| Memory Circuits |
Memory Circuits
Why Are Memory Circuits Important?
Shame on you for asking that question. Surely you know the following.
- Memory in computers is one of
the most important things you need to know about when you buy a computer.
You usually ask questions like:
- How much RAM does this
machine have?
- How much space is there
on the hard drive?
- How much cache does this
chip have?
- All of those questions relate
to memory. However, the hard drive question asks about a different kind of
memory than we are going to talk about in this lesson. Here we are going to
talk about the evanescent kind of memory, RAM - or random access memory.
Memory Circuits - An Introduction
We'll start with a
simulation of a simple memory. The simulation demonstrates several - but not
all - aspects of memory operation.
Simulation
In this simulator you can
set four bits of data (using the small red buttons to set each bit), and then
you can store the data in the memory by "clocking" it in. In the simulator
below do the following.
- Set the data you want to
enter. All four bits are pre-loaded to zero, so you need to set at least
one bit.
- Cycle the clock through one
complete cycle (two mouse clicks) bringing the clock high, then low.
- Data will be transferred
to the output stage of the flip-flops at the right when the clock cycle
is complete and the clock signal returns to zero.
A Short Note About The Four Bits Above
One interesting
thing about the four bit memory above. Memory usually comes in
bytes. A byte is eight
bits. The memory register above only has four bits, so it is half of a byte.
Strangely enoungh, there is a name for four bits of memory. Since it is half a
byte, the name assigned to four bits of memory is a
nybl. Although we make a few
jokes here and there through these lessons, it is true (albeit a little funny)
that a nybl is half of a byte - and they are both spelled with a "y".
When you buy memory in a computer, you specify things
like how much RAM (Random Access Memory) you want the computer to have. You
might want to have 512 Megabytes, for example. We can't simulate that large a
memory, so we will try something smaller. In the next simulation we give you a
four nybl memory. It's just large enough that you can experiment with it, and
you can give yourself a mental model of how larger memories work.
Simulation
In this simulation, you
can work with a four nybl memory. Note the following about this simulation.
- The data inputs are
controlled using the button/switch combinations at the left of the memory
array.
- Clicking a button once
will change the switch state from zero to one or back.
- The address is set using the
two buttons on the control panel.
- The address has two bits,
a zero bit and a one bit.
- Addresses are zero (00),
one (01), two (10) and three (11).
- Clicking the buttons
changes the state of the address bit.
- The data is
clocked into the memory
array, using the clock signal.
- When the clock signal is
high, the address line that will be used is highlighted.
- When the clock returns
low, the data bit - if it is set - will appear at the output of the
memory flip-flop.
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