Computer Memory

Data and program instruction are stored in the electric circuitry of the computer until it is their turn to be processed. This circuitry area is called memory.
It is sometimes called primary storage. Memory refers to the circuits that are directly linked to the processor. On other hand, the media that stores information that is not related to the processor is called storage (auxiliary memory). Basically al computers use the following type of memories.

a. Internal Memory/Main memory/Primary memory
b. External/Auxiliary/Secondary memory
Internal Memory:

The internal storage section is he holding area in which instructions and data are kept. The control section to control and coordinate all processing activity, it must be able to locate each instruction and data item in storage. A particular program instruction or data item that is held in storage can be located by knowing its address. Some computers can address each character of data in memory directly. Others address computer words which contain a group of characters at a single address. Each computer word contains a group of characters at a single address. Today common types of internal storage media used in are as follows: magnetic core, semiconductor, and bubble.

Magnetic Core Memory:

The magnetic core storage is made up of tiny doughnut-shaped rings made of ferrite (iron), that are strung on a grind of very thin wires . since data in computer is stored in binary from, a two-state device is needed to represent the two binary digits (bits), 0 for off and 1 for on. In core storage, each ferrite ring can represent a 0 or 1 bit, depending on its magnetic state. If magnetized in one direction, it represents a 1 bit, and if magnetized in the opposite direction, it represents a 0 bit. These cores are magnetized by sending an electric current through the wires on which the core is strung. It is this direction of current that determines the state of each core.

Bubble Storage

The latest technological is used for storage media is the introduction of bubble memory. Bubble memory consists of a very thin crystal made of semiconductor material. The molecules of this special crystal act as tiny magnets. The polarity of these molecules or "magnetic domains" can be switched in an opposite direction by passing a current through a control circuit imprinted on top of he crystal. In this manner, data can be stored by changing the polarity of the magnetic domain. Since the principle is the same as for magnetic core storage, bubble memory is considered nonvolatile. The data is retained even if here is a power failure. The process of reading from bubble memory is nondestructive, meaning that the data is sill present after being read. If we view these magnetic domains under a microscope, they would look like tiny bubbles; hence the name, bubble memory.

Semiconductor storage (Silicon Chip):

Semiconductor memory consists of hundreds of thousands of tiny electronic circuits etched on a silicon chip. Each of these electronic circuits is called a bi cell and can be in either an off or on state to represent a 0 or 1 bit, depending on whether or no current is flowing in that cell. These bi cells comes in semiconductor memory chips is known as integrated circuits (ICs). The memory basically comes in two forms.

1. ROM (Read only Memory)
2. RAM (Random access memory)


Read Only Memory (ROM):

The ROM is a major type of non-volatile memory that is used in computers. A regular ROM is constructed from hare-wired logic, encoded in the silicon itself during the manufacturing. It is designed o perform a specific function and cannot be changed. This is inflexible and so regular ROMs are only used generally for programs that are static. The ROM contains the PC's Basic input/output system BIOS is called ROM-BIOS program which basically performs POST and Boot-startup process. The different types of ROMs are follows:

a. Programmable ROM (PROM):

This is a type of ROM that can be programmed using special equipment; it can be written to, but only once. This is useful for companies that make their own ROMs from software they write without requiring expensive equipment. The PROM becomes a ROM and he stored information can only be read.

b. Erasable Programmable ROM(EPROM):

An EPROM is a ROM that can be erased and reprogrammed. A little glass window is installed in the top of the ROM package. The ultraviolet light of a specific frequency can be shined through this window for a specified period of time, which will erase the EPROM and allow it to be reprogrammed again.

c. Electrically Erasable Programmable ROM (EEPROM):

The EEPROM which can be erased under software control. This is the most flexible type of ROM, and is now commonly used for holding BIOS programs.

d. Electrically Alterable PROM (EAPROM) :

A recently developed type of memory can have programmed and used by electrical signals which is known as EAPROM. It is provided an easy means to load and store temporary or permanent information in a form of ROM memory. Information loaded in this memory can be retained for many years without any power supplied. One of its new applications is as a backup to RAM memory whose content is lost in a power failure. When power is turned, the EAPROM memory can be used to replace he list contents of RAM memory and the computer can continue working on he same work.

Random Access Memory

It is volatile memory, meaning all data is lost when power is turned off. The RM is used for temporary storage of program data, allowing performance to be optimum. It is also called a working memory or user memory. The different types of RAMs are follows:

Types Of Ram :
Static RAM (SRAM):

This RAM will maintain its data as long as power is provided to the memory chips. I do not need to be rewritten periodically. SRAM is very fast but much more expensive than DRA. SRAM is often used as cache memory due to its speed.

Dynamic RAM (DRAM) :

The DRAM must be continually rewritten I order to maintain its data. This is done by placing he memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small.

Fast Page Mode DRAM (FPMDRAM):

The FPM DRAM is only slightly faster than regular DRAM. It uses a slightly more efficient method of calling data from the memory. FPM DRAM is not used much anymore due to its slow speed, but it is almost universally supported.

Extended Data Out DRAM ( EDO DRAM):

The EDO allows one access o begin while another is being completed. While this might sound ingenious, the performance increase over FPM DRAM is only slight. EDO DRAM must be properly supported by the chipset, but it is the most common type of memory for most users.

Burst EDO DRAM (BEDO DRAM) :

This is basically EDODRAM with combined pipelining technology. The result is a much faster EDO memory chip capable of working with faster bus speeds. Support for the BEDO technology is rather sparse. SDRAM has caught o faster.

Synchronous DRAM (SDRAM) :

SDRAM is the developing new standard for PC memory. Its speed is synchronous, meaning that it is directly dependent on the clock speed of the entire system. It works at the same speed as the system bus, up to 100MHz. SDRAM is faster but most users are working on a relatively slow 66MHz bus speed, which doesn't use the SDRAM to its full capacity.

RAMBus DRAM (RDRAM):

This is a technology still being developed by Intel that may prove to surpass SDRAM. Its goal is to get rid of the latency, the time taken to access memory. It does this by actually narrowing the bus path and treating the memory bus as a separate communication channel.