SBC-386EX-2

SBC-386EX

A single board computer built around the Intel 80386EX, an embedded 386 with most of a PC's support chips on the die. It plugs into a RetroBrew ECB backplane or runs on its own, and with the BIOS in this repository it boots MS-DOS 6.22 and COHERENT 4.2, Mark Williams Company's UNIX-like operating system.

The board was designed by John Coffman for RetroBrew Computers. Its project page, with the boards, the history and the community around it, is the SBC-386EX wiki page. This repository holds the board's design files, its BIOS, and the software that runs on it.

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The board

Version 2.0, from John Coffman's guide to the schematic (Hardware/SBC386ex2-schematic-guide.pdf):

The KiCad project, the schematic and board PDFs, a bill of materials and the PLD equations are in Hardware/.

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The BIOS

SBC386/bios is the board's ROM BIOS: John Coffman's, from 2018, which brought the board up and laid the foundations, taken by Dan Werner in 2026 to the point where it boots DOS. It gives the board a PC-compatible BIOS interface on hardware that is not a PC:

• POST with a memory test and sizing, the ROM checked by CRC16 and then shadowed into DRAM, and the CPU clock measured
• SETUP, by pressing a key during POST, with its settings kept in the DS1302's NVRAM: disk geometry, floppy drives, the serial console, date and time, the boot device
• Disks: CompactFlash and IDE through INT 13h, CHS and the LBA extensions; floppy drives on the ECB Disk I/O V3, which has no DMA and is driven polled
• Console: the serial port, with VT100 translation, and the ECB VGA3 colour display with a PS/2 keyboard, both at once
• Clock: INT 1Ah on the DS1302
• A debug monitor, SETUP entry 5, which every piece of hardware on the board was brought up with (commands in upper case, numbers in hex)

Notes for using it are in SBC386/bios/0README.TXT. Two worth knowing first: three Ctrl-^ in a row on the serial console restart the board (it has no Ctrl-Alt-Del), and HIMEM.SYS needs /MACHINE:PS2, since there is no 8042 for it to drive A20 through.

How the BIOS went from never having run INT 13h to a DOS prompt, and why things are as they are, is told in 0BRINGUP.md; what is left is 0TODO.md.

Building the BIOS

The build runs on Linux. It needs:

1. The helper programs (exe2rom, hex2bin, bin2hex, copt) are in SBC386/tools, with their sources. Build them if the ones there do not run on your system:

   cd SBC386/tools
   make

2. The ROM:

   cd SBC386/bios
   make

   This builds rom064.hex, the 64 KB image for an EPROM, and rom128.hex, rom256.hex and rom512.hex, the same BIOS placed at the top of a 128, 256 or 512 KB flash part. They are Intel HEX, for any device programmer. make MONITOR=0 leaves the debug monitor out, which frees about 25 KB of the 64 KB.

3. Program the part that matches the ROM jumper, fit it, and connect a terminal to the serial port at 9600 baud, 8 data bits, no parity. POST runs there; press a key during it for SETUP.

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The SD card under DOS

SBC386/sdcard has a DOS driver for the on-board microSD socket, and the program it was brought up with:

• SD.SYS: DEVICE=SD.SYS in CONFIG.SYS gives the card's first FAT12 or FAT16 partition a drive letter (DOS 4 or later)
• SDTEST.COM: the bring-up program, which tests the socket a step at a time, from the pins to timed reads and a write test on a sector outside the partition

Both build with make in that directory (GNU make and NASM). How the socket was made to work, given a serial unit that only moves 16-bit words and a clock line that floats between transfers, is set down in the comments of sdtest.asm.

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COHERENT

Coherent/ is a port of COHERENT 4.2 to the board. It comes up multi-user, with a login on the serial port and another on the VGA3 screen and keyboard when one is fitted. Its drivers cover the board's IDE port and CompactFlash, the ECB floppy controller (read, write and format), the VGA3 console, and the microSD socket, so files move to and from DOS on SD cards and diskettes.

It is installed on the board from five diskettes, the original 4.2.10 kit with a boot diskette and a supplement made for the board. The diskette images, the installation directions and everything about building it are in Coherent/README.md.

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What is where

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Credits

• John Coffman designed the SBC-386EX, wrote its first BIOS and the tools it is built with, and brought the board up in 2018. See the RetroBrew Computers wiki.
• Dan Werner took the BIOS to MS-DOS in 2026, and ported COHERENT with the board's drivers, including the SD cards.
• Mark Williams Company wrote COHERENT, released as open source in 2015.
• The RetroBrew Computers community, whose ECB bus and boards this one shares.

Licence

The BIOS is free software under the GNU General Public License, version 3 or later: see SBC386/bios/COPYING. COHERENT's own licence, and that of the work on it here, are described in Coherent/README.md.