[pups] 2.11BSD "make unix" aborts Error 141
cdl at mpl.ucsd.edu
Thu Mar 20 04:58:37 AEST 2003
> From: "Steven M. Schultz" <sms at 2BSD.COM>
> To: pups at 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 <dfevans at bbcr.uwaterloo.ca>
> > 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 lowenstein marine physical lab u.c. san diego
clowenst at ucsd.edu
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