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Random Access Memory

Random Access Memory (RAM) is a type of computer data storage. Its mainly used as main memory of a computer. RAM allows to access the data in any order, i.e random. The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data. You can access any memory cell directly if you know the row and column that intersect at that cell.
    Most of the RAM chips are volatile types of memory, where the information is lost after the power is switched off. There are some non-volatile types such as, ROM, NOR-Flash.

SRAM: Static Random Access Memory
SRAM is static, which doesn't need to be periodically refreshed, as SRAM uses bistable latching circuitry to store each bit. SRAM is volatile memory. Each bit in an SRAM is stored on four transistors that form two cross-coupled inverters. This storage cell has two stable states which are used to denote 0 and 1. Two additional access transistors serve to control the access to a storage cell during read and write operations. A typical SRAM uses six MOSFETs to store each memory bit.
    As SRAM doesnt need to be refreshed, it is faster than other types, but as each cell uses at least 6 transistors it is also very expensive. So in general SRAM is used for faster access memory units of a CPU.

DRAM: Dynamic Random Access Memory
In a DRAM, a transistor and a capacitor are paired to create a memory cell, which represents a single bit of data. The capacitor holds the bit of information. The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state. As capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh process, it is a dynamic memory.
    The advantage of DRAM is its structure simplicity. As it requires only one transistor and one capacitor per one bit, high density can be achieved. Hence DRAM is cheaper and slower, when compared to SRAM.

Other types of RAM

FPM DRAM: Fast page mode dynamic random access memory was the original form of DRAM. It waits through the entire process of locating a bit of data by column and row and then reading the bit before it starts on the next bit.

EDO DRAM: Extended data-out dynamic random access memory does not wait for all of the processing of the first bit before continuing to the next one. As soon as the address of the first bit is located, EDO DRAM begins looking for the next bit. It is about five percent faster than FPM.

SDRAM: Synchronous dynamic random access memory takes advantage of the burst mode concept to greatly improve performance. It does this by staying on the row containing the requested bit and moving rapidly through the columns, reading each bit as it goes. The idea is that most of the time the data needed by the CPU will be in sequence. SDRAM is about five percent faster than EDO RAM and is the most common form in desktops today.

DDR SDRAM: Double data rate synchronous dynamic RAM is just like SDRAM except that is has higher bandwidth, meaning greater speed.

DDR2 SDRAM: Double data rate two synchronous dynamic RAM. Its primary benefit is the ability to operate the external data bus twice as fast as DDR SDRAM. This is achieved by improved bus signaling, and by operating the memory cells at half the clock rate (one quarter of the data transfer rate), rather than at the clock rate as in the original DDR SRAM.


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>> Introduction
>> Gate Primitives
>> Delays
>> Examples


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