DOS ain't dead

ROM-BIOS leaves ds and es limits beyond 64 KiB (Developers)

posted by tkchia , 06.11.2025, 17:29

Hello ecm,

> I can read from the word at segment 0 offset B8000 to read the text mode
> buffer. I just checked again, ds and es have higher limits, cs, ss, fs, and
> also gs do not. Also, the ROM-BIOS identifies itself as "Award Medallion
> BIOS v6.0", "Copyright (C) 1984-2000", running an "ASUS MED 2001 ACPI BIOS
> Revision 1009". At the bottom of the startup screen it reads
> "06/12/2001-VT694X-MED 2001".

Interesting finding!

Since this is real mode, I suppose another thing to try is to use the 0x40:0x67 + CMOS 0x0f trick (as outlined in RBIL) to reset the CPU segment registers to something like their power-on state, and see what happens from there...

(I recall it might be necessary to enable A20 for that, by the way.)

Thank you!

---
https://gitlab.com/tkchia · https://codeberg.org/tkchia · 😴 "MOV AX,0D500H+CMOS_REG_D+NMI"

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