u/maniek-86

Some of you might have heard of and remember the M8SBC-486 (well, the news talked about it at the beginning of this year). This was truly amazing for a project that was meant to be experimental homebrew rather than "kinda PC compatible". Exactly a year ago, I received the PCBs for this homebrew and today I'm happy to tell you that I got this computer to run Windows 95!

To recap: the M8SBC-486 is missing a second interrupt controller (PIC) and a DMA controller. This breaks compatibility with software that expects these. One of them is, of course Windows 95. How do I know, it has some hardcoded references to these - well, it was created for standard PCs. But even apart from that, the M8SBC-486 theoretically meets the minimum Windows 95 requirements: a 386 (or better) class CPU and 4 MB of RAM. So how did I do it on a "kinda PC compatible" homebrew?

At the beginning of August, I stumbled across this Reddit post by u/Mike1978uk about running Windows 95 on an Intel Inboard/386 (nice project by the way). When I read the description of what it took to get it running, I noticed that he had removed references to "high IRQs not found on XT class machines". This sounded familiar - the M8SBC-486 doesn't have a second PIC and therefore doesn't have the higher interrupts either! (I even remembered that some people called the M8SBC-486 a 486-class XT). I decided to try it out of curiosity and see if his mod had any chance of working on my board.

I downloaded the first released image, modified it a bit to remove Inboard/386-specific stuff, downsized it from 2 GB to 500 MB, added the QEMM 7.5 memory manager (as it currently works better on the M8SBC-486 than HIMEM) and booted the image in my copy of 86Box with the M8SBC-486 added (my experimental local fork). To my surprise, it booted! I could even use the mouse (when I booted Windows 3.1 on the M8SBC-486 I couldn't use mouse there because of some issues). So, I flashed the image onto an HDD and... it booted! Windows 95 on an i486 board built from scratch, with its own FPGA chipset and a BIOS. Project that wasn't even meant to be "PC compatible" - I can't believe I've got this far.

After playing around a bit with System properties, Paint and Minesweeper, the system crashed - oops. It was working, but there seemed to be some kind of memory/disk corruption. I started suspecting my BIOS disk handler because one thing common to these crashes was that the disk activity LED would get stuck lit (meaning the disk controller was stuck as well). I proceeded to rewrite the disk subsystem in my BIOS. With that rewrite, I also added some BIOS improvements, including slave IDE support, CHS-LBA translation and more. After that, the system started working almost flawlessly! I mean, changing some system settings (especially those related to hardware/drivers) still caused crashes, but applications started working perfectly.

Here is what I've tested:

• ISA graphics card driver (SVGA driver) - I managed to install the built-in Trident driver in combination with my Trident TVGA8900C ISA card to get higher resolutions in Windows. I went from 640x480 with 16 colors to 800x600 with 256 colors!
• NE2000 network card & Internet Explorer - After messing around with drivers and configuration, I actually got the built-in NE2000 driver and Internet Explorer to work! It wasn't fast, but I could browser the Internet!
• CPU-Z Vintage Edition - It can detect the installed 486 CPU and correctly identify the fixed 24 MHz FSB. Since the BIOS doesn't provide any identification features, it can't detect anything beyond that. Still it's nice to see it working.
• Microsoft Office 95 (Word and Excel tested) - Both work fine. I can write documents and spreadsheets without any problems. I had to extract all the floppy disk contents to the hard drive to install it. The installation took about 30 minutes (disk to disk).
• Printing - I have an old parallel LX-300 printer. Windows 95 doesn't have built-in drivers for it, but it works fine when installed as an LQ-810 (which is available in the built-in drivers). I can print documents from the Office 95 applications mentioned above without any problems.

I also tried early versions of Notepad++ and 7-Zip, but both failed because of missing libraries in this Windows 95 OSR1 installation. I could install updates, but I suspect that doing so would break the patches.

When it comes to speed - it's not bad, but it's not great either. For example, applications take some time to open. Office runs slowly, but it's usable. The problem is the fixed 4 MB of RAM, which forces Windows 95 to swap a lot (this also explains the crashes I experienced when the disk subsystem was broken: memory was getting corrupted because of swap corruption).

I'll leave a link to my website, where I describe everything above in more detail, along with some extra stuff. There's also a YouTube video showing the boot process, CPU-Z, Office 95, and printing.

Links:

• My website about running Windows 95 on M8SBC-486 (and the overall project's page)
• YouTube showcase video
• M8SBC-486 GitHub
• Windows 95 on the Intel Inboard/386 project GiHub

I've been studying the PCI bus on my own and one evening I got an idea about making a homebrew computer with an actual PCI bus. I've never seen one, so I decided to try it myself. I also wanted to learn about the PCI bus in a practical way. I chose the Motorola 68000 CPU because of its balance in complexity - it's easy to interface, has nice performance/capabilities and has official GCC toolchain support.

The project began literally a month ago when I got a scrap lot from my friend Shadowdestroyer777 (huge thanks!) containing an industrial Motorola 68000 SBC and some old dead motherboards. I desoldered the SBC for the CPU and a few other parts. From one motherboard I grabbed connectors, ISA/PCI slots and many other things. One evening when I had everything on my desk (the CPU, the PCI slots and even a spare prototype PCB), the idea was born. Right before this project, I worked on an experimental PCI FPGA card, so I had a bit of practical PCI experience beforehand. I even read most of "PCI System Architecture" by MindShare!

The specs of this homebrew:

• Motorola 68000 @ 10 MHz
• 1 MB of SRAM
• 4 KB of ROM (just to fit an serial bootloader)
• MC68681P DUART for UART, timing and I/O
• Two 32-bit 5V PCI slots, interrupt capable, no bus mastering (DMA). Runs at the same frequency as the CPU (10 MHz)
• Spartan 2 FPGA XC2S100 as the PCI host bridge
• XC95144XL CPLD as the glue logic
• ATMega128 (used solely to quickly load the FPGA bitstream on boot)

As the base, I used an 18x12 cm prototype PCB. Connections were made using enameled wire, just like in my previous 486 homebrew project. To fit the PCI slots into the prototype board, I actually had to bend the PCI slot pins to fit the 2.54 mm grid! Don't even ask about this manually adapted FPGA... It's another story - it just works.

I won't go into detail here on exactly how the PCI bus works and what I had to do, but generally, the bridge translates CPU accesses into PCI accesses. It maps memory 1:1 and for the I/O and configuration spaces, I do some translation tricks. It's all done inside the XC2S100 FPGA in VHDL. In addition to basic accesses, PCI hardware interrupts also work. I omitted DMA mainly because I ran out of pins on the FPGA. There were various challenges in this process, like the different endianness (PCI is designed around little-endian systems, while the Motorola 68000 is big-endian). I’ve shared all the source code on GitHub and more details on my website.

So, thanks to my efforts, I managed to support 3 types of card:

• Serial PCI card: I had a Moschip NM9835 dual serial port card. It was a nice test target because it's 16C550 UART compatible, making it relatively simple to write a driver for. With this card, I also got hardware interrupts to work so on serial receive, the CPU handles it nicely.
• VGA graphics cards: For me, these are one of the coolest parts. I managed to "wake up" three cards: a Cirrus Logic GD5440, an S3 Trio64V2/DX (S3 86C775) and partially an IGS IGA 1680_A. The thing is, I can't execute their VBIOSes because of the architecture mismatch. However, there is some documentation on the internet that shows how to wake these cards up. After waking them up, they program like standard VGA cards - it all comes down to standard VGA initialization (like programming the CRTC, DAC, etc). I got 80x25 16-color text and 320x200 256-color graphics modes to work perfectly!
• RTL8029 network card: With the help of AI (as I was exhausted from working on the homebrew - soldering, designing the logic, etc. and I just wanted to see what was possible with the correct software), I got this homebrew on the internet! I got an RTL8029 driver, a simple network stack, telnet and a http server to work. I can say that I got a 10 MHz CPU from the 80s to serve a web page over a 32-bit PCI network card from the end of 90s.

The lack of DMA is quite limiting as I can't use cards that depend on it, such as later network adapters, SCSI controllers, USB controllers and more. However, for my first project like this, I think it's pretty good anyway. Honestly, it's crazy to think about (even for me) that I got an '80s CPU to work well with late '90s PCI hardware and serve a webpage. All in one month of hard work.

I also wrote a massive, detailed article on my website about this project, including my previous experimental PCI card, a brief PCI introduction, the build process and the results. This is just a very short description of the project, but I wanted to share it!

Project site: https://maniek86.xyz/projects/homebrew_m68k_pci.php