From: james.harris.1@gmail.com   
      
   On 19/07/2021 18:01, Joe Monk wrote:   
   >   
   >> A20 has to be enabled before entering PMode. Processor operation is   
   >> undefined otherwise.   
   >   
   >> If you want processors to perform to spec you have to ensure A20 is   
   >> enabled before entering PMode.   
   >   
   > All modern PC's start with A20 enabled.   
      
   Er, how are you defining "modern"?   
      
   If the code is started in 32-bit or 64-bit mode I would agree that   
   there's no need to run any A20-enable routine.   
      
   But if one is writing 16-bit code there are two choices:   
      
   1) Assume a 'modern' PC.   
      
   2) Check that it's modern enough.   
      
   The assumption is unnecessary as checking is easy. And the check is   
   probably best as a check for A20 state anyway.   
      
   Furthermore, - as you may have noticed - Paul is targetting PCs so old   
   they would not fit your idea of modern.   
      
   >   
   > As you can see from the provided example: http://www.scs.stanf   
   rd.edu/05au-cs240c/lab/i386/s10_05.htm there is no check to see if A20 is   
   enabled before going to protected mode.   
      
   I wouldn't expect code in the 80386 manual to do such a check as the A20   
   Gate was part of the PC architecture, not part of the Intel CPU spec.   
      
      
   --   
   James Harris   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)