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From: liberte@uiucdcs.UUCP
Newsgroups: net.lang.forth
Subject: Re: Re: single-stepping in Forth - (nf)
Message-ID: <30900008@uiucdcs.UUCP>
Date: Tue, 8-May-84 16:05:00 EDT
Article-I.D.: uiucdcs.30900008
Posted: Tue May  8 16:05:00 1984
Date-Received: Wed, 9-May-84 23:44:25 EDT
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Nf-ID: #R:uwvax:-24100:uiucdcs:30900008:000:2039
Nf-From: uiucdcs!liberte    May  8 15:05:00 1984

#R:uwvax:-24100:uiucdcs:30900008:000:2039
uiucdcs!liberte    May  8 15:05:00 1984

Although the single stepping methods suggested by Dave Anderson may be 
helpful, particularly in bringing up a new system, neither of them do
what I had in mind, as stated.

They are both "global" stepping, in that they would step through everything
(except code words).  What I wanted was "local" stepping that would step
through just the words I choose.  To do this by changing (modifying the
compiled code of) either the inner interpreter (;S, I believe) or the docol
word, one would also have to have a table of words that are being stepped
through and the stepper would have to continuously determine whether it
is supposed to step through the current word.  Blechkt!

The alternative is, as I suggested, to change just the CFA of those words
which you wish to step through and have the stepper control the interpreting
of those words.  Then the inner interpreter would always return to the
stepper as each subword finished.  A schematic of the stepper follows.

: step  ( instead of docol for stepped words )
  magic words to initialize
  begin
   load next subword address
   display name of subword
   prompt for command
   if step then execute subword
   ( return here )
   more magic words
  again
;


Another approach is to have the stepper change the subword address following
the next subword to reference the stepper again so that after the next
subword returns, the stepper is activated again.  This would act like a
series of breakpoints.  The problem here, as with breakpoints, is that the
address replaced by the stepper address must be saved and restored, probably
on the return stack.  Another more serious problem is that finding that
address depends on what the next subword does.  Literals stored in the code
would have to be jumped over correctly.  This is an argument for keeping
literals in a separate place, as in Smalltalk, another threaded interpreted
language of more appropriate design.


Daniel LaLiberte,  U of Illinois, Urbana-Champaign, Computer Science
{moderation in all things - including moderation}