First post, by nztdm
I've been having some fun with 486 overclocking. I have two boards, a Biostar MB-8433UUD, and an ASUS VL/I-486SVGOX4.
The Biostar (BIOS UUD2014) is great, and is a well known board. It even POSTS and can run some of the DOOM benchmark running at 2x83=166MHz and one of the 2X-capable Am5x86 chips (Am486DX2-66V16BGC). But it has graphical corruption and instability (same CPU is stable at 3x60=180).
But even running at 4x40MHz with fastest RAM/cache, or 3x60MHz and fastest RAM and slowest L2 or disabled L2, it is unremarkable in DOOM, getting 1268 realtics, but good in Quake, getting 19.7 fps.
The ASUS (BIOS 0402.002) is odd, as it's Rev1.2, which has incomplete documentation, and somewhat incompatible jumper settings with the later revs. If you find the correct settings for your CPU, take a pic in case you can't find it again. Another board that is almost identical is the VL/I-486SV2GX4. I got nowhere with ASUS via email trying to find manual for the GO (or is it G0).
The board was the non-X4 version, so I added the missing parts.
I used a lower dropout regulator (MIC29302) rather than the correct part (LT1084), along with an adjustment trimpot for voltage. The lower dropout is needed for the 4.0-4.5V range.
I bodged on an Si5351 module and Arduino Nano to allow any bus speed.
TLDR:
The board supports up to 64MB RAM officially.
The SiS 85C471 chipset supports 128MB, but this isn't implemented on this board.
I wish to use this board for a build, as I have some nice VLB cards to go with it, and gets faster benchmarks than the Biostar PCI board.
I'd rather have at least 64MiB of RAM than 32MiB I'm currently stuck with. 2x32MiB modules is asking for trouble at overclock speeds as 32MiB 72-pin SIMMs are dual-bank, putting almost twice as much capacitance on the bus.
The RAM slots (both 72-pin and 30-pin slots) have A11 disconnected, limiting you to 32MiB 72-pin SIMMs and 4MiB 30-pin SIMMs. (Due to missing A11, SIMMs of 64MiB = 16MiB, 128MiB = 32MiB, 30pin_16MiB = 4MiB).
As per the datasheet page 23, pin 138 is RAS3 when register 50, Bit 2, is 0. This pin is unconnected on the board. (The chipset has up to 7 RAS's).
Setting that register to 1, will change that pin function to MA11 (A11 for RAM).
If I connect pin 138 to RAM slots A11, there is no change. The pin stays High always. (I may need to put A11 through an F245 buffer like A0-10 go through (buffers which have DIR and OE fixed to 5V and GND)).
MODBIN shows the BIOS as having register 50, bit 2, set to 0 by default.
Changing this to 1 in MODBIN doesn't work either. I suspect you need to do something else to the BIOS to get this change to stick...
Any other ideas? Perhaps another way to modify the BIOS.