640-KB / megabios

MEGA-BIOS V1.0 Reconstruction Project

MEGA-BIOS V1.0 Reconstruction Project

Display Telecommunications Corporation (DTC) MEGA-BIOS V1.0

Copyright © 1983 Display Telecommunications Corporation (DTC)

Copyright © 1985-1986 International Business Machines Corporation (IBM)

Author: Don K. Harrison

The Project

This project is the work of @FinnJorgensen, @sergioag and @640-KB to reconstruct MEGA-BIOS V1.0 from the printed source code listing originally sold by DTC.

More information about DTC, MEGA-BIOS and it's legacy can be found here and here.

Goals

1. Preserve the original source code exactly as it was printed.
2. Document the source build process.
3. Provide fixes and updates to known bugs.

Build instructions

Prerequisites:

1. A PC running real mode DOS, or emulator: MartyPC, VirtualXT or 86Box are suggested. Note: since the LINK86/LOC86 command line requires STDIN redirection, DOSBox will not work.
2. Intel ASM86 Macro Assembler complete tools: ASM86.EXE, LINK86.EXE, LOC86.EXE and OH86.EXE (here or here).
3. (Optional) Tool to convert Intel HEX output to binary. Example: objcopy or similar.
4. Tool to compute and add valid checksum to file offset 0x1fef.

Steps:

1. Make sure ASM86 directory is in your PATH environment variable (ex: PATH=C:\ASM86 or whever it is located).
2. Run BUILD.BAT in a DOS environment, which will produce MB.HEX in Intel Hex format.
3. (Optional) Convert to binary file format using one of the following methods:
   • objcopy: objcopy --input-target=ihex --output-target=binary --gap-fill=255 mb.hex mb.bin
   • TODO: instructions for other programs.
4. Compute 8 bit file checksum in mb.bin and replace byte at 0x1fef so that sum is 0. TODO: include program to do this.

Known Issues

V1.0

1. INITIAL.ASM: POST DMA Refresh test checks for a counter reading between 270H-290H after a busy Loop of 0A00H iterations. This will likely fail on anything other than a 4.77MHz 8088.
2. VIDEO.ASM: Light pen incorrectly checks for trigger as active high, but is actually active low.
3. VIDEO.ASM: During CGA snow/blanking wait, incorrectly jumps to outer loop instead of inner loop.
4. VIDEO.ASM: Mov Bl,OffsetTable[Bx] ASM 2.x requires Mov Bl,Byte Ptr OffsetTable[Bx]. TODO: confirm this is a change in behavior to ASM 1.x.
5. VIDEO.ASM: POST Memory test display occurs prior to video option ROM scan, so is not shown on EGA/VGA. Error messages are also displayed if an EGA/VGA card is installed.