XWin Architecture

[Harold L Hunt II]

Jeremy,

Jeremy Wilkins wrote:

> Hi,
> 
> I'm curious about how the cygwin XWin server works?

Curious, or interested in helping?  If just curious, then please wait a 
few days or dig through the archives... I have described it in detail a 
few times.  Perhaps the following answers will be all you need.

> My understanding is it is similar to an X Server on linux but instead of 
> rendering to the graphics card framebuffer, it is rendered to an 
> offscreen surface and then bitblt'd to an on screen pixmap using direct 
> draw, or GDI if dd is unavailable.

That is correct.

> Does the MacOS X Xserver work in the same way?

Yes.

> Does this use a lot of existing code, eg from either vnc or xvfb servers?

Yes, but not from vnc or xvfb.  There is a generic layer written by 
Keith Packard called "fb" that draws to framebuffers using the cpu. 
This "fb" layer replaced the old "cfb" and "mfb" layers that did the 
same thing but were crufty and designed for the cpu being the constraint 
(think 1980's) rather than the buses to and from memory and other 
peripherals being the constraint (think today).

> How does the rootless stuff work, does this just figure out where the 
> top level windows are and only bitblt those but into separate 
> surfaces/windows.

Well, that is sort of a dangerous question.  There is the original 
"rootless" mode that Kensuke wrote a while back.  That mode just keeps 
track of the top-level X11 windows and clips transfers to our single 
Win32 window to the region that is occupied by X11 windows, thus 
preventing the root window from being drawn.  You will note that all of 
the X11 windows appear to float in the same plane when you use -rootless 
in the current versions of XWin.exe.

There is also multi-window, but that is a little more complex in that it 
has an internal window manager and it creates a Win32 window for each 
top-level X11 window and provides management of those windows.  This one 
does bitblt to particular Win32 windows, whereas the aforementioned 
rootless mode only had a single Win32 window.

> I've read somewhere that there is some generic rootless code in the xc 
> tree, where abouts is this?

That is in xc/programs/Xserver/miext/rootless.  Kensuke's latest work 
has centered around getting this to work.  He has a new window manager 
written that is more complete than the current multi-window window 
manager.  However, there are still significant bugs in this 
implementation and we are not even building releases from the source 
tree with that code in it yet.

Torrey Lyons just sent me a draft description of how miext/rootless 
works.  Hopefully he will finish it soon so I can point you to it.  It 
really is an interesting layer and it allows optimization of some simple 
cases that will have quite a large performance impact.

Harold