DOS ain't dead

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

posted by tkchia , 06.11.2025, 23:07

Hello ecm,

> This? https://fd.lod.bz/rbil/cmos/other/r0f.html

Yes, this is it! And https://fd.lod.bz/rbil/memory/other/m00400067.html .

Robert Collins has a more detailed discussion at https://www.rcollins.org/articles/pmbasics/tspec_a1_doc.html .

The 0x40:0x67 + CMOS 0x0f + CPU reset mechanism is the way to return from protected mode to real mode on a 286 PC. But it might possibly also work with your Pentium's BIOS. And I think you do not actually need to switch to protected mode to be able to use it.

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