I have begun mapping out the SXL2-relevant pins on the M912's PGA socket, in particular, S4, B16, C15, A13, A12, R17, and A15. These are the pins that the SXL data book mentions have some difference between the i486SX and the SXL2. I have done this now. I also want to check where these pins go on a board known to work with the SXL2, then try to copy over the working mapping onto the M912 board. Not having the pinouts for the M912's chipset will be an issue, but hopefully there shouldn't be much difference between the 486S and the SXL2.
The purpose of this exercise was to see if my SXL2-66 chips run properly at 80 MHz. So here's the snag... On boards which support the SXL2, the clock input is a 2x clock source, e.g. 80 MHz. On 386 motherboards, the crystal oscillator frequency gets halved by the CPU and the CPU runs at 40 MHz. On the SXL2, when you put it in 2x mode, it is actually taking away the halving. If I were to get the SXL2 running on the M912, it would run at 20 MHz by default, then when put in clock double mode, it would run at only 40 MHz. This pretty much defeats the purpose of getting the SXL2 running on the M912. The only way to get the SXL2 running at 80 MHz on the M912 would be if the PLL clock generator on the M912 contained a 2x output mode. If it did, I would have to rewire the system to use 2X to the CPU, and 1X to the VL bus and everything else. And that is assuming the UM9515 PLL clock generator on the M912 contained a 2X output mode and the signal was clean enough for use. I doubt the traces on this board were designed for anything past 50 MHz.
I assume that hybrid PGA132 / PGA168 boards have some jumper which selects 1X clock or 2X clock, or they always have 1X clock sent to the PGA168 socket and 2X clock sent to the PGA132 socket. This is probably part of the reason that we don't see any 486 boards with SXL2-PGA168 support.
Nonetheless, it would still be modestly fun to get the SXL2 running at 40 MHz on a socket 3 as merely a challenge exercise.
Plan your life wisely, you'll be dead before you know it.