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386/486 hubrid boards (with two or more sockets) questions.

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First post, by gregorem

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Hi. From time to time I see some hybrid 386/486 motherboards. They usually have two sockets (for a 386 and a 486), though boards with three sockets (486 + 386 + 387) also very rarely show up.

Since I already have one 486 build with PCI slots, I'm thinking about building a 386 machine. I find these hybrid motherboards interesting because the 386DX isn't really that powerful, and I'd like to have a 386-compatible system with a bit more “juice” available on demand.

So my question is: is it possible to populate both CPU sockets and switch the active CPU just by using jumpers (or, even better, switches mounted outside the case)? Or is that idea completely ridiculous — i.e. could keeping a CPU in the inactive socket damage something, or would the motherboard simply fail to POST?

Of course, I'm talking about switching the active CPU only while the PC is powered off, not during runtime.

Reply 1 of 6, by Nvm1

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Most hybrids disable the 386 when a 486 is placed, some have a jumper to select which is the active cpu.
Just be aware then those hybrids have horrible performance with a 486 since the chipsets have some limits from the 386 architecture.

Reply 2 of 6, by gregorem

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I'm not planning to build a 386DX-33 system with a Cyrix 5x86 or AMD 5x86 running at over 100 MHz. I'm thinking more along the lines of a “386+” configuration — so an Intel or AMD 386DX paired with something around a 486DX2-66. That would still be roughly four times faster than a 386 at the same clock speed.

I know Cyrix DLC chips exist, but they might be too fast for many older programs and games, and there’s no easy way to slow them down.

Reply 3 of 6, by gregorem

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How can I tell which motherboards disable CPU1 when CPU2 is installed in the socket?

Reply 4 of 6, by DOSDays

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It's also worth noting that it's rarely just one jumper you need to change - I have several of these hybrid motherboards, and there are at least 4 other jumpers that need changing (or at least verifying are correct) out of the 20 or so on the boards I have. So it's a bit of a hassle if you think you'd be switching CPUs regularly.

Reply 5 of 6, by DaveDDS

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Funny ... this just came up in another thread -- wwwaaaaayyyy back in the day, my first multi-core system was TWO separate processors.

IIRC It was a "Tyan MP" board which took two AthlonMP devices.
My friend called it "the Whopper" (WarGames reference)

As far as I can recall, the only desktop mainboard I ever saw with two separate CPU sockets that were active at the same time!
(I once had a huge Compaq rack-mount server which IIRC could have 8 CPUs in it - but not a typical desktop)

- Dave ; https://dunfield.themindfactory.com ; "Daves Old Computers" ; SW dev addict best known:
ImageDisk: rd/wr ANY floppy PChardware can ; Micro-C: compiler for DOS+ManySmallCPU ; DDLINK: simple/small FileTrans(w/o netSW)via Lan/Lpt/Serial

Reply 6 of 6, by MikeSG

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I have an AcerPower 386dx with an Acer V5 motherboard.

In this all three 386, 387, 486 can sit in their sockets. There's one jumper for CPU (386/486) selection, a 169th pin on the 486 CPU socket, and a 25MHz/33MHz XTAL selection via jumper.

Intel DX4 ODP-100 CPUs work well. They have the 169th pin.. but I don't think many motherboard rely on it for disabling the other CPU.

In Quake it scored the same as a real 486 motherboard using the same CPU, video card. I think burst mode is mainly for PCI cards.. Memory speed is faster real 486s but L2 cache speed is the same..

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