Elf2Dos

Cross-compile a 32-bit DOS program with GCC!

• Use any compiler that targets x86 ELF. This means you can use the version of GCC or Clang already installed on your x86 Linux system.

• Write 32-bit protected mode code. You do not have to worry about near or far pointers, and you are not limited to 64K.

This is a horribly ill-advised way to write code and it will surely erode your sanity. You just have to write a DOS program without using anything in the standard library, and this program will convert it into a 32-bit LE “Linear Executable” which can be loaded with DOS/32 Advanced. The input must be a 32-bit ELF executable with the relocations preserved. The GNU linker will do this with the --emit-relocs flag. Only R_386_32 and R_386_PC32 relocations are supported.

An example “Hello, World!” program is available in the examples directory.

A blog post is forthcoming.

Getting This to Work

• You should probably be using DJGPP instead of Elf2Dos.

• For obvious reasons you cannot use the standard library. Link with -nostdlib and compile with -ffreestanding.

• GCC will still emit calls to memcpy, memmove, memset, and memcmp, so you may need to define these.

• A linker script is a good idea. An example is available in examples/link.ld.

• The _stack_end symbol defines the initial value of the stack pointer esp. This must be defined in the input. A natural way to do this is with a linker script,

  .stack : ALIGN(0x10) {
      _stack_start = .;
      . += 0x8000;
      _stack_end = .;
  }
  

• The es segment will refer to the PSP at program start. Copy ds to es at some point or your string instructions won’t work.

• DOS/32 Advanced by default uses 16-byte alignment. Don’t bother aligning anything to pages unless you change that.

Future Work

• Combine executable with stub without having to run DOSBox.

License

Elf2Dos is licensed under the terms of the MIT license. See LICENSE.txt for details.