Computer Memory

Computer memory is where computer stores its information, it's a very important concept in computing but sometime we get confused about the types of memory and it's use in computers. In this article we are going to know about different types of memory with example. We will focus on primary memory of computer.

Computer memory is mainly divided in to two types.

1. Primary Memory
2. Secondary Memory

Primary Memory:

Primary memory is computer memory that is accessed directly by the CPU. This includes several types of memory, such as the processor cache and system ROM.Primary Memory is again divided in to two kinds. They are volatile(Ram) and Non-volatile memory(Rom).

1. RAM
2. ROM

RAM (Random Access Memory) is an kind of volatile memory.Volatile memory are those which stores information temporarily. The programs and data that the CPU requires during execution of a program are stored in this memory. It looses the all the data when power supply is discontinued to the computer. 

RAM can be classified into the following types:

• DRAM (Dynamic Random Access Memory).
• SRAM (Static Random Access Memory)
• DRDRAM (Direct Rambus Dynamic Random Access Memory)

DRAM or Dynamic random-access memory is a type of random access semiconductor memory that stores each bit of data in a memory cell consisting of a tiny capacitor and a transistor, typically a MOSFET. Since capacitor can store some charge, this memory cell needs to be refreshed at a certain interval to keep the data alive. Generally RAM used in computer and other digital devices like phone is DRAM (More specifically Synchronous DRAM or SDRAM).

SRAM or Static Random Access Memory refers to the memory type that retains the data as long as the power is supplied. It does not use capacitor like DRAM so it does not need to be refreshed. It is mainly constructed with 4-6 transistor. SRAM provides faster access to data and are very expensive. The cache memory in computer system is a good example of SRAM.

Key differences between DRAM and SRAM:

1. DRAM is available in larger capacity where SRAM is found in smaller capacity.

2. SRAM is expensive whereas DRAM is cheap.

3. The cache memory is an example of SRAM and the main memory is an example of DRAM.

4. SRAM is faster than DRAM

5. Power consumption is higher in DRAM than SRAM.

Read Only Memory (ROM) are type of non-volatile memory that retains data stored in it even after power supply is lost. ROM is used to permanently store the data in it which often do not need any change. The computer firmware (firmware is a kind of low level program that bridges the gap between the hardware and higher level software) is stored in ROM.

There are mainly three kinds of ROM. They are:

• PROM (Programmable Read Only Memory)
• EPROM (Erasable and Programmable Read Only Memory)
• EEPROM (Electrically Erasable and Programmable Read Only Memory)

ROM usually means "Mask programmed Read Only Memory or simply masked ROM" and refers to those memory chips where the contents are actually built onto the chip and can never be changed by the user. This type of ROM are the first of these kinds of devices to become available.

1. PROM (Programmable read-only memory) – It can be programmed by user only once. Once programmed, the data and instructions in it cannot be changed. PROM is a way of allowing a user to tailor a microcode program using a special machine called a PROM programmer .Typically, each bit of memory is stored with “fusible links,” and you “burn” the fuses to set that bit to a 1 or 0 using a programmer.The PROM chip was commonly used in earlier computers’ BIOS systems.

2. EPROM (Erasable Programmable read only memory) – This was an improvement over PROM. Where we can reuse the ROM by programming it over and over again. We could erase the content of the ROM by exposing the EPROM to UV-light through some medium like glass and keep it in a black-box for a period of time to erase all of the content stored in it and then reprogram it. To keep the data in the ROM being accidentally deleted by the UV-Light of sun it used to be kept in a foil paper. EPROM is used more commonly than PROM because it allows the manufacturers to modify or reprogram the chip.

3. EEPROM (Electrically erasable programmable read only memory) – This type of ROM is used in computers, micro-controllers and other devices to stores small amount of data. It allows data to be erased and reprogrammed electrically. Improved version of EPROM. The data in it could be erased and reprogrammed without using UV-Light. It was done electrically. So we need not remove it from the board where it is installed to erase and reprogram it.

Flash Memory: Flash memory is an improved version of EEPROM where we could modify each of its bit individually without needing whole of the data being wiped out and rewrite it. Making it possible to add normal memory to a circuit, and have it non-volatile, keeping its contents between uses, and without power. It was immediately used to store images from digital cameras, and then smartphones, in those little SD, Micro-SD cards. Flash memory is also the basis for SSD (Solid State Drive) which are used in the new laptops, tablets, and smartphones to provide permanent memory with no moving parts, and in a smaller size, than the hard disks they replace.

Sometimes we get confused when people refers to Mobile phone ROM. Technically we can call it ROM because it's kind of flash memory and it originates from EEPROM. The operating system on this ROM resides in a special area which is kept hidden and normally not accessible by the user. 

Secondary Memory:

Secondary Memory: Secondary memory are those storage devices where data is stored for a longer period of time. Magnetic Disks - Hard disk, Magnetic tapes, Optical Disks (CD, DVD) , Floppy disks are all secondary storage devices or secondary memory.

Sometimes RAM is classified as primary memory and ROM is called as secondary  memory. Hard disks, CD, Floppy disks etc are just simply called storage device.