DOS ain't dead

What does DOS undocumented function 55h? (Developers)

posted by ecm  , Düsseldorf, Germany, 02.02.2026, 23:01

> This guy tries to run Win 3.x native under Linux environment (not
> wine/qemu/etc...). He has some partial success:
> https://www.youtube.com/watch?v=xT-ZvToZDLw
> He'd like to know what DOS func. 55h/int 21h do, he just made some bypass,
> is it documented somewhere?

Yes, of course, it's in the old Interrupt List: https://fd.lod.bz/rbil/interrup/dos_kernel/2155.html

Here's how to use it: https://hg.pushbx.org/ecm/ldebug/file/952433a1600d/source/debug.asm#l7643

Note how the debugger resets the current PSP to its own, because (as the list states) the function 55h call changes the current PSP to be the new one.

Also, the source is the current PSP regardless the caller's CS. Function 26h may use the CS as its source instead (on MS-DOS).

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l

Complete thread:

• What does DOS undocumented function 55h? - RayeR, 02.02.2026, 22:47 (Developers)
  • What does DOS undocumented function 55h? - ecm, 02.02.2026, 23:01
    • What does DOS undocumented function 55h? - RayeR, 02.02.2026, 23:25