Absolute addresses of certain memory-mapped devices are mapped differently under djgpp, which is protected-mode environment. You can't just poke any address, that's what protected mode is all about. In non-DPMI mode, the entire graphics video memory is mapped 1:1 starting at 0xD0000000 in the program's address space; the djgpp paging mechanism understands how SuperVGA's map their memory onto the AT bus and automatically swaps pages as the program tries to access them. The program sees a linear range from 0xd0000000 to 0xd0100000 that corresponds to each pixel in the 256-color modes of SuperVGAs. For this to work correctly, you will have to set the GO32 environment variable to the graphics driver suitable for your SuperVGA card, like this:

  SET GO32=driver c:\djgpp\drivers\ati.grd gw 640 gh 480 tw 132 th 43