Support roms with no CHR-ROM but CHR-RAM

Author: pherrymason

src/nes/gamePak/factory.go

@@ -38,8 +38,15 @@ func CreateGamePakFromROMFile(romFilePath string) GamePak {
 	prgLength := int(inesHeader.ProgramSize())*0x4000 + 16
 	prgROM := data[16:prgLength]
 
-	chrLength := int(inesHeader.CHRSize()) * 0x2000
-	chrROM := data[prgLength : chrLength+prgLength]
+	var chrROM []byte
+	chrLength := int(inesHeader.CHRSize())
+	if chrLength == 0 {
+		chrLength = 8 * 0x2000
+		chrROM = make([]byte, chrLength)
+	} else {
+		chrLength *= 0x2000
+		chrROM = data[prgLength : chrLength+prgLength]
+	}
 	return CreateGamePak(
 		inesHeader,
 		prgROM,

src/nes/gamePak/iNes.go

@@ -68,6 +68,7 @@ func (ines INesHeader) TvSystem() byte {
 }
 
 func CreateINes1Header(prgRomSize byte, chrRomSize byte, flag6 byte, flag7 byte, flag8 byte, flag9 byte, flag10 byte) INesHeader {
+	// If the header CHR-ROM value is 0, we should assume that 8KB of CHR-RAM is available.
 	return INesHeader{
 		prgROMSize: prgRomSize,
 		chrROMSize: chrRomSize,