History of this MR.BIOS Version
I obtained a DEC DECpc LPV Socket 1 486 motherboard in fairly good condition.
The board was pre-cleaned and powered on with a known-good CPU and RAM.
However, the POST-card consistently displayed only 0D 09 across all power-on attempts, with no further progress.
As it turned out, the culprit was the original DEC BIOS, which is extremely picky regarding memory compatibility.
I decided to try MR.BIOS for the SiS 85C461 chipset used on this motherboard
(such ROM images are available on TRW for various boards based on this chipset).
For the initial test to succeed, I had to use jumpers to disable the onboard S3-805 graphics controller and install a standard discrete ISA VGA card.
ROM Sizes and the First Boot
Original DEC BIOS: 128 KB
MR.BIOS: 64 KB
For the initial power-on, I simply doubled the MR.BIOS image by concatenating two identical 64 KB files into a single 128 KB file. In this configuration, the motherboard POSTed successfully and initialized the display.
However, my goal was to get a fully functional motherboard with the onboard S3-805 graphics working. So, I came up with the idea to integrate the VGA BIOS into the MR.BIOS ROM image.
Step-by-Step Integration Process
1) Extracting the VGA BIOS:
I opened the original DEC BIOS in a hex editor. The 32 KB VGA BIOS was located right at the beginning, from offset 0000h to 7FFFh.
I extracted and saved it as a separate file.
2) Analyzing Reference ROMs:
I examined an MR.BIOS firmware image that natively supports S3 graphics, using the MR.BIOS for the Intel TC430HX (Tucson) motherboard as a reference.
In that image, the VGA BIOS is also located from 0000h to 7FFFh, followed by free space, a recovery boot-block, and the actual MR.BIOS code at the very end.
3) Structuring the New ROM Layout:
I replicated this layout for my specific MR.BIOS build in the following order:
0000h - 7FFFh: Inserted the 32 KB S3-805 VGA BIOS extracted in Step 1.
8000h - FFFFh: 32 KB of padding (filled with 00 null bytes).
10000h - 1FFFFh: Injected the 64 KB MR.BIOS image for the SiS 85C461 chipset.
Results
That was essentially it. The resulting custom MR.BIOS image works flawlessly on this motherboard. It resolved all memory module compatibility issues, adds support for hard drives up to 128 GB, is Y2K bug-free, and offers user-friendly configuration options (including various chipset and memory timing tweaks).
Performance-wise, it is quite impressive—the memory and L2 cache benchmarks are on par with much newer 486 chipsets designed for VLB/PCI buses.
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