From: james.harris.1@gmail.com   
      
   On 12/07/2021 22:51, muta...@gmail.com wrote:   
   > On Monday, July 12, 2021 at 11:45:34 PM UTC+10, muta...@gmail.com wrote:   
   >   
   >> It occurs to me that in that situation I can have   
   >> a "desperation mode" where I instead set up   
   >> 1 MiB regions of free memory, which should   
   >> satisfy quite a lot of applications.   
   >   
   > Actually I can do all that while in protected mode.   
   >   
   > I start with memory below 1 MiB being made available   
   > to PDOS/386, and then attempt to enable A20 via the   
   > official BIOS interrupt.   
      
   A20 has to be enabled before entering PMode. Processor operation is   
   undefined otherwise.   
      
   Rod checked up and found that to be true for both Intel and AMD. See his   
   various comments in   
      
   https://groups.google.com/g/alt.os.development/c/uRhADf8_nS8/m/aYn-cJ7klicJ   
      
   >   
   > Then if I detect that A20 is not enabled, I can present   
   > the user with options:   
   >   
   > 1. Manipulate hardware to try to enable A20 (your funeral).   
   >   
   > 2. Enable "desperation mode" (technically sound).   
   >   
   > 3. Enable desperation mode and then email Jens to carpet   
   > bomb Taiwan for making you lose half your memory and   
   > fragmenting what remains into 1 MiB chunks so that you   
   > can no longer run a 3 MB executable like gccwin (recommended).   
      
   If you want processors to perform to spec you have to ensure A20 is   
   enabled before entering PMode.   
      
   As for how, I think Rod already pointed you to this table   
      
      http://aodfaq.wikidot.com/mc-a20-controls   
      
   Contrast the "KBC Method" column with that for "BIOS Control".   
      
      
   --   
   James Harris   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)