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From: asente@adobe.com (Paul Asente)
Newsgroups: comp.windows.x
Subject: Re: OW as a PostScript X server...
Message-ID: <8245@adobe.UUCP>
Date: 15 Nov 90 01:34:32 GMT
References: <9011131842.AA08663@islanders.> <1990Nov14.012626.3685@mcs.anl.gov>
Sender: news@adobe.COM
Organization: Adobe Systems Inc.
Lines: 37

In article <1990Nov14.012626.3685@mcs.anl.gov> korp@atlantis.ees.anl.gov (Peter Korp) writes:
>As I am not privy to source, exactly how is the DP extension to X implemented?
>Are the X proto requests interpreted by the X-server and basically converted
>to standard X calls? Is systemdict shared by all processes?

You don't need to be privy to source, just documentation!

The way an X extension works is that each particular instance of an X server
supports some number of extensions, possibly 0.  The extensions that a server
supports is normally determined when the server is built.  (Run-time loading
complicates this somewhat.  The server must be able to respond to the
XListExtensions request and give back all the extensions it supports, so it
must know about extensions that it is willing to load.)

Requests destined for an extension look exactly like normal X requests.  They
have a request code, length, etc.  The server's event dispatcher sees these
requests, notices that they are for an extension, and passes them off to the
extension code.  In DPS's case, what the extension does with the request is
execute it as PostScript code and image into a drawable.

Each X server connection that also uses DPS/X (the name for the Display
PostScript extension to X) must get a PostScript execution context.  The
normal case is one context per display connection.  Part of a PostScript
execution context is a virtual memory (VM), and normally each execution
context has its own VM (although they can be shared).  There is also a
designated VM called the shared VM, visible to all contexts.  When the
extension looks for something, it looks in both the private VM for the
currently executing context and the shared VM.  Usually a context only
updates its own VM, but it can enter a mode where it can update the shared
VM; an example use would be to install a new font.  Systemdict lives in the
shared VM.

If you're interested, you can order the Display PostScript documentation
binder by calling Adobe Developer Support at (415) 961-4111.

	-paul asente
		asente@adobe.com	...decwrl!adobe!asente