DOS ain't dead - DS and ES matching

DS and ES matching (Developers)

posted by ecm , Düsseldorf, Germany, 18.10.2025, 12:48

> Since we have the normal non-MT source code, I was just
> wondering how definite it is that DS and ES match. ie will
> they always be set the same, or are there some special
> conditions? Is there a deliberate attempt to make them
> the same (seems a bit odd actually, but that's ok).

https://hg.pushbx.org/ecm/msdos4/file/51ad27d225a8/src/DOS/EXEC.ASM#l796

They're always pointing to the PSP upon application entry. I believe this is documented, as well.

However, for an extension I would suggest adding a new entrypoint at an offset within the executable's image, perhaps with a new signature. This would allow to load old executables in a compatible way with ds = es => PSP and new executables with the new entrypoint.

---
l

Post reply

Complete thread:

DS and ES matching - kerravon, 18.10.2025, 11:29
- DS and ES matching - ecm, 18.10.2025, 12:48
  - DS and ES matching - kerravon, 18.10.2025, 14:43
    - DS and ES matching - PDB meaning - ecm, 18.10.2025, 18:32
      - DS and ES matching - PDB meaning - kerravon, 18.10.2025, 19:04
        DS and ES matching - PDB meaning - ecm, 18.10.2025, 19:41
        DS and ES matching - PDB meaning - kerravon, 18.10.2025, 19:59
        DS and ES matching - PDB meaning - kerravon, 18.10.2025, 20:01
- DS and ES matching - kerravon, 19.10.2025, 10:32