China DOS Union

Make sure config.sys contains
device=c:\dos\himem.sys
dos=high
There absolutely must not be emm386.exe.
This ensures the CPU is in real mode rather than V86 mode, and that A20 is also enabled.

Then you can do whatever you want in the program, for example
xor ax,ax
mov ds,ax
mov eax,12345678h
mov bx,
mov esi,40000000h
dec dword ptr

Unfortunately, none of the high-level language development tools under DOS support this kind of data access method, so
you can only use assembly.

Would it be possible to write a patch program for real-mode C compilers like TC and BC?

In real mode you can use the XMS interface to access it
but I still recommend using Watcom or djgpp.

Question for DWING in post #3: what method can WATCOM use to do this? The function is very simple, I just need to access data in memory beyond the 1MB range. I currently have no idea where to start.

I already figured out how to access it, and now I'm starting to appreciate how powerful WATCOM is.