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From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.lang.misc
Subject: Re: Displays (Was:  Var scoping in Wirth-type languages)
Message-ID: <4554@aw.sei.cmu.edu>
Date: 14 Mar 88 14:50:22 GMT
References: <3821@ihlpf.ATT.COM> <2791@enea.se> <3949@ihlpf.ATT.COM> <7562@sol.ARPA> <4426@june.cs.washington.edu> <7629@sol.ARPA>
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Reply-To: firth@bd.sei.cmu.edu.UUCP (Robert Firth)
Organization: Carnegie-Mellon University, SEI, Pgh, Pa
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In article <7629@sol.ARPA> crowl@cs.rochester.edu (Lawrence Crowl) writes:

>Displays require one more load-to-register to implement than do static chains.
>(Assuming you have enough registers to implement the display.  Most new
>processors do.)

This statement is true ONLY for languages that do not have parametric
procedures or procedure variables.  Invoking a parametric procedure
can require an arbitrary amount of a display to be reset, whereas the
cost of maintaining the static chain is still very small.  Moreover,
the cost in other areas of reserving 5 or 6 registers for a display can
be pretty high - you have that many fewer for locals, temporaries,
parameters &c.

By the way, what is the compiler supposed to do when a user nests
procedures more deeply than you've allowed for in the display registers?
Crash disgustingly?