Computer memory and its types

As we all know about two memory devices RAM and ROM used in computer, below are the some terms used to describe memory and which are being given technical names. Have a look

Random access memory (RAM)--an essential component for running personal computers--determines the computer's speed. Computers possess five key functions: input, output, processing, storage and retrieval. RAM is designed for storage and retrieval functions. RAM provides a location for a computer's central processing unit (CPU) to access current information. Typically, RAM is referred to as a computer's memory.

        RAM is composed of transistors and capacitors. Transistors act as switches, while capacitors store information. Transistors and capacitors are paired and create a memory cell. Each memory cell can hold one bit of information. The capacitor loses information quickly and the central processing unit (CPU) must read the information and write the information back. As a result, you can consider RAM short-term memory. Information is stored in RAM to facilitate the transfer of information to long-term memory or to run applications and programs.

Computer memory is either volatile or non-volatile. Compared to the human mind, volatile memory is short-term while non-volatile memory is long-term. Volatile memory requires a power source to maintain the stored information. Non-volatile memory is not limited by power restrictions and retains information for extended time periods. RAM is volatile memory and, as a result, serves as a short-term storage location.

How RAM works? 

Computers use RAM to store information about currently running applications, documents and data. For example, while you are using word processing software, your CPU stores data to RAM each time you type and during auto-save. Once you are finished and save the file to a location, the information is moved to a long-term storage option like your hard disk.

DDR RAM

RAM comes in the market as DDR1 RAM, DDR2 RAM and DDR3 RAM. 

Double data rate memory is a type of computer memory built off integrated circuit technology. DDR memory achieves just under double the data rate of single data rate memory, which it superseded by using a double pumping data transfer technology. This technology doubles the data transfer rate without increasing the clock speed of the memory. 

History

DDR SDRAM (DDR1 RAM)  was the first memory type to use DDR technology. It superseded the SDRAM memory specification in terms of both speed and functionality. DDR memory was originally released with a data bus clock frequency of 100 MHz however through technological advancements and dropping research and production costs speeds of up to 200 MHz were achieved before the DDR2 memory specification became standard.

Main Memory 

Refers to physical memory that is internal to the computer. The word main is used to distinguish it from external mass storage devices such as disk drives. Another term for main memory is RAM.

This is the part of the computer that stores operating system software, software applications and other information for the central processing unit (CPU) to have fast and direct access when needed to perform tasks. It is called "random access" because the CPU can go directly to any section of main memory, and does not have go about the process in a sequential order. 

RAM is one of the faster types of memory, and has the capacity to allow data to be read and written. 

Main memory is available in two types: Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM).

DRAM: Dynamic random access memory (DRAM) is the most common kind of main memory in a computer. Dynamic random access memory is constantly restoring whatever information is being held in memory. It refreshes the data by sending millions of pulses per second to the memory storage cell.

SRAM: Static Random Access Memory (SRAM) is the second type of main memory in a computer. It is commonly used as a source of memory in embedded devices. Data held in SRAM does not have to be continually refreshed; information in this main memory remains as a "static image" until it is overwritten or is deleted when the power is switched off.

Physical Memory on a Computer

Physical memory, usually referred to as RAM or random access memory, is a form of dynamic computer storage. This memory allows the computer to store data for quick retrieval and run multiple programs at once. Unlike built-in memory, RAM is stored on circuit boards. The type and capacity of the physical memory depends on the computer's motherboard.

Volatile and Non Volatile Memory

Memory that loses its contents when the power is turned off, it is called Volatile  memory.

All RAM except the CMOS RAM used for the BIOS is volatile. ROM, on the other hand, is nonvolatile.

Basically, non volatile memory stores data without power. Two examples of devices that use non volatile memory are hard drives and CMOS chips (the chip that holds your BIOS information).

Non-volatile memory is important for long term storage of essential system information and data. 

EPROM (erasable programmable read only memory) is another form of non-volatile memory. CDs and DVDs are also a form of non-volatile memory.

Resident Memory: 

It means Permanently in memory. Normally, a computer does not have enough memory to hold all the programs you use. When you want to run a program, therefore, the operating system is obliged to free some memory by copying data or programs from main memory to a disk. 

This process is known as swapping.

Some programs are considered as being memory resident, which means that the operating system is not permitted to swap them out to a storage device; they will always remain in memory.

The programs and data used most frequently are the ones that should be memory resident. This includes central portions of the operating system and special programs, such as calendars and calculators, that you want to be able to access immediately. Another term for memory resident is RAM resident. 

Virtual Memory

Volatile memory is  also known as volatile storage in computer. This is An imaginary memory area supported by some operating systems (for example, Windows but not DOS) in conjunction with the hardware. 

You can think of virtual memory as an alternate set of memory addresses. Programs use these virtual addresses rather than real addresses to store instructions and data. 

The purpose of virtual memory is to enlarge the address space, the set of addresses a program can utilize. Virtual memory is used when the RAM is filled.

When the program is actually executed, the virtual addresses are converted into real memory addresses.

To facilitate copying virtual memory into real memory, the operating system divides virtual memory into pages, each of which contains a fixed number of addresses. Each page is stored on a disk until it is needed. When the page is needed, the operating system copies it from disk to main memory, translating the virtual addresses into real addresses. 

As multitasking operating systems emerged, virtual memory became a necessity, especially on computers with lower amounts of physical memory. The file or files used as virtual memory are sometimes called 'swapfiles' or 'pagefiles.'

Virtual memory may be implemented in an operating system in a variety of ways. Most versions of Windows store virtual memory data in a single file called "pagefile.sys." Mac OS X, being an operating system based on UNIX, uses several files, named sequentially: "swapfile0," "swapfile1" and "swapfile2," for example.

The process of translating virtual addresses into real addresses is called mapping.

The copying of virtual pages from disk to main memory is known as paging or swapping. 

The virtual memory manager might issue the message:

"Your system is low on virtual memory. Windows is increasing the size of your virtual memory paging file." This happens if it is required to swap out a page from RAM to the pagefile while all pages in the pagefile are already taken. With that message, it will allocate more space to the pagefile and use the added space to store the newly- swapped-out page (and subsequent pages).

One case that might cause the system to want to enlarge the pagefile is that too many processes are running.

Flash Memory

Flash memory refers to a particular type of EEPROM, or Electronically Erasable Programmable Read Only Memory. It is a memory chip that maintains stored information without requiring a power source. It is often used in portable electronics, like MP3 players, and in removable storage devices. 

It is non-volatile memory.