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Computer Organization and Assembly Language. Week 10. Memory Cell. We are following the Von-Neumann Machine Architecture. Previously you have learnt about processors, their design, types and the operations and simplifications they perform. Now we will jump on Memory Cell.
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Memory Cell • We are following the Von-Neumann Machine Architecture. • Previously you have learnt about processors, their design, types and the operations and simplifications they perform. • Now we will jump on Memory Cell. • The truth table that represents the operation of memory devices have to include a few symbols in order to represent the functionality of the devices. For example a memory cell capable of storing a binary value either a 1 or a 0. • A new symbol however is needed to represent the stored value.
Edges/ Transitions • Many devices use as their input a change in the signal rather than a change in the level of signal. • For example when you switch on your computer, it is not the binary 1 or 0 that is working behind the transition. If this was the case, as soon you removed your finger the computer should have turned down. • It changes from the state. • There are two truth table symbols that represents transition from one logic to another. • If you go from 0 to 1 the transition points upwards and if you go from 1 to 0 the transition points downwards.
Edges/ Transitions (Continued) • Figure below presents a binary signal with points where transitions occur identified with these two new symbols. • If a memory cell is powered on, it contains a stored value. if the logic circuit uses the stored value of that cell as an input we need to have a way of labeling it so that it can be used within a Boolean Expression.
Unidentified Values • Not all the conditions are true in real time operations, some conditions are impossible to reach or they should be avoided because their use can cause problems. They are represented by U • Consider the binary circuit that operates the light inside a microwave oven. The inputs to this circuit are a switch to monitor whether the door has been opened and a signal to indicate whether the magnetron is on or off. Note that the magnetron never turns on when the door is opened, so this is a false condition.
The S-R Latch • Computer Memory is made up of array of cells, each of which is capable to store a single bit, either a 0 or 1. • The purpose of the circuitry is to send a logic 1 or 0 to a device and leave it for a period of time and after some time observe whether the value is still there or not. • A simple wire can’t do this but if you leave a value on the wire it will do. How if the value disappears it will quickly loose the charge and will vanish. • Early memory stored data is small doughnut shaped rings of iron. Wires that were woven through the centers of the iron rings were capable of magnetizing the ring in one of two directions.
The S-R Latch (Continued) • With the advent of digital circuitry the magnetic circuitry was replaced with gates. • A circuit is not impossible to be developed where the output could be routed back around to the circuits input in order to maintain the stored value. • The ring provided the feedback which allowed the circuits current data to drive the future data and thus maintain its condition.
The S-R Latch Circuit • The output of the first inverter in the circuit is fed into the input of the second inverter. Since the inverse of an inverse is the original value, the input to the first inverter is equal to the output of the second inverter. If we connect the output of the second inverter to the input of the first inverter, then the logic will be maintained until power of the circuit is removed.
Issue • The problem is that there is no way to modify the value that is stored. • There is a need to replace either one or both the inverters with the device that has more than one input. • But one that can also operate the same way as the inverter during the periods when we want the data to be stable. • It turns out that NAND gate can do this.
Replacement of an Inverter with NAND Gate • As long as the free inputs to the two NAND gates remain equals to one, the circuit will remain stable since it is acting as a pair of inverters connected together in series.
The D Latch • Every logic gate based memory has an S-R Latch embedded to it. • Now we will treat Latch as a black box. • The typical data storage latch is referred to as a data latch or D Latch. There are slight variations between different implementations of the D Latch.
The D Latch (Continued) • The rest operations remains the same. Two new inputs are D and Clock. • D is data to be stored • And clock tells when the data has to be stored.
