From: invalid@nospicedham.lkntrgzxc.com   
      
   On Fri, 5 Apr 2019 19:41:13 +0100   
   James Harris  wrote:   
      
   > Does anyone here know what would likely happen to cache lines and   
   > buses if one were to read addresses which had no RAM - with or   
   > without caching enabled for those addresses? Ditto writing such   
   > addresses? Would the reads/writes cause problems? What value would be   
   > read? All zeroes? All ones?   
   >   
   > And would such operations be slower than accessing real RAM? I guess   
   > not but I don't know. Accessing non-existent RAM is not something one   
   > gets to do too often!   
   >   
      
   No, I don't.   
      
   I know an older machine returned all ones for when reading the PS/2   
   port, which it doesn't have.   
      
   The same machine has a BIOS that could create a memory hole at 1MB,   
   which I believe was for a video card (perhaps AGP?).   
      
   It also allowed you to copy ROM in RAM for speed.  So, writing to the   
   BIOS address range is probably not a good idea.  Maybe, the BIOS region   
   is protected from writes?  I'm really not sure what happens here.   
      
   I doubt I have any machines old enough that would have a memory gap   
   between 640K and 1MB, i.e., assuming memory is probably back-filled   
   with RAM if no card is installed in the region, or a gap between 512K   
   and 640K.   
      
   Most likely your best best is to pull memory modules or chips to reduce   
   installed memory in a real machine below the peak address.   
      
      
   Rod Pemberton   
   --   
   The lesson for Boeing.  You push the nose of a plane down, and planes   
   go down.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)