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RDRAM Confusion, please help

Last response: in Memory
February 16, 2003 1:53:27 AM

I have an old Intel VC820 motherboard that supports 100mhz or 133mhz fsb. I have an Intel PIII 1.ghz processor can run at 133mhz fsb. The memory i have is rambus ram that runs at 2x400mhz which i always thought was PC800. I can't get the system to run at 133mhz bus, it stays at 100mhz and is set automatically, tx intel.
My thoughts are that the rdram i have can't run at 133mhz so i was looking to buy ram that can run at 133mhz fsb. I get all confused by modules that say they are RDRAM pc800, RDRAM 800MHz, PC1066 and 1066MHZ. I thought i would need pc1066 but i can find modules that are RDRAM 800MHZ that run at fsb 133mhz. does anyone know the clarification of these?

More about : rdram confusion

February 17, 2003 12:37:37 AM

The good question does your mothersboards support 133 FSb

Just next to the lab and the bunker you will find the marketing departement.
February 17, 2003 12:37:37 AM

The good question does your mothersboards support 133 FSb

Just next to the lab and the bunker you will find the marketing departement.
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February 18, 2003 5:32:21 PM

The Intel VC820 Motherboard supports both 100mhz fsb and 133 mhz fsb. The PIII 1Ghz processor can do both. I have two 128Mb PC800 Rdram but the system boots using only the 100mhz fsb. What Rdram would i need to run it at 133mhz fsb???
Since this is an intel board it doesn't support forcing a clock or bus multiplier...the board chooses it automatically at boot.
February 19, 2003 12:53:20 AM

The board you have supports PC600, PC700 and PC800 RDRAM.
It also supports 100MHz and 133MHz system bus. (FSB)
Download and install the Intel Processor Frequency ID Utility to determine your CPU's core speed and system bus speed.
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There are many busses in your computer. The two busses concerned in your post are the system side bus (aka FSB) and the memory bus. The busses connect to the northbridge (Intel calls the northbridge a Memory Controller Hub [MCH]). The AGP bus and bridge interface bus (Hub Interface Bus [HIB]) also connect to the northbridge.
Your chipset uses the 82820 northbridge. If you are using a 1.0GHz PIII with a 133MHz processor bus, PC800 RDRAM and a 4X AGP card your respective bus frequencies will be:
System Side Bus (FSB) 133MHz or 1064MB/s
Memory Side Bus 400MHz (PC800) 1600MB/s
4xAGP Bus 266MHz or 1064MB/s
Hub Interface Bus 66MHz or 266MB/s
The bus frequencies are multiples of the clock generator on your mother board. The crystal frequency used in the clock generator (frequency sythesizer) is 14.381MHz. All frequencies used in the computer are subdivided or multiples of the clock generator crystal. However the real time clock uses its own crystal. Also the 14.381MHz clock generator crystal has been used since IBM made the first PC in 1981. This crystal frequency is used on all motherboards manufactured today.
In many personal computers the System bus and memory bus run asynchronously. That simply means they do not run at the same speed. This is most common in systems using a P4 and DDR memory.
But the P4 was designed to use RDRAM. Using my system as an example I have 2.40B P4 cpu, 850E chipset and PC1066 RDRAM. In my computer the System Bus (FSB) and memory bus run synchronously at 533MHz. This is the optimum performance setting because the theoretical data throughput from the CPU to the MCH and memory to the MCH are matching at 4.2GB/s. This means the cpu does not have to waste clock cycles waiting for data in memory. FSB is old terminology. It should be called System Side Bus. The System Bus or FSB for legacy minded individuals is clocked from the system clock on the motherboard. So if you are using a P4 it will be 400MHz or 533MHz or if you use an Athlon XP it is 266MHz or 333MHz.
P4 cpu's are made to be used with RDRAM. The P4 with it's quad-pumped processor bus transfers data four times per clock cycle.
Athlon cpu's are made to be used with DDR-SDRAM. The Athlon with it's double-pumped processor bus transfers data two times per clock cycle.
Inside the P4 core is the processor bus. The processor bus interconnects all the components inside the core. The processor bus inside a P4 core is quad-pumped. The actual core frequency from the system clock is 100MHz/133MHz and data is transfered four times per clock cycle giving an effective rate of 400MHz/533MHz. The system bus is the bus between the Northbridge and the cpu. The frequency of this bus will be 400MHz/533MHz. Somtimes the system bus is called the processor side bus. Which makes things even more confusing.
In a P4 cpu the internal processor bus is quad-pumped. The external cpu bus or system bus or front side bus is not. This is the physical bus that runs between the cpu and the northbridge.
So when you run the Intel Processor Frequency ID Utility it gives you the CPU core frequency and the System Bus frequency.

Getting back to your post. I suspect the cpu you have supports a 100MHz system bus. Use the link I provided and install the English version of the Intel Processor Frequency ID Utility. It will tell you your processor speed, system bus speed, amount of cache, cache speed, cpu stepping, etc.

The term Front Side Bus comes from when cpu's had external L2 Cache memory. The L2 Cache was connected to the cpu using a Back Side Bus. The BSB ran at half the cpu clock speed for Intel CPU's and 1/3 the cpu clock speed for AMD CPU's. The FSB is used to connect the CPU to the Northbridge. The term Front Side Bus is slowly phasing out and more use of the term System Bus is phasing in as the L2 cache is now integrated into the cpu core on modern processors.

<font color=red><i>Doctor Hooter</i></font color=red> <A HREF="" target="_new"><b>(·Y·)</b></A>
a b } Memory
a b V Motherboard
February 19, 2003 5:42:31 AM

The PIII 1Ghz processor can do both

No. It can do either, or. There were two bus speed versions. The more common version ran at 133x7.5, the less common kind ran at 100x10. If your computer is running at 1000MHz with a 100MHz bus, you can't do 133MHz bus. This is because the multipliers of all PIII CPU's are locked. In other words, if you're running at 10x100, you would have to run at 10x133, aka 1333MHz, in order to use the higher bus speed.

You see, Intel has 3 bus speeds for all their CPU's over the past 8 years. 66MHz, 100MHz, and 133MHz. In order to get 400MHz out of a PII, the CPU ran a multiple, 4x times the 100MHz bus speed. And that 4x mutliple was locked on the CPU. As far as I know, the last CPU to NOT be locked by Intel was the PIII 333.

So getting back to you. If your CPU is a 1000MHz PIII, and it's only runinng at 750MHz, you need to change it to 133MHz bus in order to make it 1000MHz. But Intel boards are made to AUTO DETECT bus speeds, so it's very doubtfull your CPU is running at 750MHz.

Therefore if it's running at 1000MHz and 100MHz bus, you have the 10x multiplier, as previously explained. Since you can't change your multiplier, you can't change your bus speed to 133, because that would force your CPU to run at 1333MHz, and only Crashman can overclock that far, no mear mortals need try!

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