General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

Example 443 (Address decoder logic for 2-bit addresses)
c○: Michael Kohlhase 269
Now we can combine an n-bit address decoder as sketched by the example above, with n D-flipflops
to get a RAM element.
Storage Elements
Idea (Input): Use a D-flipflop connect its E input to the ADL output.
Connect the D-input to the common RAM data input line. (input only if addressed)
Idea (Output): Connect the flipflop output to common RAM output line. But first AND
with ADL output (output only if addressed)
Problem: The read process should leave the value of the gate unchanged.
Idea: Introduce a “write enable” signal (protect data during read) AND it with the ADL
output and connect it to the flipflop’s E input.
Definition 444 A Storage Element is given by the following diagram
c○: Michael Kohlhase 270
So we have arrived at a solution for the problem how to make random access memory. In keeping
with an introductory course, this the exposition above only shows a “solution in principle”; as
RAM storage elements are crucial parts of computers that are produced by the billions, a great
deal of engineering has been invested into their design, and as a consequence our solution above
is not exactly what we actually have in our laptops nowadays.
Remarks: Actual Storage Elements
The storage elements are often simplified to reduce the number of transistors.
For example, with care one can replace the flipflop by a capacitor.
Also, with large memory chips it is not feasible to connect the data input and output and
write enable lines directly to all storage elements.
Also, with care one can use the same line for data input and data output.
Today, multi-gigabyte RAM chips are on the market.
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