From: james.harris.1@gmail.com   
      
   On 13/02/2022 21:14, wolfgang kern wrote:   
   > On 13/02/2022 16:50, James Harris wrote:   
      
      
   >>>> Why could it not be in PM running a 16-bit code segment?   
   >   
   > at which address would you see such PM16 code?   
      
   Within range 0 to 64k. See below.   
      
   > while trueRM is limited to FFFF:FFFF (aka HMA minus 16),   
   > PM16 blocks can reside anywhere within 4GB.   
      
   I don't think so: PM16 is limited to 64k code segments. The following is   
   from the 80286 documentation.   
      
   The programmer views the virtual address space on the 80286 as a   
   collection of up to sixteen thousand linear subspaces, each with a   
   specified size or length. Each of these linear address spaces is called   
   a segment. A segment is a logical unit of contiguous memory. Segment   
   sizes may range from one byte up to 64K (65,536) bytes.   
      
   ...   
      
   >> Further, loading CS with a selector for a 32-bit ('big') descriptor   
   >> will only affect the code segment. One or more data segments could   
   >> still be 16-bit.   
   >   
   > been there :) it crashed if you leave data selectors RM styled.   
      
   It should work, AFAICS.   
      
   >   
   >> All this would make Real mode little more than a subset of Protected   
   >> mode. Or, put another way, one could say that Real mode *is* Protected   
   >> mode with:   
   >   
   >> 1. certain values in the segment descriptors   
   >> 2. different rules as to what it means to load a segment register   
   >   
   > just a point of view matter ?   
      
   To me this is more about gaining an insight into what is likely   
   happening inside the processor, and thereby making it easier to understand.   
      
   ...   
      
   >>>> Furthermore, you could consider that accesses off DS would also   
   >>>> include checks but that the internal descriptor would have the limit   
   >>>> set to 0xffff so nothing would be out of range.   
   >   
   >>> You could setup smaller than 64K limits on Unreal Data segments.   
   >>> this might raise a real mode exception because still in RM  :)   
   >   
   >> Or PM16. :-)   
   >   
   > :) look at the AMD pages which lists RM<>PM instruction differences,   
   > and then check on a few to see if it is in RM or PM16.   
   > hint: some instructions are privileged and a few aren't allowed.   
      
   What about RM code? ISTM that a lot of RM code could work unchanged in PM16.   
      
      
   --   
   James Harris   
      
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