targets: add support for GameBoy Advance

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

Makefile

@@ -124,6 +124,7 @@ smoketest:
 	tinygo build -size short -o test.elf -target=stm32f4disco        examples/blinky1
 	tinygo build -size short -o test.elf -target=stm32f4disco        examples/blinky2
 	tinygo build -size short -o test.elf -target=circuitplay-express examples/i2s
+	tinygo build -size short -o test.elf -target=gameboy-advance     examples/gba-display
 ifneq ($(AVR), 0)
 	tinygo build -size short -o test.elf -target=arduino             examples/blinky1
 	tinygo build -size short -o test.elf -target=digispark           examples/blinky1

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)
+}

targets/gameboy-advance.json

@@ -0,0 +1,29 @@
+{
+	"llvm-target": "thumb4-none-eabi",
+	"cpu": "arm7tdmi",
+	"build-tags": ["gameboyadvance", "arm7tdmi", "baremetal", "linux", "arm"],
+	"goos": "linux",
+	"goarch": "arm",
+	"compiler": "clang",
+	"linker": "ld.lld",
+	"cflags": [
+		"-g",
+		"--target=thumb4-none-eabi",
+		"-mcpu=arm7tdmi",
+		"-Oz",
+		"-Werror",
+		"-fshort-enums",
+		"-Wno-macro-redefined",
+		"-Qunused-arguments",
+		"-fno-exceptions", "-fno-unwind-tables",
+		"-ffunction-sections", "-fdata-sections"
+	],
+	"ldflags": [
+		"--gc-sections",
+		"-Ttargets/gameboy-advance.ld"
+	],
+	"extra-files": [
+		"targets/gameboy-advance.s"
+	],
+	"emulator": ["mgba-qt"]
+}

targets/gameboy-advance.ld

@@ -0,0 +1,72 @@
+OUTPUT_ARCH(arm)
+ENTRY(_start)
+
+MEMORY {
+    ewram   : ORIGIN = 0x02000000, LENGTH = 256K /* on-board work RAM (2 wait states) */
+    iwram   : ORIGIN = 0x03000000, LENGTH = 32K  /* in-chip work RAM (faster) */
+    rom     : ORIGIN = 0x08000000, LENGTH = 32M  /* flash ROM */
+}
+
+__iwram_top = ORIGIN(iwram) + LENGTH(iwram);;
+_stack_size = 3K;
+__sp_irq    = _stack_top;
+__sp_usr    = _stack_top - 1K;
+
+SECTIONS
+{
+    .text :
+    {
+        KEEP (*(.init))
+        *(.text)
+        . = ALIGN(4);
+    } >rom
+
+    /* Put the stack at the bottom of RAM, so that the application will
+     * crash on stack overflow instead of silently corrupting memory.
+     * See: http://blog.japaric.io/stack-overflow-protection/ */
+    .stack :
+    {
+        . = ALIGN(4);
+        . += _stack_size;
+        _stack_top = .;
+    } >iwram
+
+    /* Start address (in flash) of .data, used by startup code. */
+    _sidata = LOADADDR(.data);
+
+    /* Globals with initial value */
+    .data :
+    {
+        . = ALIGN(4);
+        _sdata = .;        /* used by startup code */
+        *(.data)
+        *(.data*)
+        *(.iwram .iwram.*)
+        . = ALIGN(4);
+        _edata = .;        /* used by startup code */
+    } >iwram AT>rom
+
+    /* Zero-initialized globals  */
+    .bss :
+    {
+        . = ALIGN(4);
+        _sbss = .;         /* used by startup code */
+        *(.bss)
+        *(.bss*)
+        *(COMMON)
+        . = ALIGN(4);
+        _ebss = .;         /* used by startup code */
+    } >iwram
+
+    /DISCARD/ :
+    {
+        *(.ARM.exidx)      /* causes 'no memory region specified' error in lld */
+        *(.ARM.exidx.*)    /* causes spurious 'undefined reference' errors */
+    }
+}
+
+/* For the memory allocator. */
+_heap_start = ORIGIN(ewram);
+_heap_end = ORIGIN(ewram) + LENGTH(ewram);
+_globals_start = _sdata;
+_globals_end = _ebss;

targets/gameboy-advance.s

@@ -0,0 +1,33 @@
+.section    .init
+.global     _start
+.align
+.arm
+
+_start:
+    b       start_vector
+    .fill   156,1,0                // Nintendo Logo Character Data (8000004h)
+    .fill   16,1,0                 // Game Title
+    .byte   0x30,0x31              // Maker Code (80000B0h)
+    .byte   0x96                   // Fixed Value (80000B2h)
+    .byte   0x00                   // Main Unit Code (80000B3h)
+    .byte   0x00                   // Device Type (80000B4h)
+    .fill   7,1,0                  // unused
+    .byte   0x00                   // Software Version No (80000BCh)
+    .byte   0xf0                   // Complement Check (80000BDh)
+    .byte   0x00,0x00              // Checksum (80000BEh)
+
+start_vector:
+    mov     r0, #0x4000000                 // REG_BASE
+    str     r0, [r0, #0x208]
+
+    mov     r0, #0x12                      // Switch to IRQ Mode
+    msr     cpsr, r0
+    ldr     sp, =__sp_irq                  // Set IRQ stack
+    mov     r0, #0x1f                      // Switch to System Mode
+    msr     cpsr, r0
+    ldr     sp, =__sp_usr                  // Set user stack
+
+    // Jump to user code (switching to Thumb mode)
+    ldr     r3, =main
+    bx      r3
+