Questions about CPUs

[PrayForDeath]

Hi, there's something I can't understand about Processors, when I read an acrticle on an Intel CPU, I read in the specifications that it has a800 Mhz FSB for example, and in another article on an AMD CPU, it says that it has a 1600 Mhz FSB (Athlon 64), but when people talk about overclocking, they say for example: my CPU has a 200Mhz FSB and the multiplier is set to 13.5x, so my processor's speed is 2700Mhz!
How come the FSB is 200mhz and 800 Mhz at the same time, are there two kinds of FSB in the processor or what? If that's wrong, then if you have an Athlon 64 with a 1600Mhz FSB you can set the multiplier to 2x and get a 3200Mhz processor!! I am so confused!
And there's another thing I wanna ask you about, it's the Athlon Mobile CPUs, I hear people saying they are going to buy a 2500+ Athlon mobile CPU cuz it's cheap and very good for overclocking! What I don't understand is that isn't the mobile CPU designed for laptops??! How can you install it in a desktop PC? And what about the Barton, is that a desktop CPU or a mobile one?
So Please explain for me if you can.

 

[VIAN]

A FSB (Front Side Bus) is the connection between the CPU and the Memory Controller located in the Northbridge. The CPU Clock Speed, is found by Multiplying a number by the FSB.

An Athlon XP has a 200MHz FSB, but it is 128-bit IIRC, meaning it can ship 128-bits per cycle. RAM is only 64-bit so you would need twice the clocks of a 64-bit part to match up to a 128-bit part at half the clocks.

Theoretically, you could say that the Athlon XP has a 400MHz FSB that is 64-bit. DDR RAM that is 200MHz that is 64-bit will be able to match up to that FSB because DDR means Dual Data Rate shipping twice the information per clock. Theoretically, this means DDR-SD-RAM 200MHz equals SD-RAM, normal RAM, at 400MHz. They don't make normal RAM anymore. DDR RAM is best described as PC2700 or PC3200. Those numbers are the bandwidth in MB/s. You can figure this out easily by doing the following calculations.

For instance DDR133 at 64-bit. Remember to multiply 133 x 2 because it's DDR. (133x2)x(64/8) The 8 is there because - 8 bits equals one byte. We want to know how many bytes per second. This will give you about 2100MegaBytes per second. This is correct because this is is PC2100.

The Intel P4 has a 200MHz FSB, but it is 256-bit. So theoretically, it can be said that it has a 800MHz FSB shipping at 64-bits per sec.

The Athlon 64 doesn't have a FSB because the Memory Controller is inside the chip. So instead they have a HyperTransport Bus. I'm not quite sure how this works but the 1600MHz you are talking about is that it can do 800MHz input and 800MHz output from the CPU.

About the mobile processors I have no clue.

Barton just means that the chip has 512kB of cache for either desktop or mobile.

 

[JSSheridan]

The FSB is the connection between the CPU and the northbridge, and both the P4 and Athlons have a FSB of 200MHz. The processor speed is the product of the multiplier and the FSB. Intel can claim their FSB is 800MHz because their CPU's recieve 4 bits of data during a clock cycle. 4 * 200MHz gives 800MHz equivalent. AMD claims a 400MHz FSB because their CPU's recieve 2 bits of data per cycle. I believe the width of the data bus is 8 bytes (64 bit), but whatever it is, the bandwidth is the product of the equivalent FSB and the bandwidth. The article you read about the A64 was probably referring to the speed between the CPU and the on-die memory controller which runs at the same speed as the CPU. HTH. Peace.

 

[PrayForDeath]

Thanks for the info, guys

 

[VIAN]

The FSB is the connection between the CPU and the northbridge, and both the P4 and Athlons have a FSB of 200MHz. The processor speed is the product of the multiplier and the FSB. Intel can claim their FSB is 800MHz because their CPU's recieve 4 bits of data during a clock cycle. 4 * 200MHz gives 800MHz equivalent. AMD claims a 400MHz FSB because their CPU's recieve 2 bits of data per cycle. I believe the width of the data bus is 8 bytes (64 bit), but whatever it is, the bandwidth is the product of the equivalent FSB and the bandwidth. The article you read about the A64 was probably referring to the speed between the CPU and the on-die memory controller which runs at the same speed as the CPU. HTH. Peace.

No. 4 bits of data per cycle? That's kind of funny because the smallest addressable unit is 8 bits. Plus look at the bandwidth. If it were 4-bit per cycle then it would be (4x200MHz)/8= 100MBps. That isn't it. Look at my example. (256/8)x200MHz=6400MBps. That's the real answer.

 

[Soulkeeper]

people need some good articles/reviews and a cup of coffee
haha that should fix this thing in like half an hour

 

[JSSheridan]

Now you've gone and done it, you've disagreed with me. 'You fell victim to one of the classic blunders, the most famous of which is "Never get involved in a land war in Asia", but only slightly less well known is this: "Never go in against a Sicilian, when *death* is on the line.". Hahahahahah. [Falls over dead]'

My math works out just fine, say thankya. The product of effective speed and bus width gives bandwidth for a parallel device. Both the HyperTransport bus the A64 uses and the FSB of the AthlonXP and Pentium 4 are parallel.

( 4 * 200MHz ) * 64 bits / 8 (bits/byte) = 6400MBps

Originally posted by: Soulkeeper
people need some good articles/reviews and a cup of coffee
haha that should fix this thing in like half an hour

Good idea, I'm off to get some.

AT Faq containing bandwidth calculation

AT's article on Hypertransport and the CPU clock

AT Article on the Pentium 4
And to provide a quote from the article for your convience, "The difference between the Pentium 4?s AGTL+ bus and the AGTL+ bus used by the Pentium III is that the Pentium 4?s bus is a quad pumped 100MHz bus effectively running at 400MHz."

Peter speaks on the quad-pumped bus in a HT thread.

And last but not least. The Hypertransport FAQ

 

[VIAN]

( 4 * 200MHz ) * 64 bits / 8 (bits/byte) = 6400MBps

That's not what you said before.

The 4 can be shifted to also make this equation:

200MHz*[4*(64/8)]

If the bus is 4 in parrallel running at 200MHz, then it is a 256-bit bus.