Squashed commit of the following:

commit df2183f
Author: pokemium <bluejapan73+dev@gmail.com>
Date:   Wed Apr 7 11:36:34 2021 +0900

    feat: SRAM sav

README.md

@@ -46,7 +46,6 @@ $ ./build/darwin-amd64/mettaur XXXX.gba
 ## ToDo
 
 - [ ] Sound
-- [ ] SRAM Save
 - [ ] Window
 - [ ] Mosaic
 - [ ] Blend

cmd/main.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"os/signal"
 	"path/filepath"
+	"strings"
 	"syscall"
 
 	"github.com/hajimehoshi/ebiten/v2"
@@ -53,8 +54,9 @@ func main() {
 // Run program
 func Run() ExitCode {
 	var (
-		showVersion  = flag.Bool("v", false, "show version")
-		showCartInfo = flag.Bool("c", false, "show cartridge info")
+		showVersion   = flag.Bool("v", false, "show version")
+		showBIOSIntro = flag.Bool("b", false, "show BIOS intro")
+		showCartInfo  = flag.Bool("c", false, "show cartridge info")
 	)
 
 	flag.Parse()
@@ -72,15 +74,20 @@ func Run() ExitCode {
 
 	emu := &Emulator{
 		gba: gba.New(data),
+		rom: path,
 	}
 	if *showCartInfo {
 		fmt.Println(emu.gba.CartInfo())
 		return ExitCodeOK
 	}
 
 	emu.SetupCloseHandler()
-	emu.gba.Reset()
-	// emu.gba.SoftReset()
+	emu.loadSav()
+	if *showBIOSIntro {
+		emu.gba.Reset()
+	} else {
+		emu.gba.SoftReset()
+	}
 
 	ebiten.SetWindowResizable(true)
 	ebiten.SetWindowTitle(title)
@@ -112,6 +119,7 @@ func readROM(path string) ([]byte, error) {
 
 type Emulator struct {
 	gba *gba.GBA
+	rom string
 }
 
 func (e *Emulator) Update() error {
@@ -123,11 +131,16 @@ func (e *Emulator) Update() error {
 		}
 	}()
 	e.gba.Update()
+	if e.gba.DoSav && e.gba.Frame%60 == 0 {
+		e.writeSav()
+	}
 	return nil
 }
+
 func (e *Emulator) Draw(screen *ebiten.Image) {
 	screen.DrawImage(ebiten.NewImageFromImage(e.gba.Draw()), nil)
 }
+
 func (e *Emulator) Layout(outsideWidth, outsideHeight int) (screenWidth, screenHeight int) {
 	return 240, 160
 }
@@ -141,3 +154,18 @@ func (e *Emulator) SetupCloseHandler() {
 		os.Exit(0)
 	}()
 }
+
+func (e *Emulator) writeSav() {
+	path := strings.ReplaceAll(e.rom, ".gba", ".sav")
+	os.WriteFile(path, e.gba.RAM.SRAM[:], os.ModePerm)
+	e.gba.DoSav = false
+}
+
+func (e *Emulator) loadSav() {
+	path := strings.ReplaceAll(e.rom, ".gba", ".sav")
+	if f, err := os.Stat(path); os.IsNotExist(err) || f.IsDir() {
+		return
+	} else if sav, err := os.ReadFile(path); err == nil {
+		e.gba.LoadSav(sav)
+	}
+}

pkg/gba/gba.go

@@ -49,13 +49,14 @@ type GBA struct {
 	RAM        ram.RAM
 	inst       Inst
 	cycle      int
-	frame      uint
+	Frame      uint
 	line       int
 	halt       bool
 	pipe       Pipe
 	timers     timer.Timers
 	dma        [4]*DMA
 	joypad     joypad.Joypad
+	DoSav      bool
 }
 
 type Pipe struct {
@@ -180,11 +181,11 @@ func (g *GBA) Update() {
 	// line 227
 	g.scanline()
 
-	if g.frame%2 == 0 {
+	if g.Frame%2 == 0 {
 		g.joypad.Read()
 	}
 
-	g.frame++
+	g.Frame++
 }
 
 func (g *GBA) scanline() {
@@ -323,3 +324,12 @@ func (g *GBA) CartInfo() string {
 ROM size: %s`
 	return fmt.Sprintf(str, g.CartHeader, util.FormatSize(uint(g.RAM.ROMSize)))
 }
+
+func (g *GBA) LoadSav(bs []byte) {
+	if len(bs) > 65536 {
+		return
+	}
+	for i, b := range bs {
+		g.RAM.SRAM[i] = b
+	}
+}

pkg/gba/io.go

@@ -78,8 +78,6 @@ func (g *GBA) setRAM8(addr uint32, b byte, s bool) {
 	g._setRAM8(addr, b)
 }
 
-var ctr = 0
-
 func (g *GBA) _setRAM8(addr uint32, b byte) {
 	defer func() {
 		if err := recover(); err != nil {
@@ -132,6 +130,9 @@ func (g *GBA) _setRAM8(addr uint32, b byte) {
 		g.GPU.VRAM[ram.VRAMOffset(addr)] = b
 	case ram.OAM(addr):
 		g.GPU.OAM[ram.OAMOffset(addr)] = b
+	case ram.SRAM(addr):
+		g.RAM.Set8(addr, b)
+		g.DoSav = true
 	default:
 		g.RAM.Set8(addr, b)
 	}