DOS ain't dead

PSP environment word as a selector - How is exec handled? (DOSX)

posted by Japheth , Germany (South), 25.01.2026, 18:02

> I've heard that for some DOS extenders, the word [PSP:2Ch] may
> be replaced by a selector that points to the environment, rather than a
> segment address.

It's not just what "some DOS extenders" do, but specified in DPMI: https://www.delorie.com/djgpp/doc/dpmi/ch4.1.html

> If this is so, does the extender swap around the PSP field around exec
> calls? If DOS is given a zero in the exec block for the environment source,
> it'll retrieve the environment from the current PSP.

HDPMI emulates int 21h, AX=4B00h and in this case expects a selector. But if a real-mode interrupt is simulated ( int 31h, ax=300h ), HDPMI does nothing.

---
MS-DOS forever!

Complete thread:

• PSP environment word as a selector - How is exec handled? - ecm, 22.01.2026, 12:24 (DOSX)
  • PSP environment word as a selector - How is exec handled? - tkchia, 22.01.2026, 16:54
  • PSP environment word as a selector - How is exec handled? - tkchia, 22.01.2026, 17:24
  • PSP environment word as a selector - How is exec handled? - Japheth, 25.01.2026, 18:02
    • PSP environment word as a selector - How is exec handled? - ecm, 25.01.2026, 19:15
      • PSP environment word as a selector - How is exec handled? - ecm, 25.01.2026, 19:21