Storage Elements

• Each storage element is connected to one row and one column selector line.
• Since for every address only one row and one column selector line will be ``on'', exactly one storage element will have both its row and its column selector line ``on''.
• Therefore, a storage element can be selected (or eanbled) by AND-ing its row and its column selector line.
• This suggests that we use a D-flip for a storage element and connect its Enable input to the output of the above mentioned AND gate.
• The D-input is connected to the data input line that is common to all storage elements.
• Similarly, the output of the flipflop is AND-ed with the output of the first AND gate and then connected to a data output line that is common to all storage elements.
• This way, only the selected storage element contributes to the common output line.
• The final problem arises when data is to be read.
• The read process should leave the value of the gate unchanged.
• To prevent that data is destroyed duing a read operation, a ``write enable'' signal is introduced.
• This signal is ``on'' only when new data is to be stored.
• It is AND-ed with the output of the first AND gate and then applied to the flipflop's Enable input.
• Thus, during read operations (write enable=''off'') the flipflop is disabled and does not change its state.
• The write enable signal again is common to all storage elements.

  

Figure 3.16: Storage Element.