From: antispam@fricas.org   
      
   Anton Ertl  wrote:   
   > antispam@fricas.org (Waldek Hebisch) writes:   
   >>VAX-780 architecture handbook says cache was 8 KB and used 8-byte   
   >>lines.  So extra 12KB of fast RAM could double cache size.   
   >>That would be nice improvement, but not as dramatic as increase   
   >>from 2 KB to 12 KB.   
   >   
   > The handbook is:   
   > https://ia903400.us.archive.org/26/items/bitsavers_decvaxhandH   
   ndbookVol11977_10941546/VAX_Architecture_Handbook_Vol1_1977_text.pdf   
   >   
   > The cache is indeed 8KB in size, two-way set associative and write-through.   
   >   
   > Section 2.7 also mentions an 8-byte instruction buffer, and that the   
   > instruction fetching is done happens concurrently with the microcoded   
   > execution.  So here we have a little bit of pipelining.   
   >   
   > Section 2.7 also describes a 128-entry TLB.  The TLB is claimed to   
   > have "typically 97% hit rate".  I would go for larger pages, which   
   > would reduce the TLB miss rate.   
      
   I think that in 1979 VAX 512 bytes page was close to optimal.   
   Namely, IIUC smallest supported configuration was 128 KB RAM.   
   That gives 256 pages, enough for sophisticated system with   
   fine-grained access control.  Bigger pages would reduce   
   number of pages.  For example 4 KB pages would mean 32 pages   
   in minimal configuration significanly reducing usefulness of   
   such machine.   
      
   _For current machines_ there are reasons to use bigger pages, but   
   in VAX time bigger pages almost surely would lead to higher memory   
   use and consequently to higher price for end user.  In effect   
   machine would be much less competitive.   
      
   BTW: Long ago I saw message about porting an application from   
   VAX to Linux.  On VAX application run OK in 1GB of memory.   
   On 32 bit Inter architecture Linux with 1 GB there was excessive   
   paging.  The reason was much smaller number of bigger pages.   
   --   
                                 Waldek Hebisch   
      
   --- SoupGate-Win32 v1.05   
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