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Re: redisplay system of emacs

From: Eli Zaretskii
Subject: Re: redisplay system of emacs
Date: Wed, 17 Feb 2010 06:20:48 +0200

> Date: Tue, 16 Feb 2010 21:56:03 +0100
> From: grischka <address@hidden>
> CC: address@hidden, address@hidden
> Eli Zaretskii wrote:
> >> It's not _that_complicated.  Basically it just draws the new screen to
> >> memory, then compares it with the current screen and then updates the
> >> differences.
> > 
> > That's only a small part of the story.  It draws only part of the
> > screen, not all of it (unless optimizations fail), and it goes to
> > great lengths to minimize that part of the screen.
> Sure, thats what I wrote: It updates only the differences.

The point is, it tries to limit the region where it looks for
differences to as small portion of the screen as possible, sometimes a
single line.  Therefore ``draws the new screen to memory'' is an
exception; the rule is ``draws a small portion of the new screen to

> > The reason is that delivering to the screen is not the heaviest part
> > of the redisplay; it's that ``drawing to memory'' that is.
> That is probably true nowadays with fast graphical screens (say for PCs
> after ~1995).  However the design of emacs' "redisplay" is based on the
> contrary assumption that sending the updates to the terminal more expensive
> than their calculation.

The current display code was designed in 1999.

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