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Bistable Multivibrator:

Only the cell circuit that is addressed by the 4-bit decoder input shall be accessible through the data bus. It simple memory circuit is random-access and volatile. Technically speaking, it is known as a static RAM. Its total memory capacity is 16 bits. As it contains 16 addresses and has a data bus that is 1 bit wide, it would be designated as a 16 × 1 bit static RAM circuit. As you may see, it takes an incredible number of gates (and multiple transistors per gate) to make a practical static RAM circuit. It makes the static RAM a comparatively low-density device, with less capacity than most other types of RAM technology per unit IC chip space. Because each cell circuit consumes a definite amount of power, the total power consumption for a large array of cells may be quite high. In personal computers early static RAM banks consumed a fair amount of power and produced a lot of heat, too. CMOS IC technology has made it achievable to lower the particular power consumption of static RAM circuits, however low storage density is still an issue. To address this, engineers turned to the capacitor rather than the bistable multivibrator as a means of storing binary data. A tiny capacitor could serve like a memory cell, complete along a single MOSFET transistor for connecting it to the data bus for charging (writing a 1), discharging (writing a 0), or reading. Unluckily, such tiny capacitors have very small capacitances, and their charge tends to "leak" away through any circuit impedances rather quickly. To combat this tendency, engineers designed circuits internal to the RAM memory chip which would periodically read all cells and recharge (or "refresh") the capacitors as required. Although this added to the complexity of the circuit, it still needed far less componentry than a RAM built of multivibrators. They termed this memory circuit a dynamic RAM, due to its requirement of periodic refreshing. Recent advances in IC chip manufacturing has led to the introduction of flash memory, that works on a capacitive storage principle like the dynamic RAM, however uses the insulated gate of a MOSFET as the capacitor itself. Before the advent of transistors (especially the MOSFET), engineers ought to implement digital circuitry with gates constructed from vacuum tubes. As you may imagine, the enormous comparative size and power consumption of a vacuum tube as compared to a transistor made memory circuits like static and dynamic RAM a practical impossibility. Other, rather ingenious, techniques to store digital data without the use of moving parts were developed.

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