DOS ain't dead - Protectmode handler called from realmode

Protectmode handler called from realmode (Developers)

posted by Japheth , Germany (South), 13.08.2010, 07:31

> 1) the original SP is necessary to shift by 6 (as cm suggested) not by 4
> (as microsoft suggested)

It depends on how your FAR routine is supposed to exit: a RETF requires 4 bytes, an IRET requires 6 bytes.

> {I know, it does nothing but in first step would be enough not to crash}
> asm
> cld
> lodsd;mov es:[edi+2Ah],eax {restores CS:IP}
> lodsd;mov es:[edi+20h],ax {flags}
> add word ptr es:[edi+2Eh],6 {sets SP}
> iret
> end;

The second "lodsd" should be a "lodsw". There's a small risk that lodsd might cause a crash.

There is another risk that Int 13h might be called recursively. IIRC, Jack R. Ellis told me that USBDRIVE.SYS is supposed to do this occasionally.

---
MS-DOS forever!

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Complete thread:

Protectmode handler called from realmode - Laaca, 12.08.2010, 21:12 (Developers)
- Protectmode handler called from realmode - ecm, 12.08.2010, 21:24
  - Protectmode handler called from realmode - Laaca, 13.08.2010, 01:46
    - Protectmode handler called from realmode - Japheth, 13.08.2010, 07:31
      - Protectmode handler called from realmode - bretjohn, 13.08.2010, 18:43
        EDIT: Protectmode handler called from realmode - bretjohn, 13.08.2010, 19:25
        Volumes vs physical storage media | WAS "Protectmod handl" - DOS386, 26.08.2010, 09:33
        Volumes vs physical storage media | WAS "Protectmod handl" - bretjohn, 26.08.2010, 19:03
        Volumes vs physical storage media | WAS "Protectmod handl" - DOS386, 27.08.2010, 03:25
        Volumes vs physical storage media | WAS "Protectmod handl" - bretjohn, 27.08.2010, 16:47
        Volumes vs physical storage media | WAS "Protectmod handl" - DOS386, 28.08.2010, 01:26
        Volumes vs physical storage media | WAS "Protectmod handl" - bretjohn, 28.08.2010, 16:35
        Volumes vs physical storage media | WAS "Protectmod handl" - DOS386, 06.09.2010, 20:10