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Some doubts about bus speed

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  • Motherboards
  • Bus Speed
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Anonymous
a b V Motherboard
August 27, 2001 10:25:38 AM

I've some doubts about bus speed: I've bought a K7 1.2 GHz, 266 Mhz, the A7M266 motherboard and DDR SDRAM (266 MHz). Can I understand that the communication between micro and ram is at 266 Mhz or -since my agp graphics card runs at 133 mhz, or my pci bus runs at ??? mhz, or my hd communicate at a different speed- must I suppose that the communication is below this limit?.
In the same line: in my bios menu I can only choose 100mhz and 133 mhz. what must I do?
Another more: if I change my agp card (nvidia geforce2 200 mx) to the next (2.1Gb/s, if I remember, nvidia geforce2 400 mx), does it improve the speed?
Thank you very much

More about : doubts bus speed

August 27, 2001 1:03:05 PM

all the speeds you see on a motherboard are different and independent of each other.
FSB (Front Side Bus): The speed the processor runs its bus on, and communicates wit the chipset. I could have said memory but many chipsets decouple the CPU-RAM transactions and can allow different speeds of the two. Its used to be 60/66 MHz for Pentium, 66/100 MHz for P-II, 100/133 MHz for P-III and 100 MHz for P4. AMD processors run at 100 or 133 MHz FSB. In case of AMD the frequency is Double Rated, that is it performs twice the amount of data transactions in a single cycle by ingeneously using both the clock edges. Intel and other processors traditionally use only one edge and thus offer bandwidths exactly half that of AMD processors. P4 has a Dual Channel Double pumped FSB that makes a whopping 400 MHz and offers a huge bandwidth of 3.2 GB/sec, but its still 100 MHz physically.
This FSB is multiplied by the processor multiplier at which the processor runs internally.

AGP and PCI and ISA speed: These bus speeds are derivatives of the FSB. A PCI bus should run at 66 MHz while AGP at 133 MHz. The PCI bus is derived from the FSB by dividing it by a certain number to get 33 MHz, i.e you can find some options in the BIOS that will ask you for the PCI/Host ratio; that is the divider. Its Host/2 or Host/3 or Host/4 for Host=66 or 100 or 133 respectively.

The ISA bus is derived from the PCI clock and is close to 12 MHz.

All the latest AGP cards operate at 133 MHz.

Hard disk controllers are onboard PCI devices (or sit on newer proprietory inter bridge links like the VIA V-Link or SiS MuIOL) If they are PCI devices they operate at a speed at around 33 MHz.

Your motherboard does not seem to support overclocking, else it would have offered you options in steps of 1 or 2 MHz. In this case dont go too far, set 100 MHz for your 100 MHz FSB processor and 133 for 133 MHz FSB processor.

upgrading your AGP card will improve your display performance, the frame rate and rendering speed and all that stuff. It wont have much effect on your system performance.

girish

<font color=red>No system is fool-proof. Fools are Ingenious!</font color=red>
Anonymous
a b V Motherboard
August 27, 2001 1:26:46 PM

Thank you very much. So, I can deduce that my micro can communicate with the ddr ram at 133 MHz x 2 (266 MHz effective), since micro, motherboard and ram support this speed.
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Anonymous
a b V Motherboard
August 27, 2001 1:29:33 PM

Good answer to a good question.

Medication helps :smile:
<font color=blue>THG needs 2 change the sig' of the week errrr century!</font color=blue>
August 27, 2001 4:26:53 PM

thats right. the A7M266 is a DDR board so it really utilises the full potential of the Athlon DDR bus, and enjoy a memory bandwidth of 2.1 GHz (if you have PC2100 DDR SDRAM)

well, this bandwidth isnt used for memory transactions alone, it is also used for I/O acceses - that is to read and write from the different onboard devices, the cards and the peripherals. so it might not achieve full memory transfers, you always get somewhat lesser value in memory benchmarks.

P.S. the PCI bus speed I wrote earlier as 66 MHz is only supported by newer PCI 2.1/2.2 spec. Standard PCI busses we see on regular desktop boards is still only 33 MHz, 32 bit and offers 133 MB/sec of peak transfer rates.

advanced server boards support these 64 bit 66 MHz PCI bus that offers 528 MB/sec of transfer rates! More performance!!

girish

<font color=red>No system is fool-proof. Fools are Ingenious!</font color=red>
Anonymous
a b V Motherboard
August 27, 2001 7:00:34 PM

dude,
I think you messed up somewhere, the PCI bus runs at 33MHz not 66, and the AGP is at 66MHz, 133MHz for sideband if it is a 2x slot, 266MHz sideband if it is 4x. That equals 1/3 fsb(100MHz) or 1/4 fsb(133MHz) or 1/2 fsb (66MHz)for PCI and 2/3 fsb(100MHz) or 1/2 fsb(133MHz) or 1/1 fsb (66MHz)for AGP. This might be what you meant,but your explanation is a bit confusing about the PCI and AGP....
August 28, 2001 2:10:38 AM

I'm going to try to explain this with some links, cause that way I take no credit and no argument either :smile: . I basically disagree with Girish as well.

<A HREF="http://library.thinkquest.org/C006208/data/system-fsb.p..." target="_new">Link 1</A>

The FSB is the bus on the motherboard with which the processor communictes with the RAM, chipset and L2 cache. Speeds are usually 66,100 and 133 MHz.

Where you see 200MHz and 266MHz (with AMD systems), the <b>bandwidth</b> has been doubled (DDR) allowing 2 lots of 100MHz or 133MHz to operate. The 400MHz stuff is as above.

Memory PC66, PC100 & PC133 should be self explainitory. PC1600 & PC2100 are for the 200 & 266 DDR systems respectively.

<A HREF="http://www.zdnet.co.uk/pcdir/content/1999/09/peripheral..." target="_new">AGP</A>

DDR graphics cards take advantage of the doubled bandwidth again.

<b><font color=blue>Change the</font color=blue><font color=red> sig of</font color=red><font color=green> the week!</font color=green> :tongue: </b>
August 28, 2001 4:14:14 AM

Anyways.... your computer will work fine.

Tr@k0r.
August 28, 2001 11:08:19 AM

Quote:
The FSB is the bus on the motherboard with which the processor communictes with the RAM, chipset and L2 cache.


Not quite - the FSB is the bus that connects the CPU to the memory controller (to purist it would be RAM, but as the CPU doesn't connect directly to the RAM, and the memory controller is integrated in the northbridge chipset, we can also say that is the bus that connects to the chipset). It is not the interface with the todays ondie L2 cache.



How terrible is wisdom when it brings no profit to the wise<P ID="edit"><FONT SIZE=-1><EM>Edited by arbee on 08/28/01 07:14 AM.</EM></FONT></P>
Anonymous
a b V Motherboard
August 28, 2001 11:45:41 AM

Nice explanation! All the posts are pretty accurate in one way or another, just a little cloudy at times. Your post should clear this all up for someone who found the others a bit confusing, including mine! hehe
August 28, 2001 2:02:02 PM

I'm glad.

It took me about 2 hours of browsing (Yes I got side tracked a few times, as you do on the web) to find sites which I found explained it quite well.

Funny. I couldn't find a site which directly explained 200/266 FSB stuff. I reckon no-one wants to take any stick if they mess up.

<b><font color=blue>Change the</font color=blue><font color=red> sig of</font color=red><font color=green> the week!</font color=green> :tongue: </b>
August 28, 2001 5:35:38 PM

that is what i added in my next post that followed. there is a spec that makes the PCI run at 66 MHz and allows a width of 64 bits. i kinda forgot to write about the common 33 MHz PCI bus in the first one. 2X 133 MHz AGP is common, and 4X AGP is becoming commonplace these days. that is why 133 MHz was in my mind.

I guess I wasnt wrong, I just left out the finer details for brevity, and that might have turned out bad.

sorry guys...

girish

<font color=red>No system is fool-proof. Fools are Ingenious!</font color=red>
August 28, 2001 7:05:46 PM

Quote:
The FSB is the bus on the motherboard with which the processor communictes with the RAM, chipset and L2 cache. Speeds are usually 66,100 and 133 MHz.

Well, the FSB is the bus that the CPU communicates with outside world, not necessarily with the cache, the chipset or memory alone.

Cache is very personal to the processor, independent of memory and could run at the full CPU speed (not the FSB but at full CPU speed that is FSB*multiplier) if its on-die , or at half or a third the speed if its off-die cache or at a certain asynchronous speed if its off-die, off-module third party chips integrated on the motherboard.

Chipset these days is the processors's Personal Assistant that manages <i>EVERYTHING</i> for the processor including memory accesses, I/O accesses to interrupts! I could well make a design <i>without</i> a chipset, but then I would need to connect different chips that will work different things for the processor. The chipset also serves as a interface for any kind of memory, a kind of translator for the processor. the memory controller in the chipset can work with any kind of memory, be it a SDRAM, a DDR-SDRAM, VCRAM or RDRAM, but it will talk to the CPU in its own native bus. 64 bit DDR link for the AMD processors and 64 bit SDR for Intel P3 family processor, or 64 bit Dual channel Dual pumped bus for Intel P4 family of processors.

Memory, as I said, does not directly communicate with the processor, but the memory controller in the chipset. that is why it could run independent of the processor FSB. many controllers decouple the CPU/memory bus so that you could use a faster processor with slower memory.

The speeds are usually 66, 100 or 133 MHz, but earlier Intel Pentium processor some Cyrix processor do work with 60, 75 or 83 MHz FSB! before that, there was no FSB concept and the processor worked at the same speed as that of the bus. the first processor to use a multiplier was the 486DX2/66 that ran at a FSB of 33 MHz and clock multiplied by 2 to run at 66 MHz internally. it was followed by DX4/100 with 33 * 3, DX2/50 and DX4/75 that had 25 MHz FSB!

the DDR FSB of AMD processors is a ingeneous trick that effectively doubles the memory access speed by transferring data twice at each cycle, while traditional processors did it only once.

Quote:
Memory PC66, PC100 & PC133 should be self explainitory. PC1600 & PC2100 are for the 200 & 266 DDR systems respectively

all SDRAMs are named by their operating speed while DDR and RDRAMs are designated by their transfer rates. those are different!

PC66, PC100, PC133, PC150 are obvious that run at 66, 100, 133 or 150 MHz speeds, this speed is the maximum the module can run at reliably, you might use it at lower speed and might overclock a bit as well!

with PC800, PC1600, PC2100, 800, 1600, 2100 etc are respective transfer rates in MB/sec of the memory modules.

girish

<font color=red>No system is fool-proof. Fools are Ingenious!</font color=red>
August 28, 2001 7:15:25 PM

on what do you disagree with me?
i might correct myself.

regards,
girish

<font color=red>No system is fool-proof. Fools are Ingenious!</font color=red>
August 29, 2001 12:18:20 AM

Basically the 66MHz PCI stuff. Peace man. You've explained where you were coming from.

<b><font color=blue>Change the</font color=blue><font color=red> sig of</font color=red><font color=green> the week!</font color=green> :tongue: </b>
!