Memory Circuits
Why Is Memory Important?
Simulations
One Nybl Memory
Four Nybl Memory
Steering Circuits
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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.  Click here to review the operation of the flip-flop being used.

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.

Exercise

In the simulation above, set 1001 into memory element 2.  As you do that, assume the the lowest bit is the LSB and the highest bit is the MSB.

Steering Circuits - Under Construction
Links to Other Lessons on Digital Logic