From: joemonk64@gmail.com   
      
   > There was 16-bit 360: 360/20, it was limited to 64k of core    
   > (or maybe only 32k). Once you have more memory there    
   > is really no sense to use 16-bit: on small machines 360/370    
   > instructions were interpreted by real processor and interpreter    
   > for 32-bit instructions is only marginally more complex    
   > than interpreter for 16-bit instructions. And interpreted    
   > 32-bit instructions are faster than segmented 16-bit ones.    
   > To be more precise: both 360/20 and 360/30 stored official    
   > 360 registers in core memory. Processing only 16 bits    
   > could give some saving due to smaller number of accesses to    
   > core. But once you have segments in the mix there is no    
   > saving: you need to handle 32-bits anyway and interpreter    
   > gets more complex and slower.   
      
   The 360/20 only had 8 registers (8-F) compared with the larger models which   
   had 16 registers (0-F). Yes they were 16 bit registers, and they were NOT   
   stored in RAM.   
      
   The 360/20 was limited to 16K of RAM.   
      
   Otherwise it was pretty much a full blown 360. You still to this day can take   
   a program from a 360/20 and run it on the latest z/arch processors.   
      
   "Direct Addressing: Direct addressing is used when the high-order bit in the   
   B-field of an instruction is zero.   
   When the direct addressing method is employed, the low-order 14 bits of the   
   combined B- and D-fields are used to refer directly to byte locations in main   
   storage. The 12 binary bits in the D-field allow an address specification of   
   up to 4095. To address    
   additional (optional) storage, the adjacent two bits in the low-order pOSition   
   of the B-field are used, allowing address specification of up to 16383."   
      
   http://www.bitsavers.org/pdf/ibm/360/functional_characteristics/   
   26-5847-3_360-20_funChar_Apr67.pdf   
      
   Joe   
      
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