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BIOS update changed FSB speed

Last response: in CPUs
July 22, 2010 5:15:31 AM


I have an Abit NF7-S with an AMD Athlon XP 2600, 2 GHz, 266 MHz FSB. I just updated the BIOS from v26 to v27 so that it would see SATA drives > 500 GB. Now when I restart, the BIOS sets the FSB to 133 w/ 7.5 multiplier which runs the chip at 1Ghz. It used to correctly set the FSB at 266 x 7.5 = 2Ghz. Tried going back to the old V26 BIOS but it didn't help. There is no speed step in the BIOS. There is a CPU temp throttle but I turned it off - no effect. Went to USER DEFINED and manually set the FSB to 266, but it wouldn't boot. Had to reset to defaults with the MoBo CMOS jumper. CPUZ shows the chip running at 133x7.5=1Ghz but also says the rated FSB is 266. Any thoughts?
a c 131 à CPUs
July 22, 2010 5:24:37 AM

downgrade to previous version

bartolm said:
Tried going back to the old V26 BIOS but it didn't help.
a c 131 à CPUs
July 22, 2010 5:49:41 AM

Which bios version were you running when you tried resetting the CMOS?
July 29, 2010 7:17:46 AM

Turns out the answer is pretty simple, although not obvious. Despite what the sales literature said, the "real" FSB speed of the processor is not 266 MHz. It's 133 MHz. The following from Wikipedia sums it up:

"Athlon XP-M 2600+ 2000 MHz 512 KB 266 MT/s 15x 1.65 AXMA2600FKT4C "

"The Athlon XP uses a double data rate (DDR) front side bus meaning that the actual data transfer rate of the bus is twice its physical clock rate. The FSB's true data rate, 200, 266, 333 or 400 MT/s, is used in the tables and the corresponding physical clock rates are 100, 133, 166 or 200 MHz, respectively. The multipliers in the tables above apply to the physical clock rate, not the true data transfer rate. "

I figured this out when I realized that I had DDR 400 memory, and I could go up to 200 MHz, but no higher on the FSB setting.

So I just set the FSB to 133 and the multiplier to 15 and everything runs fine at 2GHz. Apparently I had done this before but forgot about it. Two lessons here:

1. Always write down your BIOS settings before an upgrade
2. Computer repair/upgrading activity should stop at midnight. "Let me try just one more thing" is not sufficient reason to continue to 3 AM.