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  A program that uses one of the memory models we described earlier is ready
  to run after you use LINK to create an .EXE file. But if you have a fairly
  simple program, or if you long for the good old days of CP/M compatibility
  and an absolute maximum program size of 64 KB, you can convert your .EXE
  program into a .COM file. Before you do, however, be sure your program
  conforms to the restrictions imposed by the .COM format.

  The memory model used in a .COM file places everything--executable code,
  program data, uninitialized memory, stack, and PSP--into the same segment.
  Consequently, the source code for a .COM program is simpler than the
  source code for an .EXE program. There is only one segment, with code and
  data at the bottom (starting at offset 100H). A .COM program doesn't
  contain a stack segment; instead, DOS automatically loads the .COM program
  into 64 KB of memory and locates the stack at the top.

  If your program is constructed in .COM format, you can run the DOS EXE2BIN
  utility to transform the .EXE file generated by LINK into a .COM file. Be
  forewarned, however: Few high-level language compilers use the .COM format
  because of its limitations. You can very simply and safely find out if a
  program can be converted from .EXE format to .COM format by trying to do
  it. If it works, it works. If EXE2BIN says it can't be done, however, it
  can't be done.