DOS ain't dead - LOADFIX

LOADFIX (Developers)

posted by ecm , Düsseldorf, Germany, 06.11.2025, 20:20

> I know the LOADFIX program included with early versions of DOS was supposed
> to resolve memory rollover issues that would crash some programs, but I
> don't know the exact details.

I think that LOADFIX just reserves memory in the first 64 KiB of the Low Memory Area before loading the application. This means that segmented pointer underflow doesn't happen, including when using naively anti-normalised far pointers. That is, so that applications like the original exepack stub won't underflow and access the HMA unwittingly. This is purely an A20-related problem.

I guess that MS-DOS v5 and up will disable A20 on exec, which helps exepack. Upon an int 21h A20 will be enabled to access the DOS. LOADFIX then helps if a program would depend on A20 wraparound after making any int 21h calls.

---
l

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Complete thread:

ROM-BIOS leaves ds and es limits beyond 64 KiB - ecm, 28.10.2025, 17:20 (Developers)
- ROM-BIOS leaves ds and es limits beyond 64 KiB - Oso2k, 28.10.2025, 21:46
  - ROM-BIOS leaves ds and es limits beyond 64 KiB - ecm, 28.10.2025, 22:03
    - ROM-BIOS leaves ds and es limits beyond 64 KiB - tkchia, 06.11.2025, 17:29
      - ROM-BIOS leaves ds and es limits beyond 64 KiB - ecm, 06.11.2025, 20:13
        ROM-BIOS leaves ds and es limits beyond 64 KiB - ecm, 06.11.2025, 20:23
        ROM-BIOS leaves ds and es limits beyond 64 KiB - tkchia, 06.11.2025, 23:07
- ROM-BIOS leaves ds and es limits beyond 64 KiB - bretjohn, 30.10.2025, 16:49
  - ROM-BIOS leaves ds and es limits beyond 64 KiB - ecm, 30.10.2025, 18:49
    - ROM-BIOS leaves ds and es limits beyond 64 KiB - bretjohn, 31.10.2025, 01:45
      - LOADFIX - ecm, 06.11.2025, 20:20