First post, by nztdm
Tested on SV2GX4 Rev2.0 and SVGOX4 Rev1.2
(SVGOX4 is also known as SVG0X4)
These boards often came with "X4" covered up (486SV2G and SVGO), and the voltage regulator components needed for non-5V CPUs missing.
You can add these regulator components back to support 486DX4 and Am5x86 and Cx5x86. Also update the BIOS and find the correct jumper settings for your specific board revision. They differ a lot, and if set wrong, will give 5V to CPU, or crash with L1 Write-back enabled or DMA transfers.
Here's a photo of my jumpers for an Am5x86 on a SVGOX4 Rev1.2.
Here is what I think is going on. May not be entirely correct.
The SiS 85C471 Pin 138 is configured by the BIOS to be RAS3.
The two possible modes for this pin is DRAM A11 (MA11), or RAS3 (the chip has 8 RAS lines), selected by a register bit.
The BIOS for these two boards is the same (from what I've seen), and configures the pin to be RAS3, which means they only support up to 32M SIMMs.
On SVGOX4, Pin 138 is also wired to RAS3 of SIMM6 (the upper 72-pin slot).
On SV2GX4, Pin 138 is wired to A11 of at least the lowermost SIMM slot, but the BIOS is configured to use that pin as a RAS line, preventing 64M/128M SIMMs from working.
You need to flash an 85C471 BIOS that has Pin 138 configured as MA11. I found one.
If you have a SV2GX4, you should be done now.
If you have a SVGOX4, you also need to wire upper 72-pin slot, pin 33 (RAS3) to both SIMM slot pins 29 (A11):
(Maybe you can use 16M 30-pin SIMMs if you wire to them (pin 24) as well - untested)
I couldn't figure out how to modify the official+patched ASUS BIOS (0402.002) to set the chipset register.
Changing the register bit in chipset defaults in MODBIN didn't work, so it must be being set later.
The BIOS I ended up using is from an EFA 4MHL3G (4DMS-HL3GC). (https://theretroweb.com/motherboards/s/efa-4mhl3g#bios)
It's a bit older than the latest official BIOS, but everything seems to work - my big overclocks still work.
Haven't tested HDD size limits beyond an 8GB CF card (works fine).
Doesn't support secondary IDE. You can still use secondary IDE for CD-ROMs, or use a BIOS extension such as XT-IDE BIOS to detect all your hard drives.
I've also modified the BIOS to unhide more chipset settings and enable PS/2 mouse: