x86 embedded BIOS R3

Very compact (less than 8KB of ROM space) x86 BIOS for embedded systems, FPGA, and emulators.

Tested with a hardware:

• Original Intel 8086 CPU
• Harris 80286 CPU
• Intel 80386SX25 CPU
• Intel KU80386EX25 CPU
• Cyrix Cx486SLC-V25 CPU
• Texas Instruments TI486SXLC2-G50 CPU

Some board images could be found in the pictures directory.

Tested with an emulators:

• Bochs 2.6.11 (require USE_ADDON_ROMS and USE_IDE_HDD in "config.inc" to use Bochs's Video BIOS and HDD)

Tested with a software:

• MS-DOS 3.3, 4.0, 6.22
• Windows 1.0, 2.0, 3.1, 95
• Linux 0.01, 1.3.89
• Minix 1.x, 2.x
• Most of DOS/Windows software and games working good

Compiling on a Windows machine

• Use Netwide Assembler (https://www.nasm.us/) to compile the source code.
• Edit "config.inc" file to select features you need.
• Launch "_make.bat" file to create binaries.

If your NASM location on disk is not "C:\nasm\nasm.exe" - please change the path to nasm.exe in "_make.bat" file.

Compiling on a Linux machine

• Edit "config.inc" file to select features you need. Enable USE_DEBUG_UART if you want to test it in QEMU.
• In "config.inc" file set START_SEGMENT to 0xF000 and ROM_SIZE to 65536. QEMU requires 64 KB BIOS, and this will add 56 KB of empty space to your binary.
• Install nasm and qemu:

sudo apt update
sudo apt install nasm qemu-system-x86

• Compile BIOS with nasm:

nasm bios.asm

• Download and unzip freedos.img disk image from "e86r" project or use your own image.
• Run in QEMU:

qemu-system-i386 -bios bios -machine isapc -vga std -drive file=freedos.img,format=raw,bus=0,unit=0,media=disk

The font will look different than on a modern computers: this is because we are loading only first 128 CGA glyphs 8x8 to a 8x16 VGA glyph table. The only way to load such a font is to stretch it vertically to double its size. Complete 8x16 font requires 4 KB of ROM space and this is too expensive for this tiny BIOS.

If you want to test a serial port (COM1) output: in QEMU's "View" menu switch display to "serial0". There are should be a BIOS banner.

Implemented functions and features

• Minimal initialization
• Minimal functionality ISRs 10h-1Ah
• Lower memory test with continuous mode to help debugging FPGA SDRAM controller
• Very compact Video BIOS
• Minimal SVGA functionality enough to run hi-res games like "Heroes Of Might and Magic" and "Transport Tycoon"
• Supports add-on ROM chips (see config.inc)
• BIOS disk hypercall for emulators
• SPI mode SD-card support on FPGA boards
• Very simplified USB HID device support for FPGA boards
• Good for a systems without video adapter
• Customizable SPI/USB drivers
• A20 line and PLL control (frequency multiplier for 486)

Known issues

• No hardware detection / BIOS setup (to save ROM space)
• No extended memory test
• Int 13h (BIOS disk) supports only reset/read/write functions
• Internal video BIOS doesn't support printing text in graphic mode
• Video adapter initialization incomplete so will not work properly with a real VGA chips without OEM BIOS
• QEMU's VGA adapter will only work in 0x03 (text, 16 color, 80x25 chars) and 0x13 (graphics, 256 color, 320x200) video modes due to palette and programming sequence issues in other modes.