From: nowhere@nevernet.at   
      
   On 22/02/2022 11:12, James Harris wrote:   
   > On 21/02/2022 20:42, wolfgang kern wrote:   
   >> On 21/02/2022 19:35, James Harris wrote:   
   >   
   > ...   
   >   
   >>> What I am trying to prove is that once CR0.PE is set (and prefetch   
   >>> queues flushed) the processor will be in PM, not RM, even _before_ CS   
   >>> is reloaded.   
   >   
   > ...   
   >   
   >> assume or make your RM-CS 07c0 before the switch   
   >> and try this after it:   
   >>   push cs   
   >>   pop ax   ;ax show the RM-segment and nothing else   
   >   
   > If the /user-visible/ part of CS is 07c0 then wouldn't that end up in AX   
   > in either mode?   
      
   Only in RM but not within PM, here AX would show a descriptor value.   
      
      
   >>   or try self-modify and check where the change happens:   
   >> mov word [cs:00FE],31c8  ;or whatsoever. might crash if PM   
      
   > I can't see how modifying the instruction stream would do anything. RM   
   > encodings are valid in PM16!   
      
   OK this wasn't a good example, I meant it as a crash test because   
   exceptions work quite different.   
      
   > Consider an instruction such as   
      
   >    mov ds, ax   
   >   
   > In Protected Mode that would do   
   >   
   >      DS.base = from descriptor   
   >      DS.limit = from descriptor   
   >      DS.access_rights = from descriptor   
   >   
   > Wouldn't it make sense for the same instruction in Real Mode to do as   
   > follows?   
   >   
   >      DS.base = AX shl 4   
   >      DS.limit = 0xffff   
   >      DS.access_rights = unrestricted   
   >   
   > Then the same architectural parts (the hidden parts) could be used in   
   > either RM or PM. That would keep the hardware design simpler and more   
   > consistent than having two entirely separate modes.   
      
   X86 grew up in large steps, so we see historical remains here and there.   
      
   > In fact, surely the so-called Unreal Mode only works because the CPU   
   > uses the hidden parts of the segment registers at all times - even when   
   > in Real Mode (PE=0).   
      
   yes, Unreal may not be designed by intention, but it became handy :)   
   __   
   wolfgang   
      
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