mirror of
https://github.com/ivanizag/izapple2.git
synced 2024-10-31 20:09:02 +00:00
175 lines
5.0 KiB
Go
175 lines
5.0 KiB
Go
package apple2
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import (
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"os"
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)
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/*
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See:
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"Beneath Apple DOS" https://fabiensanglard.net/fd_proxy/prince_of_persia/Beneath%20Apple%20DOS.pdf
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https://github.com/TomHarte/CLK/wiki/Apple-GCR-disk-encoding
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*/
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const (
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numberOfTracks = 35
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numberOfSectors = 16
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bytesPerSector = 256
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bytesPerTrack = numberOfSectors * bytesPerSector
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nibBytesPerTrack = 6656
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nibImageSize = numberOfTracks * nibBytesPerTrack
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dskImageSize = numberOfTracks * numberOfSectors * bytesPerSector
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defaultVolumeTag = 254
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)
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type diskette16sector struct {
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track [numberOfTracks][]byte
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}
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func (d *diskette16sector) read(track int, position int) (value uint8, newPosition int) {
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value = d.track[track][position]
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newPosition = (position + 1) % nibBytesPerTrack
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return
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}
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func (d *diskette16sector) write(track int, position int, value uint8) int {
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d.track[track][position] = value
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return (position + 1) % nibBytesPerTrack
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}
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func loadDisquette(filename string) *diskette16sector {
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var d diskette16sector
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data := loadResource(filename)
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size := len(data)
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if size == nibImageSize {
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// Load file already in nib format
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for i := 0; i < numberOfTracks; i++ {
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d.track[i] = data[nibBytesPerTrack*i : nibBytesPerTrack*(i+1)]
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}
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} else if size == dskImageSize {
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// Convert to nib
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for i := 0; i < numberOfTracks; i++ {
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trackData := data[i*bytesPerTrack : (i+1)*bytesPerTrack]
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d.track[i] = nibEncodeTrack(trackData, defaultVolumeTag, byte(i))
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}
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} else {
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panic("Invalid disk size")
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}
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return &d
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}
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func (d *diskette16sector) saveNib(filename string) {
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f, err := os.Create(filename)
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if err != nil {
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panic(err)
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}
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defer f.Close()
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for _, v := range d.track {
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_, err := f.Write(v)
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if err != nil {
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panic(err)
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}
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}
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}
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var dos33SectorsLogicOrder = [16]int{
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0x0, 0x7, 0xE, 0x6, 0xD, 0x5, 0xC, 0x4,
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0xB, 0x3, 0xA, 0x2, 0x9, 0x1, 0x8, 0xF,
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}
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var sixAndTwoTranslateTable = [0x40]byte{
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0x96, 0x97, 0x9a, 0x9b, 0x9d, 0x9e, 0x9f, 0xa6,
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0xa7, 0xab, 0xac, 0xad, 0xae, 0xaf, 0xb2, 0xb3,
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0xb4, 0xb5, 0xb6, 0xb7, 0xb9, 0xba, 0xbb, 0xbc,
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0xbd, 0xbe, 0xbf, 0xcb, 0xcd, 0xce, 0xcf, 0xd3,
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0xd6, 0xd7, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde,
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0xdf, 0xe5, 0xe6, 0xe7, 0xe9, 0xea, 0xeb, 0xec,
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0xed, 0xee, 0xef, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6,
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0xf7, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff,
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}
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const (
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gap1Len = 48
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gap2Len = 5
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primaryBufferSize = bytesPerSector
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secondaryBufferSize = bytesPerSector/3 + 1
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)
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func oddEvenEncodeByte(b byte) []byte {
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/*
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A byte is encoded in two bytes to make sure the bytes start with 1 and
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does not have two consecutive zeros.
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Data byte: D7-D6-D5-D4-D3-D2-D1-D0
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resutl[0]: 1-D7- 1-D5- 1-D3-1 -D1
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resutl[1]: 1-D6- 1-D4- 1-D2-1 -D0
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*/
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e := make([]byte, 2)
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e[0] = ((b >> 1) & 0x55) | 0xaa
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e[1] = (b & 0x55) | 0xaa
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return e
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}
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func nibEncodeTrack(data []byte, volume byte, track byte) []byte {
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b := make([]byte, 0, nibBytesPerTrack) // Buffer slice with enough capacity
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// Initialize gaps to be copied for each sector
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gap1 := make([]byte, gap1Len)
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for i := range gap1 {
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gap1[i] = 0xff
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}
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gap2 := make([]byte, gap2Len)
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for i := range gap2 {
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gap2[i] = 0xff
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}
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for physicalSector := byte(0); physicalSector < numberOfSectors; physicalSector++ {
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/* On the DSK file the sectors are in DOS3.3 logical order
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but on the physical encoded track as well as in the nib
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files they are in phisical order.
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*/
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logicalSector := dos33SectorsLogicOrder[physicalSector]
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sectorData := data[logicalSector*bytesPerSector : (logicalSector+1)*bytesPerSector]
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// 6and2 prenibbilizing.
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primaryBuffer := make([]byte, primaryBufferSize)
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secondaryBuffer := make([]byte, secondaryBufferSize)
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for i, v := range sectorData {
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// Primary buffer is easy: the 6 MSB
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primaryBuffer[i] = v >> 2
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// Secondary buffer: the 2 LSB reversed, shifted and in their place
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shift := uint((i / secondaryBufferSize) * 2)
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bit0 := ((v & 0x01) << 1) << shift
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bit1 := ((v & 0x02) >> 1) << shift
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position := i % secondaryBufferSize
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secondaryBuffer[position] |= bit0 | bit1
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}
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// Render sector
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// Address field
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b = append(b, gap1...)
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b = append(b, 0xd5, 0xaa, 0x96) // Address prolog
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b = append(b, oddEvenEncodeByte(volume)...) // 4-4 encoded volume
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b = append(b, oddEvenEncodeByte(track)...) // 4-4 encoded track
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b = append(b, oddEvenEncodeByte(physicalSector)...) // 4-4 encoded sector
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b = append(b, oddEvenEncodeByte(volume^track^physicalSector)...) // Checksum
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b = append(b, 0xde, 0xaa, 0xeb) // Epilog
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// Data field
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b = append(b, gap2...)
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b = append(b, 0xd5, 0xaa, 0xad) // Data prolog
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prevV := byte(0)
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for _, v := range secondaryBuffer {
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b = append(b, sixAndTwoTranslateTable[v^prevV])
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prevV = v
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}
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for _, v := range primaryBuffer {
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b = append(b, sixAndTwoTranslateTable[v^prevV])
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prevV = v
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}
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b = append(b, sixAndTwoTranslateTable[prevV]) // Checksum
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b = append(b, 0xde, 0xaa, 0xeb) // Data epilog
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}
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return b
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}
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