A Primer on Digital Logic Part-III

[Music] now let us look at memory cells so memory is a far denser storage technology than flip-flops and so that's the reason it's used in structures inside the processor to actually store a lot of data not just one bit or several bits but thousands or millions of bits so the two kinds of memory technologies

that are normally used are SRAM static Ram cells and DRAM dynamic Ram cells so the SRAM cell works like this that so what is the main philosophy here the main philosophy is that we want to reduce the number of transistors that are required to store a single bit because we'll be saving millions of bits

possibly billions of bits so that's the reason we need to keep the number of transistors as low as possible and we also want to provide a memory technology that is as fast as possible so what we do is that to reduce the number of transistors instead of using cross-coupled NAND gates as was the case in flip-flops

right we use a cross coupled pair of inverters which does the same thing but we need some additional circuitry as we shall see so in this case this is an example of a cross coupled pair of inverters where the output of one inverter draw is connected to the input of the other and similarly the output of

the lower inverter is connected to the input of the upper inverter so let's say value X is stored here then the complement of X is stored on the other side so this structure by itself is potent enough to store the value of one bit and how many transistors does this requires if you would recall the

structure of an inverter was that it required two transistors a PMO's transistor and an n MOS transistor so since one inverter requires two transistors we need a total of four transistors in this structure but however this much of circuitry is not enough to read and write values into this structure so what we need to do is