From: news@alderson.users.panix.com   
      
   cross@spitfire.i.gajendra.net (Dan Cross) writes:   
      
   Too many levels to be certain who actually wrote the following quoted by Dan:   
      
   >> 5) The idea of emulating one OS on another OS is questionable   
   >>     in itself.   
      
   > This really needs to be qualified, as it is common and has been   
   > done for decades.  Evaluation criteria must include a) the   
   > complexity of the emulation target, and b) its alignment with   
   > the existing system design.   
      
   Agreed.   
      
   > Consider PA1050 on TOPS-20, for example: this was a type-2 hypervisor that   
   > allowed the DECSYSTEM-20 to provide very faithful emulation of TOPS-10.  But   
   > TOPS-20 is argably closer to TOPS-10 than, say, VMS is to Linux.   
      
   Boggle.   
      
   Tops-10 and TENEX/TOPS-20 run on the same base hardware (the PDP-10), and   
   TOPS-20 shares part of its name with Tops-10, but other than that they are   
   entirely unrelated.   
      
   PA1050 (which was written by the BBN folks who created TENEX, the ancestor of   
   TOPS-20), emulates a subset of the system calls of Tops-10, in order to allow   
   utilities like the FORTRAN and COBOL compilers to run on an OS for which they   
   were not engineered.  It does this by mapping a set of routines into the user   
   program which then make TOPS-20 system calls invisibly.  These routines are   
   mapped in when the first Tops-10 system call (which is in essence an illegal   
   instruction that thereby triggers a trap to the monitor) is encountered in the   
   user program instruction stream.   
      
   PA1050 is, in this sense, not a hypervisor but something equivalent to the Wine   
   emulator on Linux ("Wine Is Not an Emulator", indeed).   
      
   > And since TOPS-20 used a different mechanism for trapping into the executive   
   > for system requests than TOPS-10, it was easy to distinguish between the two.   
      
   At the base level, no, it does not.  It simply uses a different "illegal"   
   instruction to trigger the context switch from user mode to monitor mode than   
   does Tops-10.   
      
   > On the other hand, things like gVisor, which emulates the Linux   
   > kernel interface, are very complex and difficult to get right.   
   > And of course the PDP-10 was a much simpler machine than x86_64.   
      
   Agreed.   
      
   --   
   Rich Alderson					  news@alderson.users.panix.com   
         Audendum est, et veritas investiganda; quam etiamsi non assequamur,   
   	  omnino tamen proprius, quam nunc sumus, ad eam perveniemus.   
   									--Galen   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)