targets: add support for GameBoy Advance

Only tested in an emulator (mGBA). Almost nothing is supported, but
drawing to the screen works.

src/machine/machine_gameboyadvance.go

@@ -0,0 +1,44 @@
+// +build gameboyadvance
+
+package machine
+
+import (
+	"image/color"
+	"runtime/volatile"
+	"unsafe"
+)
+
+// Make it easier to directly write to I/O RAM.
+var ioram = (*[0x400]volatile.Register8)(unsafe.Pointer(uintptr(0x04000000)))
+
+type PinMode uint8
+
+// Set has not been implemented.
+func (p Pin) Set(value bool) {
+	// do nothing
+}
+
+var Display = FramebufDisplay{(*[160][240]volatile.Register16)(unsafe.Pointer(uintptr(0x06000000)))}
+
+type FramebufDisplay struct {
+	port *[160][240]volatile.Register16
+}
+
+func (d FramebufDisplay) Configure() {
+	// Write into the I/O registers, setting video display parameters.
+	ioram[0].Set(0x03) // Use video mode 3 (in BG2, a 16bpp bitmap in VRAM)
+	ioram[1].Set(0x04) // Enable BG2 (BG0 = 1, BG1 = 2, BG2 = 4, ...)
+}
+
+func (d FramebufDisplay) Size() (x, y int16) {
+	return 240, 160
+}
+
+func (d FramebufDisplay) SetPixel(x, y int16, c color.RGBA) {
+	d.port[y][x].Set(uint16(c.R)&0x1f | uint16(c.G)&0x1f<<5 | uint16(c.B)&0x1f<<10)
+}
+
+func (d FramebufDisplay) Display() error {
+	// Nothing to do here.
+	return nil
+}