targets: add support for GameBoy Advance

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

src/examples/gba-display/gba-display.go

@@ -0,0 +1,21 @@
+package main
+
+// Draw a red square on the GameBoy Advance screen.
+
+import (
+	"image/color"
+	"machine"
+)
+
+var display = machine.Display
+
+func main() {
+	display.Configure()
+
+	for x := int16(30); x < 50; x++ {
+		for y := int16(80); y < 100; y++ {
+			display.SetPixel(x, y, color.RGBA{255, 0, 0, 255})
+		}
+	}
+	display.Display()
+}

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
+}

src/runtime/runtime_arm7tdmi.go

@@ -0,0 +1,101 @@
+// +build arm7tdmi
+
+package runtime
+
+import (
+	"device/arm"
+	"unsafe"
+)
+
+const TargetBits = 32
+
+const GOARCH = "arm"
+
+type timeUnit int64
+
+const tickMicros = 1
+
+func putchar(c byte) {
+	// dummy, TODO
+}
+
+//go:extern _sbss
+var _sbss unsafe.Pointer
+
+//go:extern _ebss
+var _ebss unsafe.Pointer
+
+//go:extern _sdata
+var _sdata unsafe.Pointer
+
+//go:extern _sidata
+var _sidata unsafe.Pointer
+
+//go:extern _edata
+var _edata unsafe.Pointer
+
+// Entry point for Go. Initialize all packages and call main.main().
+//go:export main
+func main() {
+	// Initialize .data and .bss sections.
+	preinit()
+
+	// Run initializers of all packages.
+	initAll()
+
+	// Compiler-generated call to main.main().
+	callMain()
+}
+
+func preinit() {
+	// Initialize .bss: zero-initialized global variables.
+	ptr := unsafe.Pointer(&_sbss)
+	for ptr != unsafe.Pointer(&_ebss) {
+		*(*uint32)(ptr) = 0
+		ptr = unsafe.Pointer(uintptr(ptr) + 4)
+	}
+
+	// Initialize .data: global variables initialized from flash.
+	src := unsafe.Pointer(&_sidata)
+	dst := unsafe.Pointer(&_sdata)
+	for dst != unsafe.Pointer(&_edata) {
+		*(*uint32)(dst) = *(*uint32)(src)
+		dst = unsafe.Pointer(uintptr(dst) + 4)
+		src = unsafe.Pointer(uintptr(src) + 4)
+	}
+}
+
+func ticks() timeUnit {
+	// TODO
+	return 0
+}
+
+const asyncScheduler = false
+
+func sleepTicks(d timeUnit) {
+	// TODO
+}
+
+func abort() {
+	// TODO
+	for {
+	}
+}
+
+func getCurrentStackPointer() uintptr {
+	return arm.ReadRegister("sp")
+}
+
+// Implement memset for LLVM and compiler-rt.
+//go:export memset
+func libc_memset(ptr unsafe.Pointer, c byte, size uintptr) {
+	for i := uintptr(0); i < size; i++ {
+		*(*byte)(unsafe.Pointer(uintptr(ptr) + i)) = c
+	}
+}
+
+// Implement memmove for LLVM and compiler-rt.
+//go:export memmove
+func libc_memmove(dst, src unsafe.Pointer, size uintptr) {
+	memmove(dst, src, size)
+}