muta...@gmail.com  wrote:   
   > On Monday, September 12, 2022 at 4:23:43 AM UTC+8, s_dubrovich@yahoo.com   
   wrote:   
   > > On Sunday, September 4, 2022 at 1:55:32 PM UTC-5, muta...@gmail.com wrote:   
   > > > On Monday, September 5, 2022 at 1:03:24 AM UTC+8, Scott Lurndal wrote:   
   > > > > "muta...@gmail.com"  writes:   
   > > > > >On Sunday, September 4, 2022 at 9:25:34 PM UTC+8, wolfgang kern wrote:   
   > > > > >> On 04/09/2022 06:32, muta...@gmail.com wrote:   
   > > > >   
   > > > > >Noone had done segmentation before and it   
   > > > > >would take decades of science still to   
   > > > > >work through the repercussions.   
   > > > > Several very successful systems that predate the   
   > > > > 8086 by a full decade used segmentation; examples include the PDP-11,   
   > > > > the B6500 et alia.   
   > > > They presumably didn't do 4 bit shifts or similar   
   > > > so didn't hit the a20 issue nor the flexible shift issue.   
   > > >   
   > > > The a20 is probably because of cp/m PSP though,   
   > > > not segment shift.   
   > > They presumably didn't do 4 bit shifts or similar   
   > > so didn't hit the a20 issue nor the flexible shift issue.   
   > >   
   > > The a20 is probably because of cp/m PSP though,   
   > > not segment shift.   
   > > ~~~   
   > >   
   > > Gosh, istm you have these concepts jumbled up.   
   > >   
   > > Sorry for the review but..   
      
   >   
   > So first we have the justification for address wrap.   
   > So that call 5 could be implemented.   
   >   
   > Perhaps we need to go back to the 8080   
   > and/or cp/m to see whether the call 5   
   > could have been replaced with something   
   > that could have future-proofed the   
   > situation.   
      
   No "perhaps".  Call 5 was for CPM compatibility.  Original   
   CPM was for 64kB address space, so no problem _preserving_   
   compatiblity with respect to segment manipulation/wraparound:   
   this was new stuff.  8086 was different instruction set   
   so if was possible to implement equivalent functionality   
   in different way.   
      
   Even if you insist on having exactly the same functionality,   
   in PSP it is easy to do without wraparound: just put extra far   
   jump at end of BIOS ROM and in PSP have jump to this extra jump.   
   Cost is extra 4 bytes of ROM used by the jump and extra   
   machine cycles to execute extra far jump per each call 5.   
      
   Of course, once BIOS without this extra call was in the wild   
   DOS had to support such BIOS-es and gate A20 was logical   
   solution...   
      
   --   
                                 Waldek Hebisch   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)