3 files changed, 160 insertions, 0 deletions
diff --git a/cart/cart.go b/cart/cart.go
new file mode 100644
index 0000000..2438c1b
--- /dev/null
+++ b/cart/cart.go
@@ -0,0 +1,83 @@
+package cart
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"os"
+)
+
+type Cartridge struct {
+	PRG       []uint8
+	CHR       []uint8
+	CHRIsRAM  bool
+	MapperID  uint8
+	Mirroring Mirroring
+	Battery   bool
+
+	Mapper Mapper
+}
+
+func Load(path string) (*Cartridge, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	return Parse(data)
+}
+
+func Parse(data []byte) (*Cartridge, error) {
+	if len(data) < 16 || !bytes.Equal(data[:4], []byte("NES\x1A")) {
+		return nil, errors.New("cart: not an iNES file")
+	}
+
+	prgSize := int(data[4]) * 16 * 1024
+	chrSize := int(data[5]) * 8 * 1024
+	flags6, flags7 := data[6], data[7]
+
+	isNES2 := flags7&0x0C == 0x08
+	mapperID := flags6 >> 4
+	if isNES2 || bytes.Equal(data[12:16], []byte{0, 0, 0, 0}) {
+		mapperID |= flags7 & 0xF0
+	}
+
+	c := &Cartridge{
+		MapperID: mapperID,
+		Battery:  flags6&0x02 != 0,
+	}
+
+	switch {
+	case flags6&0x08 != 0:
+		c.Mirroring = FourScreen
+	case flags6&0x01 != 0:
+		c.Mirroring = Vertical
+	default:
+		c.Mirroring = Horizontal
+	}
+
+	offset := 16
+	if flags6&0x04 != 0 {
+		offset += 512
+	}
+	if len(data) < offset+prgSize+chrSize {
+		return nil, fmt.Errorf("cart: file truncated: need %d bytes, have %d",
+			offset+prgSize+chrSize, len(data))
+	}
+
+	c.PRG = data[offset : offset+prgSize]
+	offset += prgSize
+
+	if chrSize == 0 {
+		c.CHR = make([]uint8, 8*1024)
+		c.CHRIsRAM = true
+	} else {
+		c.CHR = data[offset : offset+chrSize]
+	}
+
+	m, err := newMapper(c)
+	if err != nil {
+		return nil, err
+	}
+	c.Mapper = m
+	return c, nil
+}
diff --git a/cart/mapper.go b/cart/mapper.go
new file mode 100644
index 0000000..7ddd8a9
--- /dev/null
+++ b/cart/mapper.go
@@ -0,0 +1,30 @@
+package cart
+
+import "fmt"
+
+type Mirroring uint8
+
+const (
+	Horizontal Mirroring = iota
+	Vertical
+	FourScreen
+	SingleScreenLower
+	SingleScreenUpper
+)
+
+type Mapper interface {
+	CPURead(addr uint16) uint8
+	CPUWrite(addr uint16, v uint8)
+	PPURead(addr uint16) uint8
+	PPUWrite(addr uint16, v uint8)
+	Mirroring() Mirroring
+}
+
+func newMapper(c *Cartridge) (Mapper, error) {
+	switch c.MapperID {
+	case 0:
+		return newNROM(c), nil
+	default:
+		return nil, fmt.Errorf("cart: unsupported mapper %d", c.MapperID)
+	}
+}
diff --git a/cart/nrom.go b/cart/nrom.go
new file mode 100644
index 0000000..3828c16
--- /dev/null
+++ b/cart/nrom.go
@@ -0,0 +1,47 @@
+package cart
+
+type nrom struct {
+	prg       []uint8
+	chr       []uint8
+	chrIsRAM  bool
+	prgRAM    [8 * 1024]uint8
+	mirroring Mirroring
+}
+
+func newNROM(c *Cartridge) *nrom {
+	return &nrom{
+		prg:       c.PRG,
+		chr:       c.CHR,
+		chrIsRAM:  c.CHRIsRAM,
+		mirroring: c.Mirroring,
+	}
+}
+
+func (m *nrom) CPURead(addr uint16) uint8 {
+	switch {
+	case addr >= 0x8000:
+		return m.prg[int(addr-0x8000)%len(m.prg)]
+	case addr >= 0x6000:
+		return m.prgRAM[addr-0x6000]
+	default:
+		return 0
+	}
+}
+
+func (m *nrom) CPUWrite(addr uint16, v uint8) {
+	if addr >= 0x6000 && addr < 0x8000 {
+		m.prgRAM[addr-0x6000] = v
+	}
+}
+
+func (m *nrom) PPURead(addr uint16) uint8 {
+	return m.chr[int(addr)%len(m.chr)]
+}
+
+func (m *nrom) PPUWrite(addr uint16, v uint8) {
+	if m.chrIsRAM {
+		m.chr[int(addr)%len(m.chr)] = v
+	}
+}
+
+func (m *nrom) Mirroring() Mirroring { return m.mirroring }