Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!ucsd!sdcc6!ir230
From: ir230@sdcc6.ucsd.edu (john wavrik)
Newsgroups: comp.lang.forth
Subject: Vectored Execution
Keywords: ANSI
Message-ID: <12398@sdcc6.ucsd.edu>
Date: 24 Aug 90 17:37:09 GMT
Organization: University of California, San Diego
Lines: 64


Re vectored execution, I asked:

>> II. Deferred Execution
>>     Will the proposed ANSI standard have a portable mechanism for
>>     deferred execution (like the F83 words  DEFER and IS)

Mitch Bradley replied:

> : DEFER  CREATE 0 ,  DOES> @ EXECUTE  ;
> : SET-DEFER  ( cfa cfa -- )  >BODY !  ;

I assume that this means that the ANSI Standard does not provide a 
mechanism for vectored execution and that Mitch is showing how one can 
be defined. (Such a mechanism was not in the last BASIS document I 
have and has apparently not been provided since.)

Mitch's approach is essentially a dressing up of the fig-Forth 
approach: store the cfa of a word in a variable, and later execute the 
contents of the variable. 

                                 ----

Vectored execution is one of the areas mentioned in the TC document 
posted by Jack Brown. There is a good reason for wanting words (like 
DEFER and IS) for vectored execution to be part of the language 
standard rather than user defined. Here are some timings for 100,000 
executions of a NOOP -- both directly and using various approaches to 
vectored execution: 
                        
        BARE NOOP        2.25 seconds
        FIG APPROACH     3.79 seconds
        BRADLEY APPROACH 4.89 seconds
        F83 APPROACH     2.69 seconds

In F83 the mechanism for vectored execution is coded in assembly 
language -- and the overhead for executing a word indirectly is 
surprisingly small.

                                 ----

I asked whether a simple non-exotic Forth application written to 
answer a question of Wil Baden could be written in the proposed ANSI 
Standard. Specifically: 
      1. Can the return stack be extended to make recursion viable?
      2. Is there a Standard mechanism for deferred execution?
      3. Can the user control compilation enough to define a simple
         control-flow structure?
I would like to thank for Mitch Bradley for replying. As he has shown, 
the answer is a discouraging NO on all points.

Now that the ANSI Team perceives itself to be near completion, it 
would be a good time to test the proposed Standard to find out whether 
or not it is missing essential ingredients. I don't share the view of 
those who think that an ANSI Standard is "window dressing" and is not 
supposed to support real programming.     


                                                  John J Wavrik 
             jjwavrik@ucsd.edu                    Dept of Math  C-012 
                                                  Univ of Calif - San Diego 
                                                  La Jolla, CA  92093