package apple2 import ( "os" ) /* See: "Beneath Apple DOS" https://fabiensanglard.net/fd_proxy/prince_of_persia/Beneath%20Apple%20DOS.pdf https://github.com/TomHarte/CLK/wiki/Apple-GCR-disk-encoding */ const ( numberOfTracks = 35 numberOfSectors = 16 bytesPerSector = 256 bytesPerTrack = numberOfSectors * bytesPerSector nibBytesPerTrack = 6656 nibImageSize = numberOfTracks * nibBytesPerTrack dskImageSize = numberOfTracks * numberOfSectors * bytesPerSector defaultVolumeTag = 254 ) type diskette16sector struct { track [numberOfTracks][]byte } func (d *diskette16sector) read(track int, position int) (value uint8, newPosition int) { value = d.track[track][position] newPosition = (position + 1) % nibBytesPerTrack return } func (d *diskette16sector) write(track int, position int, value uint8) int { d.track[track][position] = value return (position + 1) % nibBytesPerTrack } func loadDisquette(filename string) *diskette16sector { var d diskette16sector data := loadResource(filename) size := len(data) if size == nibImageSize { // Load file already in nib format for i := 0; i < numberOfTracks; i++ { d.track[i] = data[nibBytesPerTrack*i : nibBytesPerTrack*(i+1)] } } else if size == dskImageSize { // Convert to nib for i := 0; i < numberOfTracks; i++ { trackData := data[i*bytesPerTrack : (i+1)*bytesPerTrack] d.track[i] = nibEncodeTrack(trackData, defaultVolumeTag, byte(i)) } } else { panic("Invalid disk size") } return &d } func (d *diskette16sector) saveNib(filename string) { f, err := os.Create(filename) if err != nil { panic(err) } defer f.Close() for _, v := range d.track { _, err := f.Write(v) if err != nil { panic(err) } } } var dos33SectorsLogicOrder = [16]int{ 0x0, 0x7, 0xE, 0x6, 0xD, 0x5, 0xC, 0x4, 0xB, 0x3, 0xA, 0x2, 0x9, 0x1, 0x8, 0xF, } var sixAndTwoTranslateTable = [0x40]byte{ 0x96, 0x97, 0x9a, 0x9b, 0x9d, 0x9e, 0x9f, 0xa6, 0xa7, 0xab, 0xac, 0xad, 0xae, 0xaf, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7, 0xb9, 0xba, 0xbb, 0xbc, 0xbd, 0xbe, 0xbf, 0xcb, 0xcd, 0xce, 0xcf, 0xd3, 0xd6, 0xd7, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde, 0xdf, 0xe5, 0xe6, 0xe7, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff, } const ( gap1Len = 48 gap2Len = 5 primaryBufferSize = bytesPerSector secondaryBufferSize = bytesPerSector/3 + 1 ) func oddEvenEncodeByte(b byte) []byte { /* A byte is encoded in two bytes to make sure the bytes start with 1 and does not have two consecutive zeros. Data byte: D7-D6-D5-D4-D3-D2-D1-D0 resutl[0]: 1-D7- 1-D5- 1-D3-1 -D1 resutl[1]: 1-D6- 1-D4- 1-D2-1 -D0 */ e := make([]byte, 2) e[0] = ((b >> 1) & 0x55) | 0xaa e[1] = (b & 0x55) | 0xaa return e } func nibEncodeTrack(data []byte, volume byte, track byte) []byte { b := make([]byte, 0, nibBytesPerTrack) // Buffer slice with enough capacity // Initialize gaps to be copied for each sector gap1 := make([]byte, gap1Len) for i := range gap1 { gap1[i] = 0xff } gap2 := make([]byte, gap2Len) for i := range gap2 { gap2[i] = 0xff } for physicalSector := byte(0); physicalSector < numberOfSectors; physicalSector++ { /* On the DSK file the sectors are in DOS3.3 logical order but on the physical encoded track as well as in the nib files they are in phisical order. */ logicalSector := dos33SectorsLogicOrder[physicalSector] sectorData := data[logicalSector*bytesPerSector : (logicalSector+1)*bytesPerSector] // 6and2 prenibbilizing. primaryBuffer := make([]byte, primaryBufferSize) secondaryBuffer := make([]byte, secondaryBufferSize) for i, v := range sectorData { // Primary buffer is easy: the 6 MSB primaryBuffer[i] = v >> 2 // Secondary buffer: the 2 LSB reversed, shifted and in their place shift := uint((i / secondaryBufferSize) * 2) bit0 := ((v & 0x01) << 1) << shift bit1 := ((v & 0x02) >> 1) << shift position := i % secondaryBufferSize secondaryBuffer[position] |= bit0 | bit1 } // Render sector // Address field b = append(b, gap1...) b = append(b, 0xd5, 0xaa, 0x96) // Address prolog b = append(b, oddEvenEncodeByte(volume)...) // 4-4 encoded volume b = append(b, oddEvenEncodeByte(track)...) // 4-4 encoded track b = append(b, oddEvenEncodeByte(physicalSector)...) // 4-4 encoded sector b = append(b, oddEvenEncodeByte(volume^track^physicalSector)...) // Checksum b = append(b, 0xde, 0xaa, 0xeb) // Epilog // Data field b = append(b, gap2...) b = append(b, 0xd5, 0xaa, 0xad) // Data prolog prevV := byte(0) for _, v := range secondaryBuffer { b = append(b, sixAndTwoTranslateTable[v^prevV]) prevV = v } for _, v := range primaryBuffer { b = append(b, sixAndTwoTranslateTable[v^prevV]) prevV = v } b = append(b, sixAndTwoTranslateTable[prevV]) // Checksum b = append(b, 0xde, 0xaa, 0xeb) // Data epilog } return b }