Memory classification

Memories are divided into several families and subfamilies

They allow you to temporarily store data currently in use by the processor. They are fast, but lose their contents when they are no longer powered.

Dynamic memory (DRAM): This technology relies on the capacity of a capacitor to store electrical energy to conserve a bit. This technology makes it possible to obtain a very high integration density. Its main fault is linked to electrical leakage currents which, even very weak, gradually discharge the capacitor over time. To overcome this undesirable phenomenon, it is essential to maintain the electrical charge periodically (every 1 to 20 milliseconds, or 500 to 1,000 times per second) by recharging the capacitor. This operation is called refreshing.
High Bandwidth Memory (HBM ): New generation of DRAM which returns to more efficient bases by reducing the frequency but widening the communication channels. Used primarily in graphics cards and processors, it has high bandwidth, allowing faster data transfers between the processor and memory. The HBM uses a 1024-bit interface which means it can transfer more data simultaneously, improving performance. . It is based on stacking technology to achieve higher memory density while reducing the space occupied on the card. It is promised to be widely disseminated thanks to the development of AI.
Static memory (SRAM): Faster than DRAM (because it requires almost no refreshing), but more expensive and less dense, it is often used as cache memory: Located between the processor and RAM, it accelerates access to frequently used data.

ROM memory (Read Only Memory): Read-only but non-modifiable memory (impossible to write to it). The content is written into the component during its manufacture. An example is simply commercial audio compact discs which are simply readable CDs, hence their name “CD–ROM”.
PROM memory (Programmable Read Only Memory):Programmable read-only memory. The manufacturer of the memory component does not write anything into it, and leaves the possibility of writing a program or data only once which will no longer be modifiable or erasable later. This process is called “programming”.
EPROM memory (Erasable Programmable Read Only Memory): Programmable and erasable read-only memory. Same as PROM but with the ability to erase content using UV radiation. Erasing and programming operations are carried out on a special bench outside the system.
EEPROM or E²PROM memory ( Electrically Erasable Programmable Read Only Memory): Electrically erasable and reprogrammable read-only memory, it stores permanent data. Electrical pulses are sent to the EEPROM, causing it to be written or erased.
FLASH memory: Commonly used in USB sticks and SSDs but also to store the BIOS of machines, it retains data even without power supply. It is based on EEPROM technology, but has a much higher density.

Capacity or size: it corresponds to the number of information it can contain.</ span>
Access time: This is the time necessary to access the information in memory, often expressed in nanoseconds (ns) or billionths of a second.
Volatility: it characterizes the permanence of information in a memory, that is to say the amount of time that memory is able to reliably retain information.
A so-called volatile memory loses its contents when the power is cut, it therefore needs a constant supply of electrical energy to preserve its information. For example the computer’s working memory.
The memory cycle: this is the minimum time elapsed between two successive accesses to the memory. It is longer than the access time, because the proper functioning of the memory requires some maintenance operations, stabilization of the signals in the circuits, synchronization, etc.

Classification of memories