Memory

Types of Memory:
RAM - Stands for Random Access Memory. It is a semiconductor-based memory where the CPU or the other hardware devices can read and write data. It temporarily stores the data and it is a volatile memory. Once the system turns off, it loses the data. As a result, RAM is used as a temporary data storage area.

ROM - Stands for Read Only Memory. It stores the data permanently and it is a non-volatile memory. It does not losr the data even after the system turns off. As a result, ROM is a permanent data storage area.

Types of ROM:

PROM - Stands for Programmable Read - Only Memory. It stores programs permanently and is a non-volatile memory. Programming the ROM is sonetimes referred to as burning and it requires a special machine called a device programmer or ROM burner. Each binary 1 bit can be thought of as a fuse; when a prom is programmed, a higher voltage blows or burns the fuses at the de sired locations thus turning any given 1 into a 0. Due to this, PROM chips are often referred to as One Time Programmable ( OTP ) chips.

EPROM - Stands for the Erasable Programmable Read-Only Memory. Ultra-Violet ( U-V ) rays can remove the programs from this memory. It can be easily recognized by the clear quartz crystal window set on the top of chip. An EPROM eraser is a device that contains a UV light source that erases the chip by causing a chemical reaction, which essentially melts the fuses back together.

EEPROM/Flash ROM - Stands for the Electrically Erasable Programmable Read-Only Memory. Electrical signal removes the programs from this memory. EEPROM can be erased by an electric field, rather than exposed to UV. Also, the data can be erased bit by bit allowing only selected portions of the code to be replaced. But in case of Flash ROM, data is rewritten in blocks of 512 bytes where as EEPROM erases individual bytes.

Types of RAM:
RAM is the main memory of the computer. It holds the data until the system is turned off. Once the system is switched off, the data is lost. As a result, it is known as the temporary data storage area. There are two types of RAM, Static RAM ( SRAM ) and Dynamic RAM ( DRAM ).

SRAM ( Static Random Access Memory ) - SRAM is a type of semiconductor memory. It is a temporary data storage area. SRAM is faster in accessing data. The data accessing of SRAM makes it behave like a cache memory.

Cache RAM -
cache RAM is a type of SRAM which enables the CPU to send data at a very high speed. It is placed between the main memory and the CPU and used for a temporary storage. It is the buffer between CPU and DRAM which provides the information without the use of DRAM. Cache RAM is very fast and expensive compared to RAM. Date can be processed easily without waiting for it to be retrived from the DRAM if it is stored in cache RAM. The fastest cache RAM is known as L! and is placed inside the CPU.

DRAM ( Dynamic Random Access Memory ) - The lifetime of the data in DRAM is very short. It is approximately for fifteen milliseconds.

Types of DRAM -
SDRAM ( Synchronous Dynamic Random Access Memory ) - SDRAM synchronizes the memory speed with the CPU clock speed.The data transfer speed of SDRAM is measured in nanoseconds and megahertz units. It runs with an average speed of 133 MHz.
There are two modules of SDRAMs:
2-clock SDRAM - In a 2-clock SDRAM, each clock signal controls two different DRAM chips.
4-clock SDRAM - In a 4-clock SDRAM, each clock signal controls four different DRAM chips. The recent computers use 4-clock SDRAM.

DDR-SDRAM ( Double Data Rate Synchronous DRAM ) - It is the latest version of SDRAM. DDR is synchronous with the system clock. This memory chip consumes less power. DDR memory supports Error Correction CAoad ( ECE ) and non-parity.
There are three types of DDR-SDRAMs.
DDR - These are avelable 128MB to 1GB
DDR2 - These are avelable 256mb to 4GB
DDR3 - These are avelable 2GB to 4GB

What is Virtual Memory ?
virtual memory is a part of the hard disk which is used as a memory. It has a set of memory addresses and stores the instrctions or the data. When the processor executes the instructions, it converts the vertual memory addresses into read memory address. The main use of the virtual memory is to increase the address space.
Operating system such as windows supports virtual memory area. DOS operating system does not support the concept of the virtual memory area.