TYPES OF RAM AND ROM

In this article, we will take a deeper look at different types of RAM and ROM. For more information on memory technologies, refer to the article "An Introduction to Memory Technologies" on this blog.

RAM

There are two types of volatile memory that come under RAM, these are as follows:

SRAM(Static Random Access Memory)
DRAM(Dynamic Random Access Memory)

There are many combinations and next-generation memory components that build on these two technologies. But it’s important to understand the basics of SRAM and DRAM before delving into newer technologies built on top of them.

SRAM

Static RAM is a random access memory type that retains information as long as power is provided to the SRAM. It does not have to be periodically refreshed. SRAM is volatile but if the system is powered, SRAM retains data values without recharging its cells.

But why is it termed as ‘Static’? This is because the data is held statically without any need of refreshing, i.e. the information in the memory is retained in the memory as long as power is supplied.

SRAMs typically uses six transistors per bit to prevent the information from being disturbed when read. SRAM needs only minimal power to retain the charge in standby mode.

SRAM cell

Image courtesy: https://en.wikipedia.org/wiki/Static_random-access_memory

The above figure is of a typical SRAM cell made up of six MOSFETs. Each bit in an SRAM is stored on four transistors (M1, M2, M3, M4) that form two cross-coupled inverters. The two storage cells denote two stable states 0 and 1.As there exist 6 transistor cells there are many more, for example, the other kinds of SRAM chips use 4, 8, 10 (4T, 8T, 10T SRAM), or more transistors per bit.

SRAM consists of 3 states if operations which are : standby(it stays idle) , read(takes valid input ) and lastly write(updates the content).

DRAM

The Dynamic RAM, also called DRAM, is the most common type of RAM in the computer. It is termed as ‘Dynamic’ because the system needs to be activated frequently, or made ‘dynamic’ so that it retains its information. DRAM is considered a volatile memory.

DRAM uses a capacitor to store each bit of data, and the level of charge on each capacitor determines whether that bit is a logical 1 or 0. DRAM uses a capacitor to store each bit of data in it. The capacitors do not hold their charge forever, thus the data needs to be refreshed periodically in order to maintain the data it's holding. As a result of refreshing it periodically, it gains its name of being a dynamic RAM. DRAM is the form of semiconductor memory that is often used in equipment including personal computers and workstations where it forms the main RAM for the computer.

ROM

PROM

The Programmable Read-Only Memory (or the PROM) is a type of ROM that can be programmed only once after its manufacturing. The creation of ROM chips was not only time consuming but also expensive, which led the developers to create a PROM.

After the initial programming, no other information can be altered, and the information written on the PROM is permanent. This memory is also described as FPROM (Field Programmable read-only memory) or an OTP (One-Time Programmable) memory.

Typically PROM cells are initially is charged to 1, in order to change a cell to 0 you use a programmer to send a specific amount of current to the cell. Thus, this process of programming the PROM memory is known as burning. To burn information into a PROM, we need to provide a file containing the required contents to be entered into the PROM. A Gang Programmer/Gang Burner then configures each connection as presented in the file.

If a PROM faces an error it's simply discarded by replacing it with a new chip. An example of a PROM is a computer BIOS(Basic Input/Output System) in early computers. Today, PROM in computers has been replaced by EEPROM.

A Gang Programmer/Burner

Image courtesy: https://www.computerhope.com/jargon/p/progrrom.htm

EEPROM

The Electrically Erasable Programmable Read-Only Memory, also recognized as the EEPROM or the E²PROM, is another type of ROM which is extensively used in computing systems, which is considered as non-volatile memory. This type of ROM is not only programmed electronically but also erases the information in the memory electronically. The data stored in EEPROM is not lost even if the power is turned off. Unlike the PROM and therefore the EPROM chips, the EEPROM memory chips needn't be removed for programming, thus eliminating any possible delays of correcting or updating the info contained in the memory.

The problem which arises with EEPROM is that the entire contents of the memory need to be rewritten, and not selective erasure. While reprogramming EEPROM the number of times it is altered is limited. Modern EEPROMs can be programmed around a few million times. EEPROM was a replacement for PROM and EPROM chips.

Image courtesy:https://www.tech-faq.com/eeprom.html

EPROM

This is an Erasable Programmable Read-Only Memory. These semiconductor devices are often programmed then erased at a later time. This is normally achieved by exposing the semiconductor unit itself to ultraviolet. The EPROM chip is placed under UV light for some time, ranging between 5 to 30 minutes, after which the memory can be rewritten. An EPROM comes with a small quartz circular window which allows the UV rays to reach the chip.

The PROM stores its data as a charge on a capacitor. There is a charge storage capacitor for every cell and this will be read repeatedly as needed. The EPROM can be reprogrammed only for a limited number of erasures, as excessive erasing makes the EPROM unreliable. However, it's found that after a few years the charge may leak away and therefore the data could also be lost. Nowadays EPROM chips are not used in the computer, and these EPROM chips replaced by EEPROM memory chips.

Image courtesy: https://technobyte.org/memories-digital-electronics/

Flash Memory

Flash memory is an electronic non-volatile memory data-storage medium that will be electrically erased and reprogrammed. Flash memory retains data for an extended period of time, regardless of whether a flash-equipped device is powered on or off. Flash memory could also be considered as a development of EEPROM technology. Data are often erased in units called blocks and rewrites the data at the byte level.

Flash memory devices use two different logical technologies -- NOR and NAND -- to map data. NOR flash provides high-speed random access, reading and writing data in specific memory locations; it can retrieve as little as one byte. NAND flash reads and writes sequentially at high speed, handling data in small blocks called pages. Since NOR deals with a small amount of memory, it consumes more power than the NAND flash memory for writing data. Sometimes devices such as digital cameras incorporate both the NAND and NOR flash memories.

Flash memory is usually found in USB drives, iPods, MP3 players, and lots of more portable electronics. Flash memory tends to be advantageous by being the least expensive form of semiconductor memory, as well as being portable and causing lesser damage to the components by being susceptible to extreme changes in temperature and pressure.