diff --git a/nes/cassette.go b/nes/cassette.go
index f430bdb..9199aa1 100644
--- a/nes/cassette.go
+++ b/nes/cassette.go
@@ -29,15 +29,15 @@ func (m *MirroringType) IsFourScreen() bool {
 }
 
 type iNESHeader struct {
-	Magic   uint32
-	PRGSize byte
-	CHRSize byte
-	Flags6  byte // Mapper, mirroring, battery, trainer
-	Flags7  byte // Mapper, VS/Playchoice, NES 2.0
-	Flags8  byte // PRG-RAM size (rarely used extension)
-	Flags9  byte // TV system (rarely used extension)
-	Flags10 byte // TV system, PRG-RAM presence (unofficial, rarely used extension)
-	_       [5]byte
+	Magic      uint32
+	PRGROMSize byte // Size of PRG ROM in 16 KB units
+	CHRROMSize byte // Size of CHR ROM in 8 KB units (value 0 means the board uses CHR RAM)
+	Flags6     byte // Mapper, mirroring, battery, trainer
+	Flags7     byte // Mapper, VS/Playchoice, NES 2.0
+	Flags8     byte // PRG-RAM size (rarely used extension)
+	Flags9     byte // TV system (rarely used extension)
+	Flags10    byte // TV system, PRG-RAM presence (unofficial, rarely used extension)
+	_          [5]byte
 }
 
 type Cassette struct {
@@ -45,6 +45,8 @@ type Cassette struct {
 	CHR    []byte
 	Mapper byte
 	Mirror MirroringType
+
+	chrROMSize byte
 }
 
 func NewCassette(r io.Reader) (*Cassette, error) {
@@ -79,16 +81,16 @@ func NewCassette(r io.Reader) (*Cassette, error) {
 
 	mapper := (header.Flags7 & 0xF0) | (header.Flags6&0xF0)>>4
 
-	prgRom := make([]byte, int(header.PRGSize)*programROMUnit)
+	prgRom := make([]byte, int(header.PRGROMSize)*programROMUnit)
 	if _, err := io.ReadFull(r, prgRom); err != nil {
 		return nil, err
 	}
 
-	chrRom := make([]byte, int(header.CHRSize)*characterROMUnit)
+	chrRom := make([]byte, int(header.CHRROMSize)*characterROMUnit)
 	if _, err := io.ReadFull(r, chrRom); err != nil {
 		return nil, err
 	}
-	if header.CHRSize == 0 {
+	if header.CHRROMSize == 0 {
 		chrRom = make([]byte, 8192)
 	}
 
@@ -97,9 +99,22 @@ func NewCassette(r io.Reader) (*Cassette, error) {
 		CHR:    chrRom,
 		Mapper: mapper,
 		Mirror: mirroringType,
+
+		chrROMSize: header.CHRROMSize,
 	}, nil
 }
 
 func (c *Cassette) MirroingType() MirroringType {
 	return c.Mirror
 }
+
+func (c *Cassette) readCHR(index uint16) byte {
+	return c.CHR[index]
+}
+
+func (c *Cassette) writeCHR(index uint16, val byte) {
+	if c.chrROMSize == 0 {
+		// CHR RAM
+		c.CHR[index] = val
+	}
+}
diff --git a/nes/mapper0.go b/nes/mapper0.go
index 10e7dc3..2c6ac08 100644
--- a/nes/mapper0.go
+++ b/nes/mapper0.go
@@ -26,7 +26,7 @@ func (m *mapper0) Reset() {
 func (m *mapper0) Read(addr uint16) byte {
 	switch {
 	case 0x0000 <= addr && addr < 0x2000:
-		return m.CHR[addr]
+		return m.readCHR(addr)
 	case 0x6000 <= addr && addr < 0x8000:
 		return m.SRAM[addr-0x6000]
 	case 0x8000 <= addr && addr < 0xC000:
@@ -48,7 +48,7 @@ func (m *mapper0) Write(addr uint16, val byte) {
 	case 0x0000 <= addr && addr < 0x2000:
 		// https://www.nesdev.org/wiki/NROM
 		// > CHR capacity: 8 KiB ROM (DIP-28 standard pinout) but most emulators support RAM
-		m.CHR[addr] = val
+		m.writeCHR(addr, val)
 	case 0x6000 <= addr && addr < 0x8000:
 		m.SRAM[addr-0x6000] = val
 	case 0x8000 <= addr && addr <= 0xFFFF:
diff --git a/nes/mapper2.go b/nes/mapper2.go
index e268fdf..6edfd8c 100644
--- a/nes/mapper2.go
+++ b/nes/mapper2.go
@@ -25,7 +25,7 @@ func (m *mapper2) Reset() {
 func (m *mapper2) Read(addr uint16) byte {
 	switch {
 	case 0x0000 <= addr && addr < 0x2000:
-		return m.CHR[addr]
+		return m.readCHR(addr)
 	case 0x6000 <= addr && addr < 0x8000:
 		// mapper2 dont'h have PRG RAM
 		return 0
@@ -45,9 +45,9 @@ func (m *mapper2) Read(addr uint16) byte {
 func (m *mapper2) Write(addr uint16, val byte) {
 	switch {
 	case 0x0000 <= addr && addr < 0x2000:
-		m.CHR[addr] = val
+		// mapper2 don't have CHR RAM
 	case 0x6000 <= addr && addr < 0x8000:
-		// mapper2 dont'h have PRG RAM
+		// mapper2 don't have PRG RAM
 	case 0x8000 <= addr && addr <= 0xFFFF:
 		// 7  bit  0
 		// ---- ----
diff --git a/nes/mapper3.go b/nes/mapper3.go
index 9a69a06..a731b65 100644
--- a/nes/mapper3.go
+++ b/nes/mapper3.go
@@ -28,8 +28,8 @@ func (m *mapper3) Read(addr uint16) byte {
 	switch {
 	case 0x0000 <= addr && addr < 0x2000:
 		// > PPU $0000-$1FFF: 8 KB switchable CHR ROM bank
-		index := int(m.chrBank)*0x2000 + int(addr)
-		return m.CHR[index]
+		index := uint16(m.chrBank)*0x2000 + uint16(addr)
+		return m.readCHR(index)
 	case 0x6000 <= addr && addr < 0x8000:
 		// mapper 3 don't have PRG RAM
 		// but, prepare RAM for automatic testing of ppu_read_buffer