fluxengine/arch/ibm/encoder.cc

#include "globals.h"
#include "record.h"
#include "decoders/decoders.h"
#include "encoders/encoders.h"
#include "ibm.h"
#include "crc.h"
#include "sectorset.h"
#include "writer.h"
#include "fmt/format.h"
#include <ctype.h>

/* IAM record separator:
 * 0xC2 is:
 * data:    1  1  0  0  0  0  1  0  = 0xc2
 * mfm:     01 01 00 10 10 10 01 00 = 0x5254
 * special: 01 01 00 10 00 10 01 00 = 0x5224
 */
#define MFM_IAM_SEPARATOR 0x5224

/* FM IAM record:
 * flux:   XXXX-XXX-XXXX-X- = 0xf77a
 * clock:  X X - X - X X X  = 0xd7
 * data:    X X X X X X - - = 0xfc
 */
#define FM_IAM_RECORD 0xf77a

/* MFM IAM record:
 * data:   1  1  1  1  1  1  0  0  = 0xfc
 * flux:   01 01 01 01 01 01 00 10 = 0x5552
 */
#define MFM_IAM_RECORD 0x5552

/* MFM record separator:
 * 0xA1 is:
 * data:    1  0  1  0  0  0  0  1  = 0xa1
 * mfm:     01 00 01 00 10 10 10 01 = 0x44a9
 * special: 01 00 01 00 10 00 10 01 = 0x4489
 *                       ^^^^^
 * When shifted out of phase, the special 0xa1 byte becomes an illegal
 * encoding (you can't do 10 00). So this can't be spoofed by user data.
 * 
 * shifted: 10 00 10 01 00 01 00 1
 * 
 * It's repeated three times.
 */
#define MFM_RECORD_SEPARATOR 0x4489
#define MFM_RECORD_SEPARATOR_BYTE 0xa1

/* MFM IDAM byte:
 * data:    1  1  1  1  1  1  1  0  = 0xfe
 * mfm:     01 01 01 01 01 01 01 00 = 0x5554
 */

/* MFM DAM byte:
 * data:    1  1  1  1  1  0  1  1  = 0xfb
 * mfm:     01 01 01 01 01 00 01 01 = 0x5545
 */

static int charToInt(char c)
{
	if (isdigit(c))
		return c - '0';
	return 10 + tolower(c) - 'a';
}

void IbmEncoder::writeRawBits(uint32_t data, int width)
{
	_cursor += width;
	_lastBit = data & 1;
	for (int i=0; i<width; i++)
	{
		unsigned pos = _cursor - i - 1;
		if (pos < _bits.size())
			_bits[pos] = data & 1;
		data >>= 1;
	}
}

void IbmEncoder::writeBytes(const Bytes& bytes)
{
	if (_parameters.useFm)
		encodeFm(_bits, _cursor, bytes);
	else
		encodeMfm(_bits, _cursor, bytes, _lastBit);
}

void IbmEncoder::writeBytes(int count, uint8_t byte)
{
	Bytes bytes = { byte };
	for (int i=0; i<count; i++)
		writeBytes(bytes);
}

static uint8_t decodeUint16(uint16_t raw)
{
	Bytes b;
	ByteWriter bw(b);
	bw.write_be16(raw);
	return decodeFmMfm(b.toBits())[0];
}

std::unique_ptr<Fluxmap> IbmEncoder::encode(
	int physicalTrack, int physicalSide, const SectorSet& allSectors)
{
	double clockRateUs = 1e3 / _parameters.clockRateKhz;
	if (!_parameters.useFm)
		clockRateUs /= 2.0;
	int bitsPerRevolution = (_parameters.trackLengthMs * 1000.0) / clockRateUs;
	_bits.resize(bitsPerRevolution);
	_cursor = 0;

	uint8_t idamUnencoded = decodeUint16(_parameters.idamByte);
	uint8_t damUnencoded = decodeUint16(_parameters.damByte);

	uint8_t sectorSize = 0;
	{
		int s = _parameters.sectorSize >> 7;
		while (s > 1)
		{
			s >>= 1;
			sectorSize += 1;
		}
	}

	uint8_t gapFill = _parameters.useFm ? 0x00 : 0x4e;

	writeBytes(_parameters.gap0, gapFill);
	if (_parameters.emitIam)
	{
		writeBytes(_parameters.useFm ? 6 : 12, 0x00);
		if (!_parameters.useFm)
		{
			for (int i=0; i<3; i++)
				writeRawBits(MFM_IAM_SEPARATOR, 16);
		}
		writeRawBits(_parameters.useFm ? FM_IAM_RECORD : MFM_IAM_RECORD, 16);
		writeBytes(_parameters.gap1, gapFill);
	}

	bool first = true;
	for (char sectorChar : _parameters.sectorSkew)
	{
		int sectorId = charToInt(sectorChar);
		if (!first)
			writeBytes(_parameters.gap3, gapFill);
		first = false;

		const auto& sectorData = allSectors.get(physicalTrack, physicalSide, sectorId);
		if (!sectorData)
			Error() << fmt::format("format tried to find sector {} which wasn't in the input file", sectorId);

		/* Writing the sector and data records are fantastically annoying.
		 * The CRC is calculated from the *very start* of the record, and
		 * include the malformed marker bytes. Our encoder doesn't know
		 * about this, of course, with the result that we have to construct
		 * the unencoded header, calculate the checksum, and then use the
		 * same logic to emit the bytes which require special encoding
		 * before encoding the rest of the header normally. */

		{
			Bytes header;
			ByteWriter bw(header);

			writeBytes(_parameters.useFm ? 6 : 12, 0x00);
			if (!_parameters.useFm)
			{
				for (int i=0; i<3; i++)
					bw.write_8(MFM_RECORD_SEPARATOR_BYTE);
			}
			bw.write_8(idamUnencoded);
			bw.write_8(sectorData->logicalTrack);
			bw.write_8(sectorData->logicalSide);
			bw.write_8(sectorData->logicalSector + _parameters.startSectorId);
			bw.write_8(sectorSize);
			uint16_t crc = crc16(CCITT_POLY, header);
			bw.write_be16(crc);

			int conventionalHeaderStart = 0;
			if (!_parameters.useFm)
			{
				for (int i=0; i<3; i++)
					writeRawBits(MFM_RECORD_SEPARATOR, 16);
				conventionalHeaderStart += 3;

			}
			writeRawBits(_parameters.idamByte, 16);
			conventionalHeaderStart += 1;

			writeBytes(header.slice(conventionalHeaderStart));
		}

		writeBytes(_parameters.gap2, gapFill);

		{
			Bytes data;
			ByteWriter bw(data);

			writeBytes(_parameters.useFm ? 6 : 12, 0x00);
			if (!_parameters.useFm)
			{
				for (int i=0; i<3; i++)
					bw.write_8(MFM_RECORD_SEPARATOR_BYTE);
			}
			bw.write_8(damUnencoded);

			Bytes truncatedData = sectorData->data.slice(0, _parameters.sectorSize);
			bw += truncatedData;
			hexdump(std::cout, data.slice(0, 64));
			uint16_t crc = crc16(CCITT_POLY, data);
			bw.write_be16(crc);

			int conventionalHeaderStart = 0;
			if (!_parameters.useFm)
			{
				for (int i=0; i<3; i++)
					writeRawBits(MFM_RECORD_SEPARATOR, 16);
				conventionalHeaderStart += 3;

			}
			writeRawBits(_parameters.damByte, 16);
			conventionalHeaderStart += 1;

			writeBytes(data.slice(conventionalHeaderStart));
		}
    }

	if (_cursor >= _bits.size())
		Error() << "track data overrun";
	while (_cursor < _bits.size())
		writeBytes(1, gapFill);

	std::unique_ptr<Fluxmap> fluxmap(new Fluxmap);
	fluxmap->appendBits(_bits, clockRateUs*1e3);
	return fluxmap;
}