What is memory??? Computer memory refers to devices that are used to store data or programs (sequences of instructions) on a temporary or permanent basis for use in an electronic digital computer . Computers represent information in binary code , written as sequences of 0s and 1s. Each binary digit (or "bit") may be stored by any physical system that can be in either of two stable states, to represent 0 and 1. Such a system is called bi stable.
In the early 1940s, memory technology mostly permitted a capacity of a few bytes. The first programmable digital computer , the ENIAC , using thousands of octal-base radio vacuum tubes , could perform simple calculations involving 20 numbers of ten decimal digits which were held in the vacuum tube accumulators .
The next significant advance in computer memory was with acoustic delay line memory developed by J. Presper Eckert in the early 1940s. Through the construction of a glass tube filled with mercury and plugged at each end with a quartz crystal, delay lines could store bits of information within the quartz and transfer it through sound waves propagating through mercury. Delay line memory would be limited to a capacity of up to a few hundred thousand bits to remain efficient.
Two alternatives to the delay line, the Williams tube and Selectron tube , were developed in 1946, both using electron beams in glass tubes as means of storage. Using cathode ray tubes , Fred Williams would invent the Williams tube, which would be the first random access computer memory .
Volatile memory , also known as volatile storage , is computer memory that requires power to maintain the stored information . It is of two types 1. D-RAM( Dynamic random access memory ) 2. S-RAM( Static Random Access Memory )
Dynamic random access memory ( DRAM ) is a type of random access memory that stores each bit of data in a separate capacitor within an integrated circuit . Since real capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically.
Static Random Access Memory (SRAM) is a type of semiconductor memory where the word static indicates that, unlike dynamic RAM (DRAM) , it does not need to be periodically refreshed , as SRAM uses bistable latching circuitry to store each bit. SRAM exhibits data remanence ,but is still volatile in the conventional sense that data is eventually lost when the memory is not powered.
Non volatile memory
Non-volatile memory ( NVM) or non-volatile storage , is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory , flash memory , most types of magnetic computer storage devices (e.g. hard disks , floppy disks , and magnetic tape ), optical discs , and early computer storage methods such as paper tape and punched cards . It is of three types
A programmable read-only memory ( PROM ) or field programmable read-only memory ( FPROM ) or one-time programmable non-volatile memory ( OTP NVM ) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse . Such PROMs are used to store programs permanently. The key difference from a strict ROM is that the programming is applied after the device is constructed.Once inputted data cant be deleted.
An EPROM , or erasable programmable read only memory , is a type of memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile . It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in digital circuits. Once programmed, an EPROM can be erased by exposing it to strong ultraviolet light from a mercury-vapor light source. EPROMs are easily recognizable by the transparent fused quartz window in the top of the package, through which the silicon chip is visible, and which permits exposure to UV light during erasing.
EEPROM stands for E lectrically E rasable P rogrammable R ead- O nly M emory and is a type of non-volatile memory used in computers and other electronic devices to store small amounts of data that must be saved when power is removed, e.g., calibration tables or device configuration.
When larger amounts of static data are to be stored (such as in USB flash drives ) a specific type of EEPROM such as flash memory is more economical than traditional EEPROM devices. EEPROMs are realized as arrays of floating-gate transistors .