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From: leland@cs.columbia.edu (Lee Woodbury)
Newsgroups: comp.lang.postscript
Subject: Re: defeating save/restore
Keywords: images, data compression,
Message-ID: <1991Mar11.171456.10189@cs.columbia.edu>
Date: 11 Mar 91 17:14:56 GMT
References: <38972@netnews.upenn.edu> <1909@chinacat.Unicom.COM>
Reply-To: leland@cs.columbia.edu (Leland Woodbury)
Followup-To: comp.lang.postscript
Organization: Columbia University, Dept. of Computer Science, NYC
Lines: 28

In article <1909@chinacat.Unicom.COM> woody@chinacat.Unicom.COM (Woody Baker @ Eagle Signal) writes:
>In article <38972@netnews.upenn.edu>, jeffe@eniac.seas.upenn.edu (George Jefferson ) writes:
>> Is there a way to store a value at some low level which is not
>> destroyed by restore?
>I seem to remember that on at least on older version of PS, there was a bug
>that allowed you to do this.  I think it was something like if you created
>a string, then put a character into it with put, it would survive a
>save/restore pair.  I know that Don Lancaster published some thing about it,
>and even wrote code to use it, but IT IS A BUG AND IS NOT REPEATABLE
>unless you have the buggy interpreter.
>
>i.e.  Not wise to write code for others use that depends on it.

Yeah, this is referred to in a footnote on p.43 in Section 3.7, "VIRTUAL
MEMORY," of the Red Book (1st ed):

	In the current PostScript design, restore actually
	does not undo changes made to the elements of strings.
	We consider this behavior to be a defect, and we do
	not recommend that PostScript programs take advantage
	of it.

Leland Woodbury
-- 
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