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FSB Question

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December 11, 2005 2:30:39 PM

I have a question about the FSB standard. With AMD Hypertransport, the memory controller is built into the processor, so, technically, there is no front side bus. However, manufacturers give an FSB rating as a measure of the rate of data transmission from memory to the CPU.

Hypertransport boards have data paths of 16, 32 and 64 bit widths. Now, my question is, if you go from a16 bit to a 32 bit data path, does that double the FSB or is that twice as much data transmitted with the same FSB?

I believe P4 motherboards use a 64 bit front side bus. Some of the Hypertransport motherboards say 1600 MHz FSB on a 16 bit data path. That is full duplex, so it is 800 MHz up and 800 MHz down at the same time. However, if the width of the data path is not calculated into the effective FSB, that would only be comparable to 200 MHZ up on a 64 bit data path.

Can someone clear this up for me?

Thanks in advance,

Garth

More about : fsb question

December 11, 2005 4:19:37 PM

If you change a 2 lanes road to a 4 lanes road, while keeping the same speed, does that make the car to go faster or if that allow more cars to travel on that road?
December 11, 2005 4:36:04 PM

Dual Data rate RAM reads twice per cycle. This doubles the FSB at the same frequency. That is like the cars traveling at the same speed but twice as close together. The term "speed" is ambiguous.

A 200 MHz MB can run at 800 MHz FSB even if it is not dual channel. There is another doubler in there that I don't understand. DDR doubles it once, but if anybodie knows what doubles it the second time I would like to know.

Are you saying doubling the width of the data path doesn't affect the FSB standard? If so, rating a MB at 1600 FSB on a 16 bit data path is very misleading.
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December 11, 2005 6:23:22 PM

Quote:
Dual Data rate RAM reads twice per cycle. This doubles the FSB at the same frequency. That is like the cars traveling at the same speed but twice as close together. The term "speed" is ambiguous.

A 200 MHz MB can run at 800 MHz FSB even if it is not dual channel. There is another doubler in there that I don't understand. DDR doubles it once, but if anybodie knows what doubles it the second time I would like to know.

Are you saying doubling the width of the data path doesn't affect the FSB standard? If so, rating a MB at 1600 FSB on a 16 bit data path is very misleading.


The memory bus run at 200 MHz with Double Data rate. Twice the amount of data for each clock cycle.

Intel FSB 800 MHz is in fact, a 200 MHZ qith QDR. four time the amount of data for each clock cycle.

AMD HyperTransport bus is a real 800 or 1000 MHz bus. It use the base 200 MHz bus with either a 4x for 800MHz or 5x for 1000 MHz multiplier.

MHz is clock speed. DATA rate is speed at which the data are transfered. Bus width is how much data can be transfered on the same clock for each bus lane. So, the FSB speed has nothing to do with data rate or bus bandwidth.
December 11, 2005 8:53:19 PM

Thank you Pat, I know you are trying to be helpful, but some stuff still isn't adding up.

I quote you "Intel FSB 800 MHz is in fact, a 200 MHZ qith QDR. four time the amount of data for each clock cycle."


I translate that - if the clock speed is 200 MHz and you get 4 reads per second,(QDR or DDR2) then the effective speed is 800 MHz. Also, if you have dual channel and DDR400, 200 MHz times 2 for DDR times 2 for two channels, that is 800 MHz - times 64 bits or 8 bytes per read equals 6.4 GB/s bandwidth.

All fine and dandy. But, one of my boards the SOYO sy-p4vte (which I got because it was the cheapest and I didn't want to mess up a good board on my first build) is rated 800 MHz FSB, single channel and uses DDR not DDR2! My pentium IV rated for 533 MHz FSB won't run if I set the board for 200 MHz but the pentium rated for 800 MHz FSB will. All indications are it is running at 800 MHz FSB.

http://www.soyousa.com/products/proddesc.php?id=291

So again I ask - 200 MHz times 2 for DDR equals 400 MHz but what is doubling it again to 800 MHz?

Also I quote you "Bus width is how much data can be transfered on the same clock for each bus lane"

I googled - computer "bus lane" and FSB "bus lane" and found a number of interesting and informative links for DOT, but nothing relavant to this thread. Is there another term for "bus lane" I should be searching for?

finally I quote, "AMD HyperTransport bus is a real 800 or 1000 MHz bus. It use the base 200 MHz bus with either a 4x for 800MHz or 5x for 1000 MHz multiplier"

What is the nature of this multiplier?

Thank you,

Garth
December 12, 2005 1:59:28 AM

Quote:


All fine and dandy. But, one of my boards the SOYO sy-p4vte (which I got because it was the cheapest and I didn't want to mess up a good board on my first build) is rated 800 MHz FSB, single channel and uses DDR not DDR2! My pentium IV rated for 533 MHz FSB won't run if I set the board for 200 MHz but the pentium rated for 800 MHz FSB will. All indications are it is running at 800 MHz FSB.

http://www.soyousa.com/products/proddesc.php?id=291


The 533 CPU don't use a 200 MHz clock, bu a 133 MHz clock, so 4x133=533
Quote:

So again I ask - 200 MHz times 2 for DDR equals 400 MHz but what is doubling it again to 800 MHz? [


Stop thinking about the 800MHz.. the clock is still 200, it is the amount of data that is transfered that make it look like 800 MHz. You have to compare apple with apple and orange with orange. I cannot give you more info until you stop thinking that everything is MHZ.

Quote:

Also I quote you "Bus width is how much data can be transfered on the same clock for each bus lane"

I googled - computer "bus lane" and FSB "bus lane" and found a number of interesting and informative links for DOT, but nothing relavant to this thread. Is there another term for "bus lane" I should be searching for?


Search for overclocking. This will give you information about various buses because they are affected by overclocking.
Quote:


finally I quote, "AMD HyperTransport bus is a real 800 or 1000 MHz bus. It use the base 200 MHz bus with either a 4x for 800MHz or 5x for 1000 MHz multiplier"

What is the nature of this multiplier?

Thank you,

Garth


Because you don't want 36 clock generator in a computer. You put one, and you either multiply or divide its frequency to have the other frequency you need.
a b V Motherboard
a b } Memory
December 12, 2005 4:04:52 AM

OK Garret! First of all, a 533 bus processor is using Quad Data Rate for a 133MHz clock.

Second of all, your memory can run at a different bus speed than your CPU. The memory controller has two sides, the CPU side and the memory side. One side can run at 133MHz while the other runs at 200MHz, for example.

Now, if you had a 533 bus P4 and were running DDR400 at full speed, your RAM would be clocked at a higher speed but have a lower transfer rate, because the CPU side is QDR but the RAM side is DDR.

In order to double DDR you can use dual-channel mode. But that doesn't double the frequency, it doubles the bus width. DDR400 is also called PC3200 for it's transfer rate, doubling it by making the bus twice as wide gives it 6400MB/s peak transfer rates.

You see, the CPU bus is 64-bits wide (that is, 64 data pathways). DDR SDRAM is also 64-bits. DDR SDRAM in dual-channel mode is 128-bits. And you can see how 128-bits is twice as wide as 64-bits, for twice as many data pathways.

Bit width is physical, it's actually the number of wires used to tranfer data. MHz is frequency, a measure of cycles per second. Either can be doubled.
December 12, 2005 1:10:38 PM

Quote:
OK Garret! First of all, a 533 bus processor is using Quad Data Rate for a 133MHz clock..


Right, I all ready covered that. If the MB frequency is 200 MHz, at Quad Data Rate that is 200 MHz times 4 or 800 MHz. Same for 133 MHz times 4 equals 532 MHz.

But it is my understandint that DDR reads twice per cycle. DDR2 is required to get 4 reads per cycle. My MB uses DDR not DDR2. It should only get 2 reads per cycle or 200 MHz times 2 for 400 MHz. 400 MHz times 8 bytes per read is 3200 MB/s bandwidth.

Quote:
In order to double DDR you can use dual-channel mode. But that doesn't double the frequency, it doubles the bus width. DDR400 is also called PC3200 for it's transfer rate, doubling it by making the bus twice as wide gives it 6400MB/s peak transfer rates..

You see, the CPU bus is 64-bits wide (that is, 64 data pathways). DDR SDRAM is also 64-bits. DDR SDRAM in dual-channel mode is 128-bits.


Right, I already understand that. However, it doubles the bus width on the board not the front side bus. The CPU can only recieve 64 bits per cycle so the bus to the CPU has to be 64 bits wide. To transfer 128 bits on a 64 bit bus requires 2 cycles - you have to double the speed of the FSB to keep up with the memory. Hence, DDR400 times 2 (for dual channel) equals 800 MHz FSB.

However, my MB is not dual channel!

You fellows are re-explaining the stuff I already covered in my post, but the arithmatic still doesn't add up.

The board uses DDR400 single chanel and runs at 800 MHz FSB with a 200 MHz clock speed. A lot of boards out there run at 800 MHz FSB with DDR400 single channel. DDR is not quad data rate and single channel is not 128 bit data path.

http://www.soyousa.com/products/proddesc.php?id=291

200 MHz clock speed - 2 reads per cycle for DDR equals 400 MHz. What doubles it again to 800 MHz?

I'm wondering if the front side bus on these boards is only 32 bits wide.
a b V Motherboard
a b } Memory
December 12, 2005 3:39:26 PM

You already missed what I said about the bus being split. The CPU side runs at a different transfer rate than the memory bus.

CPU bus for P4 is QDR, memory bus is DDR. 800MHz refers to a QDR CPU bus using 200MHz clock rate. 800MHz doesn't refer to the memory speed.

There is DDR2 memory rated at 800MHz data rate, it runs at 400x2, rather than 200x4.

You keep getting FSB and memory speed mixed up, they haven't been locked together since the BX chipset of the late 20th century.
December 12, 2005 5:14:22 PM

Quote:
You already missed what I said about the bus being split. The CPU side runs at a different transfer rate than the memory bus.



http://www.techterms.org/definition/fsb

Quote from techterms.org

“The FSB connects the computer's processor to the system memory

The speed of the frontside bus is measured in Megahertz or Gigahertz, just like the processor. Most computers' processors run faster than their system buses, so the FSB speed is typically a ratio of the processor speed. For example, a Pentium 4 processor that runs at 2.4 GHz may have an FSB speed of only 400 MHz. The CPU to FSB ratio would be 6:1. A Power Mac G5, however, with a 2.0 GHz processor, has a 1.0 GHz frontside bus. Therefore, its CPU to FSB ratio is 2:1.”

You are confusing CPU frequency with FSB. My processor runs at 2.8 GHz. The CPU frequency is 200 MHz independent of MB frequency which could be 200 MHz or 133 MHz. On this processor the pipe line is divided up into 14 stages (16 actually but 2 stages are wait states, waiting for the signal to propagate across the chip.) The CPU can run 14 instructions through the pipe line on each cycle. 200 MHz times 14 equals 2.8 GHz.

Since you used the term “CPU bus” I googled it. This is the only relevant link I found.

http://pclt.cis.yale.edu/pclt/PCHW/BUS.HTM

• The CPU bus has a clock speed of 100, 133, 166, or 200 MHz. An Intel CPU transfers eight bytes of data four times per clock cycle, producing an effective speed of 800 MHz.
• Memory can transfer data at twice the rate of a 133, 166, or 200 MHz clock. Each transfer is 8 bytes if there is a single memory bus, and 16 bytes in a dual memory bus system. Thus the aggregate bandwidth of memory can match the bandwidth of the CPU.

You see, 200 MHz clock on the “CPU bus” (means the same as FSB) times 4 transfers per cycle equals 800 MHz effective speed. 8 bytes per transfer at 800 MHz equals 6400 MB/s bandwidth.

On the memory side, 200 MHz clock times “twice the rate” (this is DDR) equals 400 MHz effective times 16 bytes per transfer equals 6400 MB/s bandwidth. The aggregate bandwidth matches.

But 16 bytes per transfer only applies to a dual memory bus! (Dual channel)

On a single channel system, the bandwidth should be half (3200 MB/s) which is half the processor’s max bandwidth.

3200 MB/s at 800 MHz implies 4 bytes per transfer. (3200 divided by 800). If the motherboard is rated for 800 MHz FSB but the max bandwidth of the memory is 3200 MB/s, then the only way the arithmetic works is if the width of the FSB is only 32 bits.

This gets back to my original question on the FSB standard. If you double the width of the data path at the same frequency you double the rate of data transmission – you can transfer data twice as fast. The words “rate” and “fast” imply, to my mind, speed. If you double the rate of data transmission it seems fair, to me, to say you doubled the speed. So I wanted to know if FSB speed refers to “speed” in its broader context or if it only refers to frequency. It seemed to me a very simple and straightforward question.

I have spent untold hours searching the net for a straight answer to this simple question to no avail. The only answer I have received in this forum is Pat’s somewhat insinuating freeway analogy. Retailers and manufacturers rarely give you the bandwidth specs, they always quote FSB speed. Well, what the devil does FSB speed mean? If the width of the data path is not calculated into the FSB standard then to get bandwidth you should multiply FSB speed by the width of the data path. But, retailers and manufacturers rarely mention the width of the data path.

To figure out what is going on you need to be a detective.

Incidentally,

Quote:
There is DDR2 memory rated at 800MHz data rate, it runs at 400x2, rather than 200x4. .


You may be right, but this seems to contradict Pat.

Quote:
Intel FSB 800 MHz is in fact, a 200 MHZ qith QDR. four time the amount of data for each clock cycle.
a b V Motherboard
a b } Memory
December 12, 2005 5:46:26 PM

Actually I'm a writer and I can tell you that the definition you're reading needs to be revised. I should give you some history on FSB to explain why modern definitions can get screwed up:

Originally the FSB went from the CPU to the RAM through the memory controller. One speed.

In about 2000 several chipsets started splitting the Front Side bus into TWO separate "Sides", the CPU bus and the RAM bus. This made it possible to run a 133MHz FSB CPU with PC100 SDRAM, or PC133 SDRAM at full speed with a 100MHz FSB CPU. Tiawanese companies like VIA were first to do this, with Intel following on such chipsets as the i810 and i815.

This is where things took a strange turn. Intel decided to start using Rambus technologie's RDRAM for their P4, before the P4 was released. To get RDRAM on the market first, Intel released two chipsets, the i820 and i840, to support RDRAM. The most common speed for RDRAM back then was PC600, where the 600 refered to an actual data rate of 600MHz.

And things get even stranger as RDRAM used DDR technology for their own bus, so that PC600 actually ran at 300MHz using a 133MHz FSB CPU!

Around the same time, AMD introduced their Athlon series with a DDR CPU bus. Chipsets supporting it used standard PC100 and PC133 SDRAM, so you had a "200MHz FSB" CPU running to PC133 SDRAM. The 200MHz data rate was actually based on 100MHz clock rate, with the data transmitted twice per clock cycle, so in clock terms, a 100MHz bus was tied to PC133 SDRAM.

All this leads to the same point: Ever since the late 1990's the Front Side Bus has not really existed as such, instead being split into two separate buses, the CPU bus and memory bus, which are joined together by the memory controller.

You can supply that information to techterms.com and tell them to get their act together.

So now that it's apparent you have no clue as to FSB, you can re-think everything else!

CPU FSB of 800 is 200x4 and gives 6400MB/s. DDR400, aka PC3200, is 200x2 and gives 3200MB/s. If you put the memory in dual-channel, it's still 400MHz data rate but the tranfer rate doubles. So PC3200 is always running 400MHz data rate, even in dual-channel mode, but it's transfer rate (different than data rate) increases by 2x when it's in dual-channel mode.

Now, since PC3200 is running at 200MHz clock rate regardless of whether or not it's in dual-channel, you can forget your 800MHz reference. It's 400MHz data rate, using either a 64-bit or 128-bit bus depending on whether or not it's in dual-channel mode.

OK, now to your last statement, PC2-6400 runs at 800MHz data rate, with a clock rate of 400MHz, so it's 400x2. Intel's 800MHz FSB is 200x4. These do NOT contradict each other because they are NOT the same bus. FSB now refers to the bus that connects the CPU to the chipset (the memory CONTROLLER), so that the FSB can be completely different than the memory speed.

And BTW, FSB refers to data rate (frequency of transfers) between the CPU and chipset.
December 12, 2005 5:48:24 PM

I was not talking about ddr2, but ddr only.
December 12, 2005 6:07:40 PM

Quote:
I was not talking about ddr2, but ddr only.


Pat, are you saying you can get 4 reads per cycle with DDR?

Quote:
Intel FSB 800 MHz is in fact, a 200 MHZ qith QDR. four time the amount of data for each clock cycle.


That is not the information I have found on the net. I think DDR only reads twice per cycle.

http://arstechnica.com/paedia/r/ram_guide/ram_guide.par...

DDR DRAM
DDR DRAM is basically just a more advanced flavor of SDRAM, with an added twist at the data pins. If you recall the SDRAM timing diagrams from the previous RAM Guide, you'll know that SDRAM transfers its commands, addresses, and data on the rising edge of the clock. Like regular SDRAM, DDR DRAM transfers its commands and addresses on the rising edge of the clock, but unlike SDRAM it contains special circuitry behind its data pins that allows it to transfer data on both the rising and falling edges of the clock. So DDR can transfer two data words per clock cycle, as opposed to SDRAM's one word per clock cycle, effectively doubling the speed at which it can be read from or written to under optimal circumstances. Thus the "DDR" in DDR DRAM stands for "Double Data Rate," a name that it gets from this ability to transfer twice the data per clock as an SDRAM.
December 12, 2005 6:29:11 PM

call me crazy..but i think the term that you're asking for, which everyone is overlooking is dual channel

200mhzx2 is what pc3200 runs @. Installed alone only runs @ 200mhzx2 (theorhetical 400mhz)
Installed in pairs on dual channel board its 200mhzx2 x2 =the theorhetical 800mhz speed. (really still only running @ 200mhz) Makes me wonder if they had pushed rambus what our speeds would be. Yes i said rambus. PC1066 really ran @1.06ghz but was 16bitsx2 ..sigh dont get me started
December 12, 2005 6:36:14 PM

Quote:
call me crazy..but i think the term that you're asking for, which everyone is overlooking is dual channel

200mhzx2 is what pc3200 runs @. Installed alone only runs @ 200mhzx2 (theorhetical 400mhz)
Installed in pairs on dual channel board its 200mhzx2 x2 =the theorhetical 800mhz speed. (really still only running @ 200mhz) Makes me wonder if they had pushed rambus what our speeds would be. Yes i said rambus. PC1066 really ran @1.06ghz but was 16bitsx2 ..sigh dont get me started


Allready covered. Read the thread.

http://www.soyousa.com/products/proddesc.php?id=291

The board in question is single channel, DDR400 and runs at 800 MHz FSB.
December 12, 2005 7:49:16 PM

the VIA PT800 is ok, you might wanna look at the PT880. Maybe i missed something. DDR400 is pc3200 and will not run at 800mhz unless it's ran in dual channel.
So yes the FSB will be 800mhz but the memory just wont run synchronous. Memory doesnt need to run @ the same speed of the FSB. The mem will only run @ 400. If thats the case just shell out a few more bucks (PT880) and run it in dual channel. You'll be blown away by the added memory bandwidth when playing games.
hope this helps.
December 12, 2005 9:57:47 PM

Thanks Dontyouknow that was helpful.

The only reason I got the board is because it was my first build and I didn’t want to ruin a good board. It was the cheapest board on PriceWatch that would hold a Pentium IV - $30 including shipping. It actually turned out to be a decent machine too.

Actually I have a NEO PT880. I am typing this on that machine right now. I love it. My son and his friends are usually playing 3 way Doom or Brothers in Arms over the network, so I end up using the computer in the other room. A friend fished it out of the trash for me (someone had thrown it away) and I managed to find enough spare parts to make it work. 33 MHz FSB and a 500 MHz AMD processor someone gave me. Very usable though if you don’t do any gaming.

There is still some stuff I don’t understand. My PT880 is dual channel DDR 400. It gives me 6400 MB/s bandwidth at 800 MHz FSB. If the VIA PT800 is single channel DDR 400, why shouldn’t it give me 3200 MB/s bandwidth at 400 MHz FSB?

You said, “Maybe i missed something. DDR400 is pc3200 and will not run at 800mhz unless it's ran in dual channel.”

Yes, that’s what I thought and partly why I started this thread. The board is rated at 800 MHz FSB and when I set the frequency to 200 MHz it gives me 800 MHz FSB (I think). The BIOS won’t let me set the FSB separately from the Dram and, in fact, you have to set the frequency with a jumper.

You said, “Memory doesnt need to run @ the same speed of the FSB. The mem will only run @ 400”

Well, if the max mem speed is 400 why shouldn’t the FSB run at 400?

If I set the frequency to 200 MHz, it gives me 800 MHz FSB (I think) and my Pentium rated for 533 MHz FSB won’t run. If I set the board frequency to 133 MHz, I would assume I am only getting 2,128 MB/s bandwidth. Pat seems to think you can get 4 reads per cycle from DDR but I have been all over the net trying to verify that without success.

Maybe with a better board I could set the FSB frequency separately from the DRAM timing. But, with DDR400 single channel, why should the board run at 800 MHz FSB anyway? It can only read enough data from memory for a 400 MHz FSB (I think). I have to be missing something.

The BIOS shows “CPU frequency 133 MHz” but I assume that is the CPU’s FSB frequency The CPU is running 2.8 GHz which implies a 200 MHz frequency. (The multiplier is 14.)

Anyway, thank you for the courteous reply.

Garth
December 12, 2005 10:39:05 PM

AH, I think I figured it out.

The FSB frequency is 200 MHz but can somehow make 4 transfers per cycle (quad data rate) for an effective frequency of 800 MHz. For the memory to keep up with the FSB you need dual channel. If you only have one channel, you only get two reads per cycle. You can’t cut the FSB in half because that would mean 100 MHz quad data rate and DDR can only read twice per cycle. To maximize data transfer you have to run the CPU FSB frequency at 200 MHz which is 800 MHz FSB maximum. Since you are only going to get 2 reads per cycle, this is really only 400 MHz effective FSB. However, to run at 200 MHz the processor has to be rated at 800 MHz FSB.

I’m glad I got that finally. I tend to become obsessive when I can’t understand something.
a b V Motherboard
a b } Memory
December 13, 2005 4:56:22 AM

NO, I don't know who you are, but you really need to shut up. PC3200 NEVER runs 800MHz in dual channel. Not even 800MHz effective. You're handing back to him the same misconceptions he put forward earlier.

DDR400 always has a 400MHz data rate on a 200MHz bus. Dual Channel makes the BUS WIDTH twice as much, 128-bits rather than 64-bits, and that has nothing to do with frequency.
a b V Motherboard
a b } Memory
December 13, 2005 5:12:43 AM

Wow, you're getting close but you're still making completely wrong and illogical assumptions.

OK, first of all, DDR400 is never 800. Second of all, it doesn't need to be.

400 and 800 are frequencies. Frequencies as in, times per second. NOT megabytes per second, times per second. Not gigabytes per second, but times per second.

DDR400 in dual channel mode still has a 400MHz data rate, but it's on a 128-bit bus rather than a 64-bit bus.

You got most of the rest right:
1.) The P4 runs at 200MHz clock, 800MHz data rate because the bus transfers data 4x per clock cycle.
2.) The RAM runs 200MHz clock, 400MHz data rate, because the bus transfers data 2x per clock cycle.
3.) Both busses are 64-bit wide. That means the P4 has 6400MB/s bandwidth, but the memory has 3200MB/s bandwidth
4.) To make the memory have 6400MB/s bandwidth, dual channel is employed. Each CHANNEL still has 3200MB/s, but the data is split between TWO channels, half the data goes one way, half the other. That allows twice as much data to be transfered, hence PC3200 in dual channel mode can transfer 6400MB/s WITHOUT running faster.

OK, now a separate issue:
1.) The example I just used is called SYNCHRONOUS CLOCKS, where the CPU bus (FSB) and RAM bus are both 200MHz
2.) Using dual-channel mode allows synchronous bandwidth, because the memory bus, at half the data rate, also has twice the data pathways in dual channel mode.
3.) The first thing I just said is not true of many systems. I've already explained that the FSB is NOT a single bus from the CPU to the RAM as was the case LAST CENTURY. Instead, it's half of that: the FSB now ONLY refers to the bus between the CPU and chipset memory controller.
4.) The memory controller has TWO SIDES, the CPU side is called the FSB now, and the memory side is usually just refered to by the transfer rate of the memory installed. IE, 800MHz FSB, DDR400.
5.) Many systems have non-synchronous frequencies. Many 533 bus P4 systems came with PC2700, aka DDR333. That means the CPU bus (FSB) operated at 133MHz clock rate (4x for data rate), but the RAM ran at 166MHz clock rate (2x for data rate).
6.) Any normal seven year old can tell you 133 will never be equal to 166, therefor your whole line of thought on FSB is flawed.
7.) I really do write for a computer hardware site. You should read my work, rather than outdatedly wrong definitions.
a b V Motherboard
a b } Memory
December 13, 2005 5:21:57 AM

Quote:
Since you are only going to get 2 reads per cycle, this is really only 400 MHz effective FSB. However, to run at 200 MHz the processor has to be rated at 800 MHz FSB


I nearly pissed myself when I read that!

I have news for you: My friend had a 400 bus P4, back in the days when Intel used 100x4 rather than 200x4. So it had an actual clock speed of 100MHz for the FSB, but a data rate of 400MHz, you can understand at least that much. But here comes the fun part:

He had that system for a year and a half. I had some extra PC3200 laying around. I put it in his system and set it to run at 200MHz clock rate, 400MHz data rate. His board was too old to support dual-channel mode.

So, he had a clock speed of 100MHz for his FSB and 200MHz for his RAM. Yet they both had 400MHz data rate.

Using your definition this would be impossible. Using that definition of the FSB being the entire pathway between the CPU and RAM, that would be impossible. But because both are wrong, it is possible. The memory controller did an amazing thing, it converted data at 100x4 to 200x2 when transfering between the CPU and RAM.

The FSB has been split for the entire 21st century. Using 20th century definitions, even if they've been poorly updated, would leave you more clueless than you already are. Since the FSB is split into two sides now, the CPU bus and RAM bus, you can have "asynchronous clocks".

If you don't believe me, google the term "asynchronous clocks" and see what you come up with.

After NEW technology allowed the CPU and RAM clock speeds to be asynchonous, most industries have gone on to use the term "Front Side Bus" to refer ONLY to the bus beteen the CPU and chipset. The other half, that from the chipset to RAM, is no longer included in the term FSB.
December 13, 2005 3:19:32 PM

Wait a sec....no i forgot what we were talking about...
oh yeah!!
It just comes down to this. pc3200 single channel will give you around 3.2gb transfer data rate. Running dual channel depending on the chipset will give you anywhere between 5.1-5.5GB's per second. That's basically all you really need to know (you said you were getting 6GB xfer data rate and thats phenomenal!!! gratz). But be assured that you can run the FSB and memory at different speeds. Mine is set at 4/5 My FSB is 1ghz (OC'D) and my mem is @ 400mhz
Happy Tuesday everyone!!
a b V Motherboard
a b } Memory
December 13, 2005 3:41:29 PM

Well, 6400MB/s is the limit for PC3200 in dual-channel mode. Actual transfer rates suffer from inefficiencies in the memory controller, etc. I get around 4800 out of a possible 6400 on some systems, less on others.
December 13, 2005 3:42:24 PM

Ok, Crashman, granted you could fill and encyclopedia with what I don’t know about computers. I apologize for antagonizing my betters, but when I am struggling to understand something I become very obsessive and a little crazy.



Quote:
400 and 800 are frequencies. Frequencies as in, times per second. NOT megabytes per second, times per second. Not gigabytes per second, but times per second.


I have already provided links on how to calculate bandwidth from memory speed but this is from an Intel white paper.

http://www.kingston.com/newtech/MKF_520DDRwhitepaper.pd...

Peak Bandwidth is calculated as:
Memory Speed x Number of bytes transferred per channel x Number of Channels
(its data rate) (8 Bytes or 64 bits) (one or two)
For PC3200, also known as DDR400, modules on a dual-channel motherboard,
Peak Bandwidth = (400 MHz) x (8 Bytes) x (2 Channels)
= 6400 Megabytes per second (MB/s)
or 6.4 Gigabytes per second (GB/s)

I presented the above calculation earlier in the thread which, I assume, is what you are objecting to. I may be ignorant of computers but I can do arithmetic. I don’t see where I confused frequency with bandwidth.

Quote:
DDR400 in dual channel mode still has a 400MHz data rate, but it's on a 128-bit bus rather than a 64-bit bus


From the same white paper.

“Intel’s newest Pentium4 processors have front-side bus architectures operating at data
speeds of 533MHz and 800MHz. This translates to a peak data bandwidth of 4.2GB/s
(533MHz x 8 Bytes) and 6.4GB/s (800MHz x 8 Bytes) respectively”

800 MHz x 8 Bytes. That is a 64 bit or 8 byte font side bus. The board reads 128 bits at once from memory, but the data is transferred to the CPU at 64 bits per effective cycle.

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OK, first of all, DDR400 is never 800. Second of all, it doesn't need to be.


Understood, this had always been my understanding. My confusion was because I misunderstood something pat wrote. When he said Qdr (quad data rate) he was referring to FSB. When I googled Qdr I only found links about memory. So, I thought, if Pat thinks DDR can run at quad data rate, maybe he knows and I don’t.

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Using your definition this would be impossible. Using that definition of the FSB being the entire pathway between the CPU and RAM,


I never said the FSB was the entire pathway.


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The FSB has been split for the entire 21st century. Using 20th century definitions, even if they've been poorly updated, would leave you more clueless than you already are. Since the FSB is split into two sides now, the CPU bus and RAM bus, you can have "asynchronous clocks".


If you don't believe me, google the term "asynchronous clocks" and see what you come up with.

I googled “asynchronous clocks”. In a synchronous system a timing signal is sent through the whole system to keep the components in sync. This heats up the board. Also, the system can only run as fast as its slowest component.

In an asynchronous system, each component has its own clock or no clock (frequency is determined by the width of the transistors and the speed of the current.) When a signal reaches a component, it is synchronized by various means.

Components on synchronous system can also run at different rates if the rates are multiples of the clock cycle - I.e. memory at 400 MHz, FSB at 800 MHz.

Asynchronous clocks have everything to do with synchronicity but nothing to do with frequency or bandwidth. The aggregate frequency of memory still must be less than or equal to FSB.



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After NEW technology allowed the CPU and RAM clock speeds to be asynchonous, most industries have gone on to use the term "Front Side Bus" to refer ONLY to the bus beteen the CPU and chipset


I was never confused on this issue. You could say FSB is the link between the memory controller and the CPU. But, here we have the thrust of the issue. On a single channel DDR400 system the FSB runs at 200 MHz and makes 2 transfers per cycle. 2 reads per cycle is all the memory can provide. This is 400 MHz effective. Since the CPU’s FSB side must run at 200 MHz and the Pentium can handle 4 transfers per cycle, the CPU must be rated 800 MHz FSB.

But the Front Side Bus is not on the CPU it is on the motherboard. The memory controller feeds 2 transfers per cycle to the CPU at 200MHz. That is a 400 MHz FSB. The CPU could handle twice that rate, but the board only supplies 400 MHz effective and a bandwidth of 3200 MB/s.

It seems to me the specs for the board should reflect the properties of the board, not the CPU. They should say 400 MHz FSB but requires a processor rated for 800 MHz FSB.
a b V Motherboard
a b } Memory
December 13, 2005 3:53:01 PM

By the way, I had a similar problem trying to convince some guy about AGP8x/4x/2x voltages and compatiblity. He said "But this site says otherwise". I told him "I write for a site too". He kept telling me, basically, that he'd believe a published document before he'd believe me. So, I published a document on AGP compatibility! You can read it at:
http://www.sysopt.com/features/graphics/article.php/3532496
Yes, it kind of got hacked up in the editorial process, but my point got across so the guy shut up.

Now I'm thinking I just might have to do an article on CPU and memory busses, and how todays busses are different from last-century's common FSB.
December 13, 2005 4:44:14 PM

Nice article, very well writen and informative.

I have to apologize for my comments on asynchronous clocks. The links I found for “asynchronous clocks” all have to do with system components being out of phase (no system clock so they get out of sync). If I google asynchronous bus however, I see that asynchronous is used to refer to mem and cpu running at sub-multiples of the system clock.
a b V Motherboard
a b } Memory
December 13, 2005 4:57:05 PM

Of course you can't get more data through the memory than the CPU, that's why people saw little advantage to running PC133 at 133MHz on their 100MHz bus processors. How could they see any advantage at all? Higher clock speeds reduce cycle time, and CAS latency is measured in cycles. CAS3 at 133MHz is faster than CAS3 at 100MHz. That's just a little background BTW.

Anyway, the perfect match for a 533 bus P4 would be PC2100 in dual-channel mode. But when the 533 bus was in it's prime, dual channel did not yet exist for DDR SDRAM boards.

So back then people would typically use 533 bus P4's with PC2700 RAM in single-channel. PC3200 hadn't gone mainstream yet. That's a CPU bus of 133MHz x4, and a RAM bus of 166MHz x2. Obviously the RAM had less bandwidth than the CPU FSB.

Yet back then, a 533 bus P4 on PC2700 was still faster than a 400 bus P4 on PC2700. How could that be...when PC2700 in single-channel had less bandwidth than even the 400 bus P4? Isn't the limitting factor, PC2700, the same?

Well, sort of. That FSB from the CPU to the chipset also handled other types of transfers, transfers to local busses and ports like AGP. In fact, one of the main advantages added to AGP was direct access to the CPU without going through RAM. Of course RAM was still caching some stuff for the AGP card, mostly textures, hence there was also a setting called "AGP Aperture" to limit the amount of RAM that could be used as cache by the AGP.

Synchronous data rates are nice, but not always used. Synchronous clock rates between the CPU bus and RAM used to be required, but no more. And Intel uses a Quad Data Rate bus, which is properly but rarely labeled QDR, for their CPU.
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