM (DRAM) is most commonly used for system memory in a computer. It is dynamic because it has to be constantly refreshed, meaning that for every clock cycle, the data in the RAM is read and then re-written. DRAM uses a transistor and a capacitor to store data. If the data weren’t continually refreshed, the capacitors would leak their charge and the data would be permanently lost.
DRAM can be asynchronous, meaning that it responds to changes in state immediately. Another type of DRAM, synchronous (SDRAM), changes state according to the computer’s system clock. Because of the small footprint of each bit, DRAM is produced in very high density, providing for the large amount of memory capacity that can fit on just a few chips on a memory module.
Static Random Access Memory (SRAM)
Because SRAM uses six transistors for each bit, it is not as dense, but it is very fast, making it ideal for CPU caches as well as for hard drive and printer caches. Network routers, broadband modems, optical drives and other devices also use SRAM as data buffers. Because it is static, it does not need to be refreshed by a clock and can retain its contents as long as it has power from the system or a battery.
Read Only Memory (ROM)
Programmable chips called ROM are used in computers to store important programs such as Basic Input Output Systems (BIOS) that tell the computer where to look for peripherals so an operating system can load. This type of program is often referred to as firmware, and can usually be changed only through the use of special software programs. ROMs are found on computer motherboards, graphics cards, network routers, and other peripheral devices.
Non volatile memory, often called flash memory, is able to retain data even when no power is applied to it. This type of memory emulates storage devices such as hard drives, but often operates at speeds faster than a hard drive.
While flash memory is often found in a variety of embedded electronic circuits, most people recognize flash memory as a USB device used for portable storage. Solid state drives (SSD) use high density flash memory to replace mechanical hard disk storage, giving computers significantly better performance when a lot of data reads and writes are required.