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The ARM014 tutorial introduces a few new peices of the puzzle.

Firstly, as an aid to debugging now the code is getting more complex it introduces the mini UART which means we can have a basic “console”.

Finally, we get on to a tutorial that moves us away from blinking an LED as we explore the Video Core IV GPU We need some reading material for this tutorial – this is how I put the tutorial together, by reading and studying the manuals available for the Videcore, but for this tutorial mainly the information from the Cambridge Raspberry Pi Tutorials.

Yes there’s a lot of text and more than one manual – but that’s the only way you learn!

The mailbox interface is software running on the GPU which receives messages from software running on the arm and returns responses to each message after performing a task.

It’s implemented in the file that we require on the SD Card to boot the Raspberry-Pi.

If you want, go ahead and compile it now and plug the raspberry pi into a monitor or television with a HDMI interface. As we’re now including new hardware into the mix it’s possible that your monitor or TV doesn’t support the resolution and colour depth that the example is hard coded to use.

It’s an example that’s designed to be simple rather than supporting every HDMI panel out there. I’m using an old Hanns-G HUD19 monitor with DVI-HDMI adaptor.

However, don’t forget that the ARM and GPU also share the memory space, and so although we have to communicate through the mailbox, this is what negotiates settings and a framebuffer.

The framebuffer address in memory is then returned from the GPU and we can go ahead and write to the framebuffer to see graphics.

You can see their discussion about implementing the mailbox on Github The mailboxes are defined on Github.

The interface we’re interested in is the Framebuffer mailbox.

This tutorial shows how a 700MHz (or 900MHz) processor doesn’t give you carte-blanche to program in C and end up with an optimised output.

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