Pentium on a 386 motherboard

I was wondering what would be the ultimate upgrade for my 386 motherboard. It has a 386 CPU soldered-in, an unpopulated 386 PGA socket and a socket for either 387 FPU or 486 PGA or (might take a Weitek as well – not quite sure) and even might have a soldered-in 486SX PQFP. Plenty of options…

But how about hacking a Pentium in?

Obviously, the first hurdle is that my motherboard doesn’t have a Socket 2 that is required for Pentium Overdrive, let alone Socket 3. It doesn’t even have a Socket 1. It only has a hybrid socket that can hold either 486 PGA CPU or 387 FPU (in case a 386DX CPU is installed).

However, looking at the Pentium Overdrive pinout the extra row of pins doesn’t seem to be at all essential. There is a number of extra power points and some signalling pins to support L1 cache coherency when using write-back. Nothing too much to worry about. Conveniently, the PODP doesn’t use 486 WB cache controls like DX4 or 5×86 does, so the WB/WT pin remains floating and no need to configure the board for P24T like you do it on normal 486 motherboards.
I couldn’t be bothered to set any jumpers and left the board configured for 486SX.

As the socket wasn’t designed for much wider PODP it helps to use a PGA169 socket to lift the wide CPU up a bit so that the unused row of pins doesn’t interfere with jumpers and other components that are in surrounding of the CPU socket.

The resulting combo looks quite weird:

However, who cares about how it looks from bottom. It looks OK when installed.

And believe it or not, it runs fine. The MR BIOS on my board is quite confused and reports the CPU to be 586SX, which I like.

Obviously, the performance is slower than newer 486 boards, the L1 runs in write-through mode, but it runs just fine.

I thought there must be a reason for the PODP having the extra power pins. Just to play it safely I configured maximum power on my ATX2AT device and set the frequency to 25MHz. However, it turns out that idling only @ 3.6A – not too far from ordinary 486DX.

I wouldn’t probably run it for extended period of time, but it was a successful experiment showing that all those extra pins on the PODP are only optional.