izapple2/diskette16sector.go

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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
}