Computer Memory

Computer Memory

It refers to the internal storage areas in the computer. The term memory identifies data storage that comes in the form of chips, and the word storage is used for memory that exists on tapes or disks. Moreover, the term memory is usually used as a shorthand for physical memory, which refers to the actual chips capable of holding data. Some computers also use virtual memory, which expands physical memory onto a hard disk.

Every computer comes with a certain amount of physical memory, usually referred to as main memory or Random Access Memory (RAM).

Non-volatile and Volatile memory

Non-volatile memory: These are the memories which retain their contents even when power is turned off. ROM is an example of non-volatile memory whereas RAM is volatile. This term often refers to the CMOS memory in PCs that holds the BIOS.

Volatile memory: These are the memories that lose their contents when the power is turned off. All RAM, except the CMOS RAM used for the BIOS, is volatile.

Different types of memory

Random-Access Memory (RAM): This is the same as main memory. When used by itself, the term RAM refers to read and write memory; that is, you can both write data into RAM and read data from RAM. This is in contrast to ROM, which permits you only to read data. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost. When your computer boots up, it loads the operating system into its memory, or RAM. It is different than hard disk space as RAM is made up of physical memory chips, while hard disks are magnetic disks that spin inside a hard drive.

Read-Only Memory (ROM): Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to.

Programmable Read-Only Memory (PROM): A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.

Erasable Programmable Read-Only Memory (EPROM): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light.

Electrically Erasable Programmable Read-Only Memory (EEPROM): An EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge.

Flash memory: It is a special type of EEPROM that can be erased and reprogrammed in blocks instead of one byte at a time. Many modern PCs have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. Such a BIOS is sometimes called flash BIOS. Flash memory is also called flash RAM.

Dynamic RAM (DRAM): It is a type of physical memory used in most personal computers. The term dynamic indicates that the memory must be constantly refreshed (reenergised) or it will lose its contents. RAM (random-access memory) is sometimes referred to as DRAM (pronounced dee-ram) to distinguish it from static RAM (SRAM).

Static RAM (SRAM): It is also a type of physical memory which is faster and less volatile than dynamic RAM, but it requires more power and is more expensive. It is also more reliable than the more common DRAM. The term static is derived from the fact that it doesn’t need to be refreshed like dynamic RAM. While DRAM supports access times of about 60 nanoseconds, SRAM can give access times as low as 10 nanoseconds. In addition, its cycle time is much shorter than that of DRAM because it does not need to pause between accesses. Unfortunately, it is also much more expensive to produce than DRAM. Due to its high cost, SRAM is often used only as a memory cache.

SDRAM: It is the short for Synchronous DRAM, a type of DRAM that can run at much higher clock speeds than conventional memory. SDRAM actually synchronizes itself with the CPU’s bus and is capable of running at 133 MHz.

Memory card: It is an electronic flash memory storage disk commonly used in consumer electronic devices such as digital cameras, MP3 players, mobile phones, and other small portable devices. Types of memory cards include PCMCIA, CompactFlash, SD Card, MiniSD, xD-Picture Card and others.

SD Card: A Secure Digital Card or SD Card is an ultra-small flash memory card designed to provide high-capacity memory in a small size. SD cards are used in many small portable devices such as digital video camcorders, digital cameras, handheld computers, audio players and mobile phones.

PCMCIA: Short for Personal Computer Memory Card International Association and pronounced as separate letters, PCMCIA is an organisation consisting of some 500 companies that has developed a standard for small, credit-card-sized devices, called PC Cards. Originally designed for adding memory to portable computers, the PCMCIA standard has been expanded several times and is now suitable for many types of devices. In general, you can exchange PC Cards on the fly, without rebooting your computer.

Magnetic tape: It refers to a magnetically coated strip of plastic on which data can be encoded. Storing data on tapes is considerably cheaper than storing data on disks. Tapes also have large storage capacities. Accessing data on tapes, however, is much slower than accessing data on disks. Tapes are sequential-access media. Because tapes are so slow, they are generally used only for long-term storage and backup.

Cache: Pronounced cash, it is a special high-speed storage mechanism. It can be either a reserved section of main memory or an independent high-speed storage device. Two types of caching are commonly used in personal computers: memory caching and disk caching. Some memory caches are built into the architecture of microprocessors and are called Level 1 (L1) caches. Most modern PCs also come with external cache memory, called Level 2 (L2) caches. These caches sit between the CPU and the DRAM. Like L1 caches, L2 caches are composed of SRAM but they are much larger.

Virtual memory: In computing, virtual memory is a memory management technique that is implemented using both hardware and software. It is an imaginary memory area supported by some operating systems. Programs use these virtual addresses rather than real addresses to store instructions and data. When the program is actually executed, the virtual addresses are converted into real-memory addresses. The purpose of virtual memory is to enlarge the address space, the set of addresses a program can utilise.

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