Types of Computer Memory - RAM, ROM, DRAM, SRAM, PROM, EPROM

Memory in a computer is any hardware that can store data for any amount of time. If a computer memory is volatile, the information stored on the hardware is lost after the power source is disconnected. In contrast, non-volatile computer memory retains the stored information. Under these two broad categories are many subtypes of memory.

RAM (Random Access Memory)

RAM allows the user to access any part of the memory in the same amount of time. It is used to store the programs and data being used by the central processing unit (CPU) in real time. The data can be read, written, and erased any number of times.

DRAM (Dynamic Random-Access Memory)

DRAM is a volatile memory. The information is lost within a couple of seconds of the system shut off. The bits of the memory are stored in the small circuit.  In DRAM there is just one transistor and one capacitor. Capacitors slowly lose some of their charge, so a timer circuit delivers an electrical charge to the capacitor every few milliseconds.

SRAM (Static Random-Access Memory)

SRAM loses the data almost instantaneously after system shut off. A SRAM circuit is arranged in a flip-flop design which maintains the correct charge state for the entire time that the memory is connected to power. Although SRAM is by design the faster type of memory, it does come with a higher price tag than DRAM and is therefore not as popular.

ROM (Read Only Memory)

ROM is a type of non-volatile computer memory. It usually comes in the form of a chip located on the motherboard.  Data is not lost after the system is shut down. ROM is used for firmware such as system startup programs. You cannot add or modify this type of memory. Manufacturers write the memory; however, subtypes of ROM have been developed to allow user modification.

PROM (Programmable Read Only Memory)

The data is written after the memory chip has been created. The setting of each bit is locked by a fuse or antifuse, which means that PROM can only be programmed once after creation before the information becomes permanent.

EPROM (Erasable Programmable Read Only Memory)

As the name suggests, EPROM can be erased and rewritten. EPROM is used for applications where the data needs to be frequently changed. It can be identified by the quartz crystal window that exposes the chip to ultraviolet light used to reprogram the chip.

EEPROM (Electrically Erasable Programmable Read Only Memory)

This type of memory can be erased and reprogrammed repeatedly through the application of higher than normal electrical voltage. It is used for storing minimal data quantities that need to be changed regularly. The nature of this type of memory results in a shorter life span than other computer memories.

Flash Memory

Flash memory falls into the category of non-volatile memory. Invented by Toshiba, flash memory is a specific type of EEPROM that is programs and erases data in terms of blocks. Flash memory is used for easy and fast information storage in computers and various other electronic devices. It is a portable memory that acts almost like a hard drive.


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