Re: [pups] 2.11BSD "make unix" aborts Error 141 - TUHS

19 Mar 2003

...
  From: "Steven M. Schultz"
&lt;sms(a)2BSD.COM&gt;
 To: pups(a)minnie.tuhs.org
 Subject: Re: [pups] 2.11BSD "make unix" aborts Error 141
 List-Archive: <http://minnie.tuhs.org/pipermail/pups/>
 Date: Wed, 19 Mar 2003 09:46:51 -0800 (PST)

 Hi -

  From: David Evans
&lt;dfevans(a)bbcr.uwaterloo.ca&gt;
   Ugh.

   Am I naive to suggest a rewrite of as?  :)  
 	Just a little bit <g>

 	It's a thought that used to occur to me fairly often a long time 
 	ago.  I cured myself of it though after I did the rewrite into
 	its current form (if you think it's bad now you should have seen it
 	before ;)).

 	The biggest problem are the SDI (Span Dependent Instructions).  The
 	PDP-11 has 'jmp' (non conditional but no range limit) and 'br'
(many
 	flavors of conditional branches but with a +/- 128 byte range).  
 	The compiler generates pseudo instructions like 'jeq' and leaves it
 	up to the assembler to figure out if a branch will reach or if a
 	branch around a jump is needed.   That's a PITA to handle and is
 	I believe the cause of much ugliness in the assembler. 
That, of course, is a _neat feature_ of AS.  On the other hand, how
much code bloat would result if AS always emitted (bne;jmp) pairs for
'jeq', etc.  Maybe only generate (bne;jmp) for forward 'jeq' where
the
span is not yet known.  I am sure there are pathological instabilities
in this kind of code generation, where expanding one branch/jump
instruction makes another one go out of span range.

Answer:  3:1 for each unnecessary pair.  But I have no idea of the
statistics for real code.

    carl
-- 
    carl lowenstein         marine physical lab     u.c. san diego
                                                 clowenst(a)ucsd.edu