From: terje.mathisen@nospicedham.tmsw.no   
      
   R.Wieser wrote:   
   > Paul,   
   >   
   >> What do you mean by "smaller memory models"?   
   >   
   > The ones that keep DS and SS (and possibly CS) the same.   
   >   
   >> But from memory small was the default memory model for   
   >> DOS C compilers.   
   >>   
   >> How did they manage to make it work?   
   >   
   > As long as they kept SS the same as DS that one is easy - as long as you do   
   > not put (directly accessed) data in the CS segment.   No extra work is   
   > needed   
   >   
   > If not ?  Well, they would have to use segment-register overrides a lot.   
   >   
   > Assume that SS differs from DS and you have some "local string variable" (on   
   > the stack) which you want to display.  You can't use INT 21h AH=09h   
   > directly, as it expects the string to be in the data segment.   
   >   
   > So, you have to either write funtions which grab a byte a time while   
   > overriding the default segment for that register (mov dl,[ss:dx]), or wrap   
   > the call into a bit of code which sets DS to be the same as SS and   
   > afterwards restores it. (push ds | mov ax,ss | mov ds,ax | .... | pop ds).   
   >   
   > Ofcourse, if you have DS different from CS and put strings into the code   
   > segment which you want to display you would have the same problem.  Heck,   
   > the same happens when you allocate memory and want to access it : you mostly   
   > have to juggle DS and ES around to get the "movs" instruction to do its   
   > thing. :-)   
   >   
   > In other words : Its not a rocket science needing problem, but its solution   
   > is one that needs to be applied rigorously and correctly *all the time*.   
   >   
   > Which is the reason that, when I'm writing Assembly, I seldom (if ever) feel   
   > the need to pick a memory model where SS is different from DS.   :-)   
      
   For my word count code I needed a 64 KB lookup table, so I put that in a   
   separate ES segment, then I could put another largish table in the SS   
   segment and access it with BP, leaving disk buffers etc in DS.   
      
   Terje   
      
      
   --   
   -    
   "almost all programming can be viewed as an exercise in caching"   
      
   --- SoupGate-Win32 v1.05   
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