In djgpp, your program runs in protected mode. There are certain things you can't do in protected-mode programs (that's why it is called protected mode). This issue is too complex to describe here, so only a few of the more important aspects will be briefly mentioned. If you are serious about writing assembly language protected-mode code, or have a large body of existing code to convert to protected mode, you should read any of the books about protected-mode programming with 80x86 processors.

Here is a short list of some of the techniques found in many real-mode programs, which will trigger protection violation or erratic behavior in protected mode:

• Loading arbitrary values into segment registers, then using them to reference code or data.

• Referencing code with data segment register, or vice versa.

• Assuming certain locations (like BIOS area or video memory) will be found at certain absolute addresses.

• Calling DOS or BIOS services with INT XX instruction.