Raspberry Pi 1B "Pi on Fire" Demo

A mostly "bare-metal" (no Linux!) demo running directly on the original Raspberry Pi. Originally meant to showcase my pt3_lib pt3 decoder. I say mostly bare-metal as it's running on my extremely low-end VMWos operating system.

This Demo won 2nd place in the Modern Demo category at Demosplash 2019 on 9 November 2019.

Video

Disk Image

pionfire_demosplash2019.tar.gz -- 450k (9 November 2019)
Note, to run this you will need a Pi-1B, plus an SD card with Raspbian installed. Extract kernel.img. Backup your old kernel.img on the SD card, replace it with this one.

System Requirements

Tested on Raspberry Pi 1B
Sound comes out headphone jack. Composite or HDMI video.
Should work on other Pis with BCM2835 (BCM2708) core (Pi-1A, possibly the zero?)
The disk image will not run on newer Pis. VMWos will, so in theory building from scratch and copying over kernel7.img will work, but the timings will likely be off. Currently untested.

FAQ

Does this use the 3D GPU in the Pi?

No, it's just using the simple framebuffer support via the mailbox interface. I would like to use the GPU in future projects.
How big is the demo?

The demo itself is 128k, the OS kernel is only 41k. The entire operating system image (which includes the demo, the kernel, and various system utilities) is 1.6MB.

The demo itself could be much smaller. It's written in C and no attempt was made at all to optimize it.
How is the graphics implemented?

On the Pi you can request the GPU to emulate a framebuffer for you, and it will pass you a memory address you can use. So the demo requests a 640x480x24bpp framebuffer, and writes 24-bit RGB values (Actually it might be BGR) to that region.

Writing that much memory each frame is slow on the pi (I think it's non-cachable RAM for obvious reasons). So my code actually does all graphics in a seperate region of memory in 640x480x8bpp and then converts this (using a palette lookup) to 24bpp at screen update time.

You might say this sounds a lot like old-school VGA graphics, and you're right. I did a lot of VGA programming back in the day so I was used to the idea. It also means I can do some cool palette effects like fading in/out and the Doom fire effect.
How is the sound implemented?

It's a 6-channel PT3 tracker file (by DYA), and I am using my PT3 decoder plus an AY-3-8910 emulator to play the music. A PT3 file decoded to 44.1kHZ stereo is actually really large, but the Pi has excessive memory so I decode to RAM, then set up DMA to play the entire thing in the background (which is much easier than decoding with a periodic interrupt like you'd normally do).

Source Code

Source code can be found as part of the VMWos project http://github.com/deater/vmwos in the userspace/demo directory

Screenshots and Dev Notes

	Opening Screen. This dates back to my DOS programming days. Countdown is while it decodes the 6-channel pt3 file to raw 44.1kHz 16-bit stereo output. The pi plays that in the background via DMA. This is cheating; ideally interrupts would be set up and only part would decode at a time. Decoding all at once was easier, and after doing a lot of Apple II work the 512MB on the Pi seems like inifinite memory.
	Linux boot fakeout. This is a fake Linux boot, with some editorializing about everyone's favorite /bin/init.
	Falling text. Uses ANSI console code I wrote earlier for ansi2gif and vmwos.
	Linux logo. Did you know that I'm also the original author of the linux_logo utility?
	Rasterbars. I meant to do something fancier here but ran out of time.
	Mode 7 flying over pi code. Roughly based on code originally by Martijn van Iersel here. Uses all integer code (20.12 fixed point), even though in theory I figured out how to get floating point going on the Pi. An extra challenge is there is no integer divide on the BCM2835. The sky image is from the Hubble deep field picture.
	Doom fire. Just a direct implementation of Fabien Sanglard's PSX Doom Fire. The text catching on fire when scrolling was a happy accident.

Vote for this at Pouet

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