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Date: Thu, 31 May 2001 15:01:07 -0400
From: Christopher Faylor <cgf AT redhat DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Addressing Layout in 1.3.x
Message-ID: <20010531150106.O23914@redhat.com>
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References: <5 DOT 1 DOT 0 DOT 14 DOT 2 DOT 20010531090143 DOT 0263bf78 AT ks DOT teknowledge DOT com>
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In-Reply-To: <5.1.0.14.2.20010531090143.0263bf78@ks.teknowledge.com>; from rrschulz@cris.com on Thu, May 31, 2001 at 09:22:56AM -0700

On Thu, May 31, 2001 at 09:22:56AM -0700, Randall R Schulz wrote:
>Hi,
>
>[ This question can presumably only be answered by people who know the 
>internals of the code that goes into cygwin1.dll, and I hope I'm not out of 
>line for asking here rather than trying to figure it out from the source, 
>both current and historic... ]
>
>
>One of the applications ported to Cygwin is the XSB Prolog interpreter 
>(<http://xsb.sf.net/> / <http://sf.net/projects/xsb/>). The interpreter 
>uses a "tagged" scheme (like the old generation of lisp machines had in 
>hardware) to make instances of its interpreter's data types 
>self-identifying. Since there are no real tag bits available in today's 
>general-purpose processors, the XSB implementation puts its tags in 
>carefully chosen bits from ordinary words (32 bit iintegers on an x86 machine).
>
>"Carefully chosen" means bits whose values are dictated and fixed by the 
>addressing layout of the processor and operating system (and / or any 
>execution environment layered over the OS) on which XSB runs.
>
>In practice, this means bits whose value does not change regardless of how, 
>when or where the application loads into VM and regardless of how much 
>memory the application allocates during execution. Hence those bits can be 
>replaced with the interpreter's tag bits until it becomes necessary to 
>dereference that value (when the tag indicates the value is a pointer of 
>some sort). At that point, the known correct values of the usurped bits are 
>replaced with their proper values and the the deference occurs.
>
>Naturally, that means that the XSB implementation is sensitive to changes 
>in the addressing assignments of the environment that's hosting it. For XSB 
>running under Cygwin, this presumably (potentially, for all I know) 
>includes both Windows and Cygwin.
>
>
>So, after that lengthy introduction, my question is: Did the addressing 
>layout change in Cygwin 1.3.x vis. a vis. 1.1.x?
>
>
>Thanks for reading all the preliminaries to get to the question.

I read the whole thing but I still have no idea what you mean bu
addressing layout, so I don't really know how to answer this.  Certainly,
things change all of the time.  There are new globals added, the size
of the DLL changes, the location of globals within the DLL changes.
There is no way to avoid this.

cgf

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