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Types of computer memories

Les types de mémoires

Types of memories :

1- RAM :

The RAM, generally called RAM (Random Access Memory), is the main memory of the system that is to say, that it is a space allowing to temporarily store data during program execution.

In fact, unlike the storage of data on a mass memory such as the hard disk, the random access memory is volatile that is to say, that it only allows data to be stored as long as it is electrically supplied. Thus, each time the computer is turned off, all the data present in memory is irretrievably erased.

There are generally two main categories of RAM :

  • Dynamic memories (DRAM, Dynamic Random Access Module), inexpensive. They are mainly used for the central memory of the computer
  • Static memories (SRAM, Static Random Access Module), fast and expensive. SRAMs are used in particular for processor cache memories;

2- Read-only memory :

Read-only memory (ROM) is a type of memory used to read the information contained therein even when the memory is no longer supplied with electricity. Basically, this type of memory can only be accessed in reading mode. However, it is now possible to record information in certain ROM types of memories.

3- Flash memory :

Flash memory is a compromise between RAM type memories and read-only memories. Flash memory has the non-volatility of read-only memories while being easily accessible to reading or writing. In return, the access times of flash memories are greater than those of random access memory.

4- Operation of the RAM :

RAM is made up of hundreds of thousands of small capacitors that store charges. When it is charged, the logic state of the capacitor is equal to 1 otherwise, it is 0, which means that each capacitor represents a bit of the memory.

Since the capacitors discharge, they must be constantly recharged (the exact term is refresh, in English refresh) at a regular time interval called the refresh cycle. DRAM memories require, for example, refresh cycles of approximately 15 nanoseconds (ns).

Each capacitor is coupled to a transistor (MOS type) allowing to "recover" or modify the state of the capacitor. These transistors are arranged in the form of a table (matrix) that is to say, that one accesses a memory box (also called memory point) by a line and a column.

Therefore a computer with a high frequency and using memories whose access time is much longer than the cycle time of the processor must perform wait cycles (in English wait state) to access the memory. In the case of a computer clocked at 200 MHz using memories of the DRAM types (whose access time is 60 ns), there are 11 waiting cycles for a transfer cycle. The performance of the computer is reduced as much as there are waiting for cycles, so it is advisable to use faster memories.

5- Read-only memory :

Read-only memory (ROM) There is a type of memory allowing to store data in the absence of electric current, it is about ROM (Read Only Memory, whose literal translation is read-only memory) called read-only memory, sometimes non-volatile memory because it does not erase when the system is turned off.

This type of memory makes it possible in particular to conserve the data necessary for starting up the computer. Indeed, this information cannot be stored on the hard disk since the parameters of the disk (essential to its initialization) are part of this data vital to booting.

Different ROM type memories contain data essential for start-up, that is to say :

  • The BIOS is a program used to control the main I / O interfaces of the system, hence the name BIOS ROM sometimes given to the read-only memory chip on the motherboard that hosts it.
  • The boot loader: a program allowing to load the operating system in (RAM) memory and to launch it. This generally searches for the operating system on the floppy drive, then on the hard disk, which allows you to be able to launch the operating system from a system floppy in the event of a malfunction of the system installed on the disk. hard.
  • The CMOS Setup is the screen available when the computer is turned on allowing you to modify the system parameters (often wrongly called BIOS ...).
  • The Power-On Self Test (POST), a program executed automatically when the system is booted, allowing you to do a system test (this is why, for example, you see the system "counting" the RAM at startup).

ROM types :

The ROMs have gradually evolved from read-only memories frozen to programmable memories, then reprogrammable.


The first ROMs were manufactured using a process that directly records binary data in a silicon wafer using a mask. This process is now obsolete.


PROMs (Programmable Read-Only Memory) was developed in the late 1970s by the firm Texas Instruments. These memories are chips made up of thousands of fuses (or diodes) which can be "burned out" thanks to a device called "ROM programmer", applying a high voltage (12V) to the memory boxes to be marked. The fuses thus are blown correspond to 0, the others to 1.


EPROMs (Erasable Programmable Read-Only Memory) are PROMs that can be deleted. These chips have a window allowing ultraviolet rays to pass through. When the chip is in the presence of ultraviolet rays of a certain wavelength, the fuses are reconstituted that is to say, that all the bits of the memory are again at 1. It is for this is why we call this type of PROM erasable.

Electrically Erasable Read Only Memory (EEPROM) are also erasable PROMs, but unlike EPROMs, they can be erased by a simple electric current, i.e. they can be erased even when they are in a position in the computer.

6- Flach memory :

The flash memory is a semiconductor memory, non-volatile and rewritable that is to say, a memory having the characteristics of a random access memory but whose data do not volatilize during a power off. Thus the flash memory stores the data bits 60 in memory cells, but the data is kept in memory when the power supply is cut off.

Because of its high speed, durability, and low power consumption, flash memory is ideal for many applications - such as digital cameras, cell phones, printers, PDAs, laptops, or audio playback or recording devices such as mp3 players. In addition, this type of memory has no mechanical elements, which gives them great impact resistance.

Types of memory cards :

There are a large number of formats of memory cards that are not compatible with each other, carried by almost as many manufacturers. Among these most common memory card formats are :

  • Compact Flashcards
  • Secure Digital cards (called SD Cards)
  • Memory Stick cards • SmartMedia cards
  • MMC cards (MultiMediaCard)
  • xD picture cards

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