- [Instructor] Let's quickly discuss
data transfer rates for RAM.
Now on the left here,
we have a CPU and the CPU relies on cache memory internally
to store the most commonly used instruction sets.
But the cache memory is very limited.
So, all the other data
needs to be stored, for this current session, on RAM.
Because there's so much data during a computer session,
we need to make sure that the RAM is fast
and can transfer a lot of data per seconds.
How do we find that out?
Well, let's give an example here
and we'll say that this is a DDR4 memory module
and the type is DDR4-2133.
Now some people will refer to this as 2,133 megahertz
but really what this means is 2,133 mega transfers.
It can do 2,133 mega transfers
or millions of transfers per second.
Now, how do we figure out
exactly what the data transfer rate is
from the memory to the CPU?
Well, we have to think a little bit here
and we have to say okay,
the RAM needs to communicate with the CPU.
And it does that by way of the address bus.
And that is a 64-bit bus, 64-bits.
Now, what does that mean?
Let's go down to our little calculation area down here.
64-bits means that there's actually 64 circuits
and we can send 64-bits of data simultaneously per cycle.
Now, if we want to figure out the amount of actual data
that we can transfer, we want to calculate for bytes.
So 64-bits actually equals eight bytes.
Because there's eight bits per byte.
So 64-bits equals eight bytes.
Every cycle we're sending eight bytes of data
and there's a ton of cycles.
How many?
2,133 mega cycles basically.
So what are we talking about here?
Well, we wanna find out the maximum data transfer rate
and solve for bytes.
So we could do that by taking this number here,
2133 and multiplying by eight,
by the amount of bytes that are sent per cycle, that's it.
So let's bring over our trusty calculator and do the math.
If we do 2133x8, we'll get 17064
but really it's a little higher than that, it's 17,066.
Let me redo the calculation a little bit more accurately.
2133.33, because this is really a repeating decimal.
So it's 2133.33 and the threes keep going.
It's okay to just stop at two decimal points though.
So we'd say 2133.33x8 and that'll give us 17,066.
And really the answer would be 0.6666 repeating decimal.
But this is good enough for me, 17,066.
But how much are we talking about here?
Remember that we're talking about mega transfers.
So really it's 17066 megabytes per second.
So the total data transfer rate
is going to equal 17,066 megabytes per second.
That's how much data we can transfer back and forth
between the RAM and the memory controller
that's part of the CPU.
17,066 megabytes per second.
That's what this particular RAM can do.
Other types of RAM with higher mega transfers per second
will send more megabytes per second.
But basically all you're doing is using the same equation.
You'll say mega transfers per second times eight
and solve for bytes.
So that'll equal the amount of megabytes per second
and that's it, that'll be your equation
regardless of what DDR4 or five or whatever you're using.
Now, this name here, DDR4-2133,
that's the DDR4 standard
but it will correspond to a module name quite often.
And that name will be based off of
the amount of megabytes per second.
So for example, this module standard is DDR4-2133
but the module name would be PC4-17000.
And you might see it read as 17,066 also.
That would be the actual module name for this RAM.
So there's a little data transfer rate calculation
for a DDR4.