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Should memory match bus speed??

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January 29, 2001 6:11:11 AM

I am building my first system. I want to use ABIT KT7A-RAID MOBO with AMD 800 T-Bird & Mushkin PC133. The MOBO supports up to 266MHz FSB. I've read that memory speed must match the bus speed? It doesn't in this application, or am I wrong? Can anyone explain this mem/bus theory???

More about : memory match bus speed

January 29, 2001 9:02:33 PM

I'm running 100Mhz & 66Mhz RAM on my 66Mhz PII. But the RAM speed will be stunted by the bus speed in this case. I am told that having RAM all at same spec is best.
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a b } Memory
January 30, 2001 2:12:22 AM

These people are all WRONG. The Athlon uses 133MHz FSB, which is named 266 because it is running at Double Data Rate (DDR). Unless you had a board made for DDR-RAM of SDRAM, the CORRECT ram speed is 133. Your motherboard is NOT DDR-RAM compatable. Therefor your memory bus will only run 133. 133 is the memory to get. Nobady makes "266" memory, the DDR stuff is called PC2100 and does not fit your mtoherboard.

Suicide is painless...........
a b } Memory
January 30, 2001 4:11:23 AM

No, I wouldn't. But it's not the same thing. Athlons DO NOT RUN 266, AMD lied. They run 133 at DDR, which they NAMED 266. So all references to that number are deceptive. The only way you can have matching bandwidth is with DDR RAM, which his motherboard does NOT support! There is no such thing as 266MHz SDRAM. The PC2100 was originally supposed to be named PC266 in AMD fashion, but if your motherboard does not support DDR ram, you can't use it. He can't.

Suicide is painless...........
January 30, 2001 7:54:08 AM

Many thanks Crashman. After reading your reply, I remembered reading something about specs. Its rated at 266 cause its double pumped 133, right? Basically what you already said. What I was getting at, for example, mobo's that boast a 100MHz system bus (FSB)"ideally" should support SDRAM memory speeds of 100 MHz,ie PC100, right? But the 800MHz T-bird only supports 200 MHz FSB, so using PC133 will be a bit of overkill? Will it matter, or have I got this wrong again?
a b } Memory
January 30, 2001 1:46:31 PM

You've got it right so far. But one more detail-many motherboards allow separate memory and bus timing at 100/133. The new chipsets also allow 133/133 as well as 100/133 and 100/100. So your 266FSB Athlon would be set at 133/133. The 200FSB Athlon would be set at either 100/100 or 100/133. Notice the HIGHEST memory setting is 133! But you can exceed that slightly by overclocking the FSB (example FSB266 at 280 would be set at 140/140 and could still use high quality PC133). Even the multipliers on the chip are actually set to the true bus speed (a 1GHz Athlon 200FSB is actually set at 10x100) instead of the inflated numbers that represent DDR. So for any non-DDR ram AMD motherboard, PC133 is ideal.

Suicide is painless...........
January 30, 2001 2:16:52 PM

The KT7A does not allow you to run the memory and CPU bus at different speeds. The VIA KT133 chipset allowed it, but the KT133A does not. The 800 T-bird will work fine at 133Mhz FSB if you unlock it. Mushkin high performance memory is the best, however Crucial also makes good memory if you want to spend less.
January 30, 2001 4:32:15 PM

Crashman, its all becoming clear now...or was. Mr Yoda has said that the KT7A will not allow me to run memory and CPU at different speeds. So I'm going to ask him about an explanation. Again, thanks for your help and patience. I am yet another newbie attempting to design and build his own. Still in the research stage. Can't absorb this stuff fast enough.
January 30, 2001 4:42:32 PM

Ok Mr.Yoda, I thought that I had this down. Please tell me how or where I find out how to 'unlock' the FSB. Can it be done with the softmenu that is part of the KT7A?
January 30, 2001 6:37:58 PM

When I said <i>unlock</i> I was referring to the processor, not the motherboard. if you look on top of an Athlon processor you will see a bunch of <i>bridges</i>. some bridges look like two dots ' : ', others look like a line ' | '. Alongside the sets of bridges are designations like <i>L1</i>. This is the set of bridges we are interested in. to unlock the processor you must connect the two dots on all bridges that look like ' : '. That's what is meant by <i>closing the bridges</i>.

I probably confused you more than I helped you so i will refer you to <A HREF="http://www.tomshardware.com/cpu/00q3/000711/index.html" target="_new">Tom's article</A>. Now this article talks about how to modify your processor if your motherboard doesn't allow multiplier adjustments (the Abit KT7A does not have this problem), so you would only close the L1 bridges, and not have to worry about selectivly closing and opening bridges as in the article.

Now back to your original question. Get the Abit KT7A, it is an excellent motherboard. Pair it with some Mushkin or Crucial PC133 memory and unlock your 800Mhz T-bird. Set the FSB speed to 133Mhz and the multiplier to 6x. You will have your memory and CPU bus running synchronously at 133Mhz and your T-bird running at 800Mhz. At this point you could even overclock your T-bird by increasing the multiplier. 1Ghz is not abnormal for an 800Mhz T-bird.
a b } Memory
January 30, 2001 8:08:45 PM

Yoda has it right, although I am not completely sure if the 100/133 setting is no longer available as it had been. It would not apply to you anyway, because you would want 133/133.
BTW-Your memory always runs at the set speed i.e. if you have your system bus/memory set at 133/133 it will run that speed even if it is rated at 150 (as some now are). If it was set as 100/100, the same memory would run 100.
The only issue that can affect speed from there is Cas Latency, or the number of cycles it takes to get the memory to respond (aka-transfer the "first bit"). Cas 2 takes two cycles, Cas 3 takes three, so Cas 2 is a little faster.
I recomend Crucial as a memory supplier, as they have higher standards for testing than many other companies and are less likely to be dishonest concerning specifications. They are one of the less expensive suppliers of high-end memory. Even faster memory can be had from Mushkin, but the additional speed will have no affect on your system because it will only run the speed the bus is set at. It is also more expensive.

Suicide is painless...........
January 31, 2001 7:45:03 AM

Thanks for the explanation Yoda. I don't have the chip in hand yet, but I can visualize what you mean. I will refer to Tom's article as well. Thanks to Yoda and Crashman for your help and advise. It is evident that the greatest pool of knowledge lies with the people who are out there actually doing this stuff. Let the adventure continue!


Not all those who wander are lost....
January 31, 2001 1:48:44 PM

Yoda, one more detail. I understand I'll connect ONLY the L1 bridges to unlock the processor. This will bump the former 200MHz FSB to 266, right? You also said I need not worry about multiplier adjustments because the KT7A does not have this problem. Does this mean I adjust the multiplier to 6x with the KT7A's softmenu or bios?
!