101 lines
3.9 KiB
Java
101 lines
3.9 KiB
Java
package com.bytezone.diskbrowser.nib;
|
|
|
|
// -----------------------------------------------------------------------------------//
|
|
class DiskReaderGCR extends DiskReader
|
|
// -----------------------------------------------------------------------------------//
|
|
{
|
|
static final int TAG_SIZE = 12;
|
|
private final ByteTranslator byteTranslator = new ByteTranslator6and2 ();
|
|
|
|
// ---------------------------------------------------------------------------------//
|
|
DiskReaderGCR ()
|
|
// ---------------------------------------------------------------------------------//
|
|
{
|
|
super (0);
|
|
}
|
|
|
|
// ---------------------------------------------------------------------------------//
|
|
@Override
|
|
byte[] decodeSector (byte[] inBuffer, int inPtr) throws DiskNibbleException
|
|
// ---------------------------------------------------------------------------------//
|
|
{
|
|
byte[] outBuffer = new byte[BLOCK_SIZE + TAG_SIZE]; // 524 bytes
|
|
int outPtr = 0;
|
|
int[] checksums = new int[3];
|
|
|
|
// decode four disk bytes into three data bytes (174 * 3 + 2 = 524)
|
|
while (true)
|
|
{
|
|
// ROL checksum (also keep left-shifted hi bit)
|
|
checksums[2] = (checksums[2] & 0xFF) << 1; // shift left
|
|
if ((checksums[2] > 0xFF)) // check for overflow
|
|
++checksums[2]; // set bit 0
|
|
|
|
// 6&2 translation
|
|
byte d3 = byteTranslator.decode (inBuffer[inPtr++]); // composite byte
|
|
byte d0 = byteTranslator.decode (inBuffer[inPtr++]);
|
|
byte d1 = byteTranslator.decode (inBuffer[inPtr++]);
|
|
|
|
// reassemble data bytes
|
|
byte b0 = (byte) (d0 | ((d3 << 2) & 0xC0));
|
|
byte b1 = (byte) (d1 | ((d3 << 4) & 0xC0));
|
|
|
|
// calculate running checksums
|
|
outBuffer[outPtr++] = checksum (b0, checksums, 0);
|
|
outBuffer[outPtr++] = checksum (b1, checksums, 1);
|
|
|
|
if (outPtr == outBuffer.length)
|
|
break;
|
|
|
|
byte d2 = byteTranslator.decode (inBuffer[inPtr++]); // translate
|
|
byte b2 = (byte) (d2 | (d3 << 6)); // reassemble
|
|
outBuffer[outPtr++] = checksum (b2, checksums, 2); // checksum
|
|
}
|
|
|
|
// decode four disk bytes into three checksum bytes
|
|
byte d3 = byteTranslator.decode (inBuffer[inPtr++]); // composite byte
|
|
byte d0 = byteTranslator.decode (inBuffer[inPtr++]);
|
|
byte d1 = byteTranslator.decode (inBuffer[inPtr++]);
|
|
byte d2 = byteTranslator.decode (inBuffer[inPtr++]);
|
|
|
|
// reassemble checksums
|
|
byte b0 = (byte) (d0 | ((d3 << 2) & 0xC0));
|
|
byte b1 = (byte) (d1 | ((d3 << 4) & 0xC0));
|
|
byte b2 = (byte) (d2 | (d3 << 6));
|
|
|
|
// compare disk checksums with calculated checksums
|
|
if ((byte) (checksums[0] & 0xFF) != b0 //
|
|
|| (byte) (checksums[1] & 0xFF) != b1 //
|
|
|| (byte) (checksums[2] & 0xFF) != b2)
|
|
throw new DiskNibbleException ("Checksum failed");
|
|
|
|
return outBuffer;
|
|
}
|
|
|
|
// ---------------------------------------------------------------------------------//
|
|
private byte checksum (byte diskByte, int[] checksums, int current)
|
|
// ---------------------------------------------------------------------------------//
|
|
{
|
|
int prev = (current + 2) % 3;
|
|
int val = (diskByte ^ checksums[prev]) & 0xFF;
|
|
checksums[current] += val; // add to this checksum
|
|
|
|
if (checksums[prev] > 0xFF) // was there a carry last time?
|
|
{
|
|
++checksums[current]; // add it to this checksum
|
|
checksums[prev] &= 0xFF; // reset previous carry
|
|
}
|
|
|
|
return (byte) val; // converted data byte
|
|
}
|
|
|
|
// ---------------------------------------------------------------------------------//
|
|
@Override
|
|
byte[] encodeSector (byte[] buffer)
|
|
// ---------------------------------------------------------------------------------//
|
|
{
|
|
System.out.println ("encodeSector() not written");
|
|
return null;
|
|
}
|
|
}
|