From: sfuld@alumni.cmu.edu.invalid   
      
   On 11/11/2025 1:10 PM, John Levine wrote:   
   > According to Niklas Holsti  :   
   >> Speaking of indirect addressing, the HP 2100 had a special feature: it   
   >> had a 64 KB address space, but with word addressing of 16-bit words, so   
   >> addresses were only 15 bits, leaving the MSbit in each word free.   
   >>   
   >> [multi-level indirect chains]   
   >   
   > That was quite common back in the day.   
      
   Yes, as I mentioned earlier in this thread, so did the Univac 1100 series.>   
   > The Data General Nova and Varian 620i (both popular for OEM   
   > applications) did exactly the same thing, 15 bit addresses with the   
   > high bit saying indirect.   
   >   
   > The PDP-6/10 was a 36 bit machine with 18 bit addresses and a rather   
   > overimplemented addressing scheme -- each instruction had an address, an   
   > indirect bit, and an index register, so it added the address to the index   
   > register (if the register number wasn't zero), then if the indirect bit was   
   set,   
   > fetch the addressed word and interpret its address, indirect bit, and index   
   > register the same way, ad infinitum.   
      
   Yup.  Similarly the 1100 series, a 36 bit machine with 18 bit addresses,   
   had all of those features, plus one more.  If the index register   
   increment bit was set (in the instruction itself, or in each of the   
   indirect words), the upper 18 bits of the index register were added   
   (after indexing) to the lower 18 bits.  This allowed some really   
   "interesting" possible code when this was within a loop.  :-)   
      
      
   >   
   > An interesting question is what happened if a computer got into an indirect   
   > loop.   
      
      
   Yup.  The 1100 prevented an infinite loop by having a hardware timer for   
   each instruction.  If the timer expired, an illegal operation exception   
   occurred.   
      
      
      
   --   
     - Stephen Fuld   
   (e-mail address disguised to prevent spam)   
      
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