XPost: alt.folklore.computers   
   From: lynn@garlic.com   
      
   John Levine  writes:   
   > It's a 32 bit architecture with 31 bit addressing, kludgily extended   
   > from 24 bit addressing in the 1970s.   
      
   2nd half 70s kludge, with 370s that could have 64mbytes of real memory   
   with only 24bit addressing ... the virtual memory page table entry (PTE)   
   had 16bits with 2 "unused bits" ... 12bit page number (12bit 4kbyte   
   pages, 24bits) ... and defined the two unused bits to prepend to the   
   page number ... making 14bit page number ... for 26bits (instructions   
   were still be 24bit, but virtual memory used to translate to 26bits real   
   addressing).   
      
   original 360 I/O had only 24bit addressing, adding virtual memory (to   
   all 370s) added IDALs, the CCW was still 24bit but were still being   
   built by applications running in virtual memory ... and (effectively)   
   assumed any large storage locations consisting of one contiguous   
   area. Moving to virtual memory, I/O large "contiguous" area was now   
   borken into page size chunks in non-contiguous areas. Translating   
   "virtual" I/O program, the original virtual CCW ... would be converted   
   to CCW with real addresses and flagged as IDAL ... where the CCW pointed   
   to IDAL list of real addresses ... that were 32 bit words ... (31 bits   
   specifying real address) for each (possibly non-contiguous) real page   
   involved.   
      
   --   
   virtualization experience starting Jan1968, online at home since Mar1970   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)