To draw on the GBA screen you need to use the gbaScreen device:
make display gbaScreen device 4
make displayX number
make displayY number
make displayColor number
make displayFlush boolean
make message string
Then you can use standard Sniff drawing operations to tell the screen what to do:
.set displayColor to 777
.set displayX to 0
.set displayY to 0
.tell display to "move"
.set displayX to 100
.set displayY to 100
.tell display to "draw"
If you've not used these before, then you probably want to practice drawing on a PC screen first, before going to Gameboy. If you have used them, then they're exactly the same.
The new thing you do need to know about is that the the GBA has six different screen modes. Modes 0-2 are "tiled" modes, and while these are actually used for most real GBA games, they're a bit more tricky to understand, so we'll leave them for now. For regular drawing you need to use modes 3,4 or 5. Each has its own tradeoffs...
Mode 3 is 240x160 pixels with full colour. Unfortunately this uses up all of the GBA's graphics memory, which means it can't do double buffering or page flipping. In mode 3 the Sniff displayFlush has no effect, and all drawing goes directly to the screen, which means it can be very flickery unless you're careful.
To avoid flickering we need two screens worth of memory, and then we display one, while drawing to the other. Then we flip them over and all of our drawing appears instantly, without any flickering! Great, but we don't have enough memory for a full screen of full colour - something has to give:
Mode 4 is full resolution (240x160) but uses 8bits per pixel instead of 16. This isn't too bad, as Sniff generally uses a 9bit colour model(000-777). Unfortunatly this mode can also be a little slower due to the way the hardware handles memory.
Mode 5 keeps full colour but uses a lower resolution of only 160x128. Again we get the benefit of being able to keep two screens in memory, so we avoid flicker, but the low resolution looks a bit odd on screen.
Generally we like mode 4 the best, but you can run your code easily in any mode just by setting the parameter when you create the display device.
And that's all you need to know. You'll find examples of this kind of drawing in examples/GBA/window.sniff and there's GBA port of bounce out in examples/GBA/bounce.sniff
Sprites, Bitmaps, and Sound we'll save for another time.