From: bc@nospicedham.freeuk.com   
      
   On 06/04/2019 10:07, James Harris wrote:   
   > On 06/04/2019 00:38, Edward Brekelbaum wrote:   
   >> In ring 3, you'll seg fault.   
   >   
   > That's if running under an OS, presumably. But I guess it would be   
   > different if running on a bare machine (the case in point) where valid   
   > memory address ranges have not been set up in GDT/LDT or page tables.   
   >   
   >>   
   >> In ring 0, you should get all ones (IIRC). The transaction goes down   
   >> the ISA bus and times out.   
   >   
   > For some reason I, too, would most expect a read would get all ones   
   > back, but I am not sure why. Perhaps it's based on the idea of a bus   
   > which is carried off a computer by an edge connector. In such a case   
   > IIRC computers used pull-up resistors on bus lines to provide signal   
   > stability. Any line which was to be read as zero had to be driven low.   
      
   In the TTL devices I used to use, if nothing was put onto the bus lines,   
   they would be tri-stated, and generally floated high.   
      
   Although that could not be relied upon.   
      
   I remember having video memory for a 128x128 x 4-bit display (8KB total)   
   accessed as 128x128 x 8-bit (16KB range). The top 4 bits of each access   
   were considered garbage.   
      
   In this example, the address was decoded and I could have chosen to pull   
   those bits up or down. In yours I think the address is not even decoded.   
      
   What results to you actually get?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)