Gareth's was W7 now W10 Downstairs Computer wrote:

> Presumably the graphics controller fills up a frame buffer for display,
> so, if one is not at a certain point using the controller to draw
> something, could one write to the frame buffer directly?

According to various official blogs, etc, yes. AIUI, the standard linux
framebuffer api works properly on the Rpi.

> Also, AIUI, the RPi communicates with the GPU via memory-based
> message buffers, so the GPU must be running some form of background
> kernel?

Apparently, the GPU runs it's own microcode, which implements the
"VideoCore" architecture. You can find detailed documentation on this
architecture, and Broadcom's implementations for the RPi SoCs by searching
the fine web.

> I assume that this must be the purpose of only one of the GPU
> processors as running full motion video, say, in an MPEG stream,
> must take quite a lot of computer time?

Of course.

> One part of the GPU must be simply concerned with taking the frame
> buffer and emitting it as a composite video stream, so I wonder how
> the aspect ratio is set up?

I'm not familiar with the internals of the VideoCore GPU, so I can't answer
this question. Perhaps a thorough read-through of the VideoCore 3D
Architecture Reference Guide would answer this question for you. See
  https://docs.broadcom.com/docs/12358545
for details.

> Sorry, a bit unstructured in the above, just musing; it's something
> that I pick up after a few months before dropping it again; I'm
> struggling  to get to grips with the whole GPU thing.

HTH
--
Lew Pitcher
"In Skills, We Trust"

--- SoupGate-Win32 v1.05