UNDERSTANDING
THE SYSTEM CLOCK
The System Clock - Understanding what it is
and how it determines CPU and bus speeds
Many people know about the FSB (Front Side Bus) and it's relation to
the CPU and RAM. Many people also know that in order to take advantage
of fast RAM and/or CPU the FSB needs to be fast enough (and vice versa)
to prevent bottlenecking. That's all very important information to know
when building or upgrading a computer. But how are these speeds
determined? How is the FSB speed determined? How about the CPU or the
PCI bus, etc? The answer is quite simple: the System Clock. We are
going to be taking a deep look into the system clock, what it is, how
it affects the speed of your computer and how all of this ties into
overclocking.
From this point on it is important
that you have a decent understanding of how a computer works. By this I
mean you should know all the different buses and how they are used to
provide communication pathways between different components. Just a
basic understanding will suffice. Otherwise the information I present
will look greek to you (unless of course you're greek 
).
What the heck is a System Clock and
how does it work?
The system clock is a quartz crystal on the motherboard that, when very
slightly bent or disformed, produces an electric signal or pulse. When
electric current hits the crystal, it's the electrons bouncing off of
the crystal that causes it to slightly bend or fluctuate, therefore
causing the crystal to produce an electric pulse or what's known as a
clock cycle or clock pulse.
Definition of a Clock Pulse:
In synchronous digital electronics,
such as most computers, a clock signal is a signal used to coordinate
the actions of two or more circuits. A clock signal oscillates between
a high and a low state, normally with a 50% duty cycle. In other words,
the signal is a square wave. The circuits using the clock signal for
synchronization may become active at either the rising or falling edge,
or both (for example DDR SDRAM), of the clock signal.
Source: http://en.wikipedia.org/wiki/Clock_cycle
So without getting this to complicated, a clock pulse is basically
an electronic signal that tells components when it can start
sending/receiving data. It is the heartbeat of the computer. It's
pulses can be used directly or scaled up or down in frequency for
different buses. This enables different components on different buses
to communicate with each other efficiently, therefore synchronizing all
of the components together.
Understand that the System Clock doesn't just send one clock pulse
every other second, that would cause the computer to be extremely slow
and would have no real use in today's world. The System Clock generates
hundreds of millions of clock pulses a second (mhz). The most common
clock speeds of today's motherboards are 100mhz, 133mhz, and 166mhz.
However high end computers have system clock speeds of 200mhz and over.
How the System Clock affects the speed
of the FSB
Well, this actually depends on whether or not you have an Intel CPU
or an AMD CPU. An Intel CPU is designed to send data 4 times for every
CPU clock cycle at the speed of the System Clock (Quad Pumped). So if
the System Clock is set at 200mhz, the CPU sends data 4 times along the
FSB, each time at the speed of 200mhz, which is why the FSB is said to
be 800mhz. AMD CPU's are a little different, they send data 2 times
along the FSB for every CPU clock cycle at the speed of the system
clock (Double Pumped).
Important Notice: this info is
assuming the CPU's are 32-bit for simplicities sake. 64-bit CPU's work
differently and I will explain them in a later paper.
So does the type of the FSB determine
the type of RAM I should get for optimal performance?
You bet. You see, when Dual Pumped FSB (AMD) technology was introduced,
regular SDRAM couldn't keep up. That's because while data from the
processor was being sent twice per clock cycle, regular SDRAM was only
sending data once per clock cycle. So in order to avoid this
bottlenecking, DDR was soon introduced. With DDR RAM it is possible for
the RAM to send and receive data twice per clock cycle. So because of
this, the CPU and DDR RAM can now send and receive data at the same
time.
Quad Pumped FSB (Intel) technology, allows the CPU to send and
receive data 4 times per clock cycle. In order to avoid bottlenecking
Intel and the Rambus Corporation created RIMM's or Rambus memory. Like
DDR, Rambus only sent data twice per clock cycle but it was a lot
faster then DDR. Since then, Rambus has come and gone and now Dual
Channel memory is recommended for Intel's Quad Pumped FSB's, even
though it still does not truely take full advantage of Quad Pumped FSB
technology.
How the System Clock determines the
speed of the CPU
On the CPU there is this thing called a multiplier. What this does
is take the clock pulse from the System Clock and multiply it. The
value the Multiplier is set at depends on the CPU and usually cannot be
changed. The Multiplier for Pentium 4's are usually set to 14 or 15. So
if the System Clock is set at 200mhz and the Multiplier is set at 15,
then you multiply 200,000,000 by 15 which is of course 3 billion or
3ghz! How do you figure out what the multiplier is? Well, you can look
in the CMOS setup or you can divide the FSB by 4(Intel) or 2(AMD) to
get the System Clock and divide that by the speed of the CPU. So
multiplier = CPU speed / (FSB / 2 or 4) depending on if it's an Intel
or AMD. This information is important to know when overclocking your
CPU.
Important Notice: this info is
assuming the CPU's are 32-bit for simplicities sake. 64-bit CPU's work
differently and I will explain them in a later paper.
So do all the other buses use
Multipliers as well?
No. The other buses, like the AGP bus and the PCI bus for example, use
Dividers. They take the System Clock pulses and divide them to a
usuable speed for the PCI components and AGP card. You can change the
Divider values in the CMOS setup or with jumper settings on the
motherboard, but it isn't recommended unless your overclocking your
computer. The AGP bus divider is set to specifically make the bus speed
66 mhz. The PCI bus divider is set to make the PCI bus speed 33mhz.
Components that use the AGP and PCI bus are designed to run at those
speeds. So if you set the divider to make the bus speed faster then
those speeds, the components using those buses may be unusuable. On the
other hand, if you make the bus speed slower, those components won't
communicate effeciently with the rest of the components in the
computer.
If your overclocking your CPU then you must increase the System
Clock speed. Increasing the System Clock speed increases all the other
bus speeds. Basically, the only thing you should overclock is the FSB,
CPU and Memory Bus (GPU and VRAM on a video card can be overclocked as
well but I won't get into that here). All of the buses connected to the
South Bridge should not be changed. So while increasing the System
Clock, it's important to go back and adjust the AGP and PCI bus
Dividers accordingly. Unless the AGP and PCI bus are asynchronous,
which means they are automatically set to run at 66mhz and 33mhz no
matter what the System Clock is set at. It is worth noting that the AGP
bus is more flexible in terms of overclocking when compared to the PCI
bus, but you're better off just overclocking the video card.
Conclusion
So you always knew that a 2.4ghz processor running on a 533 FSB
with PC3200 RAM was a good setup, but now you know how those speeds are
determined and you are now armed with the knowledge and understanding
of how overclocking works. You now know what a System Clock is, what it
does, and why it's so important and how it allows all of the components
within the computer to communicate with each other efficiently.
Original Tutorial by The Duck
for TheTAZZone-TAZForum
Originally posted on July 28th, 2006 here
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