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As I recall, I have a PC set up like that, one CPU being a Pentium III 550E and the other a Pentium II 450.
This is on a Gigabyte GA-6BXDS motherboard.
But, in my case, there is no option to switch off the one.
The motherboard detects both CPU's on start up (you can swop them out but, this would be impractical).

Also bear in mind that older operating systems like DOS, Windows 9x and Windows XP Home will only "see" the first CPU.

So You can't choose which cpu is seen in dos or win98?

HYRO wrote:

So You can't choose which cpu is seen in dos or win98?

No, it will always see the primary CPU.
Neither operating system support dual CPU's.

But ther's a chance that different motherboard could have that option?

HYRO wrote:

So You can't choose which cpu is seen in dos or win98?

No, at least not with a conventional dual processor system.

The only situation I can see this working in is in a machine with add-in processor boards that can "take over" the resources usually used by the main processor, or that are actually add-in single-board computers to which the system's BIOS can hand control. These will either be proprietary professional systems, or certain vintage consumer systems (e.g. Amstrad Mega PC (actually two systems in one case)). In both cases you'll be limited in what software can be used, and specific drivers and OS versions may be required.

Ok amd what about the motherboard with slot1 and pga370 on board? In the manual you can find that you can not use booth cpus but does it mean there can be only one cpu installed?

HYRO wrote:

Ok amd what about the motherboard with slot1 and pga370 on board? In the manual you can find that you can not use booth cpus but does it mean there can be only one cpu installed?

Don't know. It depends on how it selects which CPU to use.

HYRO wrote:

Ok amd what about the motherboard with slot1 and pga370 on board? In the manual you can find that you can not use booth cpus but does it mean there can be only one cpu installed?

From the boards I have seen, you can not install 2 CPUs at once in those Boards. They are not Multiprocessor boards, but single processor boards that support 2 socket formats to allow upgrading.

I mean, in theory you could create a Slot 1 switch. It would really only need to be a passive switch that swaps out the connectors for each CPU. This would work for single slot boards as well.

Yes, I've been thinking about that, but it would be a lot of work to solder 2 cpu slots it to the switch and from the switch to motherboard... and what kind of switch would have so many pins?

HYRO wrote:

Ok amd what about the motherboard with slot1 and pga370 on board? In the manual you can find that you can not use booth cpus but does it mean there can be only one cpu installed?

many of these boards have both slot and socket to make easier to the user to update processors without slotkets.
If you put both processors the board would not turn on.

Boards that have both a slot-1 and socket-370 only intend one such CPU to be installed, so I doubt they would cooperate with what you're trying to do.
Assuming you could convince it to work, there's also an electrical problem. These type of boards probably only have a single VRM which is routed to both sockets. So you'd be limited to using processors that can run at the same voltage. If they are both drawing power at the same time, you'd also have to look at the combined current draw.
The requirement to have matched voltages would prevent you from safely using a combo like you suggested - a 2.8V Klamath alongside a ~1.7v Coppermine. The Deschutes P2s run at 2.0V, which a Coppermine *might* tolerate (I don't know), but I don't think many Deschutes are unlocked (if any). So basically, I think the single voltage limitation would really kill this idea.

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I believe that dual Slot-1 boards generally have a separate voltage regulator for each socket, so they should get along with having different voltages for each CPU. This is definitely something to verify for a specific board though.
These boards typically have a BIOS option to enable only one CPU, but I've never seen one that lets you control which CPU it uses. I'd be surprised if there are any that let you manually make that choice.

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I think the most feasible way to pull this off would be by modifying a dual slot-1 board.
It would need to be a board that will happily boot a single CPU from either CPU socket. You'd probably also want it to tolerate mismatched CPUs in dual mode. Some of them are tolerant of these things, and some are picky.
With such a board, you'd then need to find some way to modify it so that you can disable either of the CPUs from being detected. I'm not sure what the simplest way to do that would be.

One method might be to intercept the VID pins (I think there's 5 of them). Route them through a switch so that you can disconnect these pins from being detected by the associated VRM. When the VRM sees all those signals as floating, it should interpret this as no CPU present and disable power to that CPU. Presumably, then, this would leave the system booting with the other CPU as a single.
There might be a simpler way involving just 1 or 2 pins. Some investigation into Intel datasheets would be needed.
You might not even need to intercept CPU pins. Maybe intercepting something upstream at the voltage regulator IC would also work.

Configure the BIOS to run in single CPU mode. Use the switch on the "primary" CPU that the system normally defaults to. When that CPU is switched on, it will have precedence. When it's switched off, the other CPU will become the one that boots.

This is speculation of course, and I may easily be overlooking something.

Thank You all for the answers, I've lern something new.

shamino, Your idea with VID pins might be usefull, I have to think about it, but in the meantime I will wait, maybe some one will have some easier idea.