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BIOS settings that matter after a CPU swap

Last updated: By Tessa Quinlan9 min readBIOS & Firmware

BIOS settings that matter after a CPU swap

Swapping a processor on an older board is ten minutes of mechanical work and then half an hour in the setup screens. The board stores its settings in CMOS memory, and every one of them was chosen for the chip you just took out. Some carry over harmlessly. A few can stop the replacement chip from starting, run it slower than it should, or feed it a voltage it was never meant to see. This piece walks through the settings in the order that makes sense on a typical LGA 775 board, using ASUS and Gigabyte manuals of that era for the menu names. Other makers use different words for the same switches.

A worked example: Q6600 out, Q9550 in

Say you replace a Core 2 Quad Q6600 with a Q9550 on a P45 board. Intel's ARK pages give these figures:

Q6600 (outgoing) Q9550 (incoming)
Bus speed 1066 MT/s (266 MHz quad-pumped) 1333 MT/s (333 MHz quad-pumped)
Multiplier 9 8.5
Core clock 266 x 9 = 2.40 GHz 333 x 8.5 = 2.83 GHz
VID range 0.850 to 1.500 V 0.850 to 1.3625 V
TDP 105 W 95 W

VID is the voltage ID, the core voltage a chip asks the board's regulator for. TDP is the heat figure cooler designers plan around. Now picture two leftovers from the Q6600's settings. A bus speed fixed by hand at 266 MHz runs the Q9550 at 266 x 8.5 = 2.27 GHz, slower than the chip it replaced. And if the Q6600 had been overclocked with a manual core voltage of, say, 1.45 V, that value sits above the top of the Q9550's entire VID range. Neither problem announces itself on screen. Both go away if you start from defaults.

1. Load the defaults first

Before you change anything, put every setting back to the maker's defaults. On the ASUS P5QD Turbo the item is Load Setup Defaults in the Exit menu, or F5 anywhere in setup. Gigabyte's Award boards, such as the GA-EP45-DS5, offer two: Load Fail-Safe Defaults, which the manual describes as the safest and most stable settings, and Load Optimized Defaults, the normal starting point. Use Optimized unless the board misbehaves.

If the board will not show a picture with the replacement chip at all, clear the CMOS with the jumper or by pulling the coin cell for a minute, as page 4 of the LGA 771 to 775 guide describes. Then load defaults from setup.

One habit pays off here: photograph each setup page before the swap. You will want the boot order, SATA mode and any deliberate changes back later, and the defaults wipe them.

2. Check that the board knows the chip

Look at the setup screen's main or CPU page and compare it with the arithmetic above:

  • The model name should be the chip's own name. The processor supplies that string itself, so a correct name proves little on its own.
  • The clock should match bus speed times multiplier. A Q9550 at 333 x 8.5 shows about 2.83 GHz.
  • On the P5QD Turbo's CPU Configuration page, a Ratio Status line shows the minimum and maximum multiplier the BIOS read from the chip. If that range looks wrong, the BIOS does not fully understand the processor.
  • Leave Max CPUID Value Limit disabled. It exists so very old operating systems can boot, and enabling it hides information modern systems need.

An "unknown CPU" message, a wrong clock or missing features usually means the BIOS predates the chip. Check the board maker's CPU support list first, as our guide to checking whether a motherboard supports a processor shows, and if a newer release is needed, follow how to flash a BIOS safely with the old chip back in the socket.

3. C1E and EIST: leave them enabled

These two power-saving features confuse people after a swap, because the clock shown in Windows suddenly drops at idle.

C1E is Intel's Extended HALT state. Intel's datasheet for the Q9000 series explains that when every core has halted, the processor drops to a lower bus ratio first and then to a lower VID, with the front side bus left alone. On exit it raises the voltage first, then the ratio. The same datasheet says the state must be enabled through the BIOS "for the processor to remain within specification."

EIST, Enhanced Intel SpeedStep Technology, does the same kind of switching while the chip is working, under control of the operating system. Per the datasheet, voltage moves in steps of 12.5 mV alongside the ratio, and again the bus speed does not change.

So a Q9550 that reads 2.0 GHz on an idle desktop is fine: 333 x 6 = 2.0 GHz, and 6 is the lowest ratio in Intel's table for that bus speed. Load it up and the clock should return to 2.83 GHz. The ASUS manual lists C1E Support and Intel SpeedStep Tech as enabled by default. Switch them off only as a short test if the machine is unstable, then switch them back.

4. Core voltage: Auto, then look

Intel's datasheet makes the reason for Auto plain: each processor's VID is calibrated at the factory, so two chips of the same model and clock can ask for different default voltages. A manual value copied from the old chip tells you nothing about the replacement. On Auto, the regulator follows whatever the chip asks for.

The ASUS manual carries two warnings beside CPU Voltage: a high setting can damage the processor permanently, and a low one can make the system unstable. Leave CPU Voltage, FSB Termination Voltage, NB Voltage and CPU PLL Voltage on Auto for the first boots. One small detail in the same manual shows why Auto is smarter than it looks: the lowest FSB termination voltage it allows drops from 1.20 V to 1.10 V when a 45 nm chip is installed. The board adjusts to the processor family if you let it.

Once the chip is stable at stock, read the core voltage in the hardware monitor page under light load and keep the number. If you later want to overclock, our guide to overclocking a Xeon on an LGA 775 board covers voltages, straps and stability testing properly.

5. Memory frequency and timings

On these boards the memory clock comes from the bus clock through a ratio, and the P5QD Turbo manual notes that the DRAM Frequency options change with the FSB setting. A worked example with DDR2-800 (400 MHz):

Bus clock Ratio (FSB:DRAM) Memory clock Result
266 MHz (Q6600) 2:3 266 x 1.5 = 400 MHz DDR2-800, as rated
333 MHz (Q9550) 5:6 333 x 1.2 = 400 MHz DDR2-800, as rated
333 MHz (Q9550) 2:3 kept 333 x 1.5 = 500 MHz DDR2-1000, far past the rating

The last row is what you risk if a manual ratio survives the swap. With DRAM Frequency and DRAM Timing Control on Auto, the board reads the modules' SPD chip (the small EEPROM on each stick that lists its speeds and timings) and picks a safe combination. After defaults, check the memory speed setup reports. If Auto chose something slower than the label, set the rated speed by hand and leave the timings on Auto. Set DRAM Voltage to the value printed on the module, no higher. Our piece on DDR2 and DDR3 on LGA 775 boards goes deeper into ratios.

6. The FSB strap

ASUS calls it FSB Strap to North Bridge. The P5QD Turbo manual lists Auto, 200, 266, 333 and 400 MHz, and says Auto sets the strap from the FSB and DRAM frequency. A strap fixed at 266 MHz for the Q6600 can limit which memory ratios the BIOS offers at 333 MHz, or upset stability. Intel does not document straps for end users, so set it back to Auto and leave it there at stock speeds.

7. Fan control and the fan warning

Defaults can switch fan control on or off depending on the board. On the ASUS board, CPU Q-Fan Control has to be enabled for the CPU Fan Profile item (Standard, Silent or Turbo) to appear. Standard follows CPU temperature, which is what you want while you learn how the replacement chip behaves.

Check the CPU fan speed reading too. The manual offers an Ignored setting for the fan speed and temperature displays. Leave both reporting real values: if you changed coolers along with the chip, a fan plugged into the wrong header shows up here before it shows up as a hot processor. Sit in the hardware monitor page for a few minutes and note the idle temperature with the case closed.

8. SATA mode

This is the setting that most often makes a successful CPU swap look like a failure. The ASUS manual offers IDE, RAID and AHCI for its SATA ports. Windows installed in one mode often fails to start in another: according to Wikipedia's summary, Windows Vista through 10 do not set themselves up to load the AHCI driver at boot unless the controller was in AHCI mode during installation, and the usual symptom is a boot loop starting with a blue screen. Set the mode back to what your photograph shows. If you actually want to change it, the SATA SSD upgrade guide covers preparing Windows first.

9. Save, then save a profile

Press F10 to save and exit, boot the operating system once, and run something demanding for a few minutes while you watch temperatures. When everything checks out, store the settings on the board itself.

Board Feature What the manual says
ASUS P5QD Turbo ASUS O.C. Profile (Tools menu) Two profiles stored in the BIOS flash, plus save and load to a FAT16 or FAT32 stick
Gigabyte GA-EP45-DS5 F11 Save CMOS to BIOS, F12 Load CMOS from BIOS Up to eight named profiles

ASUS adds a warning worth reading twice: it recommends loading a profile only when it comes from the same memory and CPU configuration and BIOS version. That is the case against restoring your old Q6600 profile onto the Q9550. Make a fresh one, name it after the chip, and delete or rename the old one so nobody loads it by accident.

Finally, write a short card and tape it inside the side panel: BIOS version, CPU model, bus speed and multiplier, memory speed, SATA mode and anything you set by hand. The next time the board needs a CMOS clear, that card is quicker than any menu.

Sources and further reading

Adding a CPU to an old BIOS? read the LGA 771 Xeon microcode guide.

Headshot of Tessa Quinlan
Tessa Quinlan

Tessa covers BIOS images, microcode and firmware recovery. Her rule for every flash: two backups of the original image, a known recovery path, and nothing else running on that power strip.