From: nowhere@nevernet.at   
      
   On 02/09/2022 19:20, muta...@gmail.com wrote:   
   > On Friday, September 2, 2022 at 6:53:49 PM UTC+8, wolfgang kern wrote:   
      
   >> [...]   
   >>> No, default status is no wraparound, but some   
   >>> environments may wrap around anyway due to the fact   
   >>> that there are 20 or fewer address lines.   
   >> The KEY to wrap around at 1MB once was the A20 gate,   
   >> it's now just an historical thing found only in museums.   
   >> PCs don't even have an A20 gate anymore (==always ON).   
   >>   
   >> the HMA (0010_0000 aka FFFF:0010..) was very often used   
   >> by DOS and friends, so they needed A20 ON.   
   >>   
   >> easy to make it wrap at 1MB with segment limits.   
   >> but it will wrap only if the access is aligned while   
   >> crossing the boundary with a multi-byte instruction   
   >> end up in a Seg-Fault or with an GP-error if PM.   
      
   > Sure. What I'm looking for is a set of rules   
   > to follow, with the benefit of hindsight.   
      
   >  From memory, the PSP used when a .com file   
   > is loaded, makes use of address wrap.   
      
   I cannot confirm your assumption above,   
   .com are organized at 0100 so the PSP is at 0000.   
   therefore a .com-file is size limited to 63.75K.   
      
   Rules ?  my rules are according to given hardware.   
   I'd just recommend to always avoid address wrap [A20 ON].   
      
   the inherent backwards addressing uses overflow to make   
   an unsigned displacement look as if signed [isn't wrap].   
      
   for speed reasons my initial ESP=0, it could under-run]   
   old DOS used a misaligned SP (ffff instead 0) to force   
   a segment fault on stack under-run [but made all slow].   
      
   as you seem to work on virtual machines you can define   
   whatsoever rules you like :)   
   You cannot alter any features of existing REAL hardware.   
      
   AFAIK x86 is the only CPU family with address Shift-Add. Others   
   use either banks or two [w/o shift] registers for >64K.   
   __   
   wolfgang   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)