🚀 go-pugleaf

In article <2289@peora.UUCP> joel@peora.UUCP (Joel Upchurch) writes:
> 	One thing that no one has mentioned so far  that you could do
> 	with very large memories is table lookups.

	OK, let's talk bizarre (1/2 :-)).  Imagine the stateword of a
process as all the bits in that process's memory strung end to end.  If you
take this as a memory address, you could implement your CPU as a simple
lookup table; for each possible state there is only one possible next state
that a process can get to.  All you have to do is do a simple table lookup
to find the next state.

	Of course, you need a mighty big memory to build the lookup table
(not to mention the non-trivial amount of CPU time to calculate what values
go where in the table).  How much memory?  Well, I just did a size on
/bin/* (11/750 running 4.2 BSD).  Running the results through colex to get
the decimal total and desc to get descriptive statistics (both from Gary
Perlman's very nice UNIX|STAT package) I get (with a sample size of 60;
this excludes the shell scripts /bin/true and /bin/false):

------------------------------------------------------------
        Mean      Median    Midpoint   Geometric    Harmonic
   35343.000   27394.000   57622.000   32294.059   30172.889
------------------------------------------------------------
          SD   Quart Dev       Range     SE mean
   17635.600    7553.000   77052.000    2276.746
------------------------------------------------------------
     Minimum  Quartile 1  Quartile 2  Quartile 3     Maximum
   19096.000   24372.000   27394.000   39478.000   96148.000
------------------------------------------------------------

	The smallest program was /bin/echo (20k to echo argv!?); the
largest was /bin/as at 96k.  So, assuming most processes will fit into 100
kbytes, that means you need a LUT with an 800k bit long address.  Talk
about BFM's!  As I think I've mentioned before, it is believed that there
are approximately 2^200 electrons in the universe.  Since it is unlikely
that anybody would want to reference more things than there are electrons
in the universe, 200 bits seems like a good upper bound for the length of a
memory address.

-- 
Roy Smith, {allegra,philabs}!phri!roy
System Administrator, Public Health Research Institute
455 First Avenue, New York, NY 10016