Athlon 64 is a special beast. It was the 1st x86/x64 CPU with integrated memory controller and without the concept of a separate CPU-NB (Phenom) or Uncore(Intel) the memory frequency is linked to the CPU core frequency not the 'FSB'. It uses a whole number divider to get the memory clock form the CPU clock. This can result in quite counter intuitive situations. E.g. in case of the DDR2 mobile models that officially support only DDR2 666, an 1600 Mhz Turion64 X2 CPU with a divider 5 uses a 320 (640 DDR) MHz memory clock, but an 1800 MHz model with a divider 6 can only use a slower 300 (600 DDR) MHz memory clock because divider 5 would give an out of specification 360 (720 DDR) MHz memory clock.
So in case of Athlon 64 some CPU clocks are more ideal than others (regarding memory performance). E.g. a 2000 MHz CPU can use both DDR2 666 (divider 6) and DDR2 800 (divider 5) memory at maximum speed unlike the previously mentioned models.

Also because of this tight link between CPU and memory clock the power saving performance states can be detrimental to shared memory bandwidth of integrated VGA chips. At the minimum P-state the CPU clock is only 800 MHz and with the default divider 5 the memory clock is only 160 (320 DDR) MHz. So Aero desktop performance can be far from ideal in the lowest P-states using integrated VGA.

It is still bit confusing. CPU-Z still shows DDR speeds based on the FSB, like if I overclock from 200Mhz to 220Mhz in bios it changes memory speed from 400Mhz to 440Mhz in CPU-Z.

On the Memory tab of CPU-Z you can also see how the memory speed is calculated based on the CPU clock (e.g. CPU/6):

BTW It's an older version (1.54)
Edit:
Of course DRAM speed indirectly depends on the FSB speed since the CPU Clock itself depends on the FSB speed. But the DRAM speed directly depends on the CPU speed.
So if you change the multiplier of your CPU the DRAM speed also changes. Here's the 1800 MHz result. As you can see the FSB speed is still the same yet the memory speed is different because of the different multiplier/CPU clock:

In my pc cpu-z says that that FSB:DRAM is exactly CPU/multiplier, which would mean that no matter what the multiplier is the speed should stay the same, right?

Athlon64 doesn't have FSB. It uses Hyper Transport bus.

Well, in this case, CPU-Z is wrong.
HTT x CPU Multiplier = Core Clock
Core Clock / Memory devider = DRAM Speed.
There is no direct link between HTT and DRAM, however increasing Core Clock will increase DRAM Frequency, and there are no x.5 memory deviders.

PS. What board/CPU you have ?

Well, in this case, CPU-Z is wrong.

Not necessarily. He may have and older DDR1 Athlon 64 with DDR400 RAM. In this case the CPU multiplier and RAM divider is the same most of the cases (exceptions can be low multipliers like 4x. I don't think divider 4 exists).

HTT x CPU Multiplier = Core Clock
Core Clock / Memory devider = DRAM Speed.

So:
200 x 10 = 2000
2000 / 10 = 200 (DDR400)

200 x 11 = 2200
2200 / 11 = 200 (DDR400)
etc.

and there are no x.5 memory deviders.

AFAIK DDR1 Athlon64 does not support x.5 CPU multipliers either. So it must be some kind of BIOS bug/trickery,

Baoran wrote:

I always thought that ram speed only changes when you change FSB, but is it normal that if you just raise multiplier by 0.5x in athlon64 system without changing FSB the ram speed goes up by 100Mb/s? I am just looking at what memtest86 shows at top of the screen when booting it from floppy disk.

Athlon64.. specially on AM2 motherboards used core frequency to determine RAM speed, when speed can't match a JEDEC speed.. they used the next divider so you could end up with ram speeds slower than specified. The match between core speed and memory speed was a little confusing, so a 3200+ could be faster than a 3500+ with the same memory kit in certain benchmarks.

You could had a little underclocked chip IMHO, otherwise that little OC made the bios jump a previous divider and you get more speed...either way DDR-266 transfer speed is 2.1GB/s... so 100 mb/s doesn't seems too much of an increase, have you touch any other RAM setting?

Regards

The motherboard is Asus A8V deluxe and it has 0.5x multiplier steps in bios. Latest 1018 bios version.

Edit: CPU speed is still set in bios FSB times multiplier and cpu-z still shows memory speed as 2xFSB. Perhaps it is because I am using old version of cpu-z because newer versions don't work in win98se.

Athlon 64 don't have an FSB. They have a Base clock (100Mhz DDR, usually) that is used to derive the CPU speed via multiplier. This multiplier can have 0.5x steps. Ram speed on older (pre phenom IIRC) Athlon 64 designs is derived as a divider from the actual CPU clock to get to the valuest that are closest to the SPD speeds. This divider does NOT have 0.5x steps, only full integer.

As you can see in that bios picture second option from the top is "CPU FSB frequency" and default value for it is 200Mhz. If I set multiplier to 5.0, the cpu speed is 1000Mhz.

And you're using DDR, so the highest clocked JEDEC rating you can use is probably 200mhz (PC 3200). If you were clocked at 1100 mhz, it would probably be 1100/6 = 183 mhz.

It's even more imperative that this is done carefully with DDR2 due to higher RAM clocks.

It is still not an FSB, it is called that to ironically "avoid confusion". It is merely a base clock from which all major clock frequencies are derived via multiplier.

Read up on what an FSB actually is. No such thing exists in ANY CPU with integrated memory controller (Hint, it is the bus that connects the CPU to the memory controller on the northbridge). The closest equivalent the Athlon 64 has to an FSB is the Hypertransport link connecting the CPU to the remaining chipset parts. And that one is at 800Mhz DDR minimum (8x Base clock or higher, depending on HT version)

Agreed.

So would overclocking the base clock affect hypertransport link speed even if hypertransport link speed is set separately elsewhere in bios? If I would set base clock at 220Mhz and hypertransport link speed is set to 1000Mhz, would hypertransport link speed stay at 1000Mhz or would it be 5x220Mhz?

Yes. The HT link speed setting in the BIOS should be an option for the HT multiplier (800 mhz = 4, 1000 mhz = 5).

Most Athlon 64 BIOS versions I have seen omit HT multipliers. However Baseclock overclocking does Affect CPU and HT clock.