RawTrack.cc

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#include "RawTrack.hh"
#include "CRC16.hh"
#include "enumerate.hh"
#include "one_of.hh"
#include "ranges.hh"
#include "serialize.hh"
#include "serialize_stl.hh"
#include "xrange.hh"
#include <cassert>
 
namespace openmsx {
 
RawTrack::RawTrack(unsigned size)
{
        clear(size);
}
 
void RawTrack::clear(unsigned size)
{
        idam.clear();
        data.assign(size, 0x4e);
}
 
void RawTrack::addIdam(unsigned idx)
{
        assert(idx < data.size());
        assert(idam.empty() || (idx > idam.back()));
        idam.push_back(idx);
}
 
void RawTrack::write(int idx, uint8_t val, bool setIdam)
{
        unsigned i2 = wrapIndex(idx);
        auto it = ranges::lower_bound(idam, i2);
        if (setIdam) {
                // add idam (if not already present)
                if ((it == end(idam)) || (*it != i2)) {
                        idam.insert(it, i2);
                }
        } else {
                // remove idam (if present)
                if ((it != end(idam)) && (*it == i2)) {
                        idam.erase(it);
                }
        }
        data[i2] = val;
}
 
std::optional<RawTrack::Sector> RawTrack::decodeSectorImpl(int idx) const
{
        // read (and check) address mark
        // assume addr mark starts with three A1 bytes (should be
        // located right before the current 'idx' position)
        if (read(idx) != 0xFE) return {};
        ++idx;
 
        Sector sector;
        sector.addrIdx = idx;
        CRC16 addrCrc;
        addrCrc.init({0xA1, 0xA1, 0xA1, 0xFE});
        updateCrc(addrCrc, sector.addrIdx, 4);
        sector.track    = read(idx++);
        sector.head     = read(idx++);
        sector.sector   = read(idx++);
        sector.sizeCode = read(idx++);
        uint8_t addrCrc1 = read(idx++);
        uint8_t addrCrc2 = read(idx++);
        sector.addrCrcErr = (256 * addrCrc1 + addrCrc2) != addrCrc.getValue();
 
        sector.dataIdx = -1; // (for now) no data block found
        sector.deleted = false;   // dummy
        sector.dataCrcErr = true; // dummy
 
        if (!sector.addrCrcErr) {
                // Locate data mark, should starts within 43 bytes from current
                // position (that's what the WD2793 does).
                for (auto i : xrange(43)) {
                        int idx2 = idx + i;
                        int j = 0;
                        for (; j < 3; ++j) {
                                if (read(idx2 + j) != 0xA1) break;
                        }
                        if (j != 3) continue; // didn't find 3 x 0xA1
 
                        uint8_t type = read(idx2 + 3);
                        if (type != one_of(0xfb, 0xf8)) continue;
 
                        CRC16 dataCrc;
                        dataCrc.init({0xA1, 0xA1, 0xA1});
                        dataCrc.update(type);
 
                        // OK, found start of data, calculate CRC.
                        int dataIdx = idx2 + 4;
                        int sectorSize = 128 << (sector.sizeCode & 7);
                        updateCrc(dataCrc, dataIdx, sectorSize);
                        uint8_t dataCrc1 = read(dataIdx + sectorSize + 0);
                        uint8_t dataCrc2 = read(dataIdx + sectorSize + 1);
                        bool dataCrcErr = (256 * dataCrc1 + dataCrc2) != dataCrc.getValue();
 
                        // store result
                        sector.dataIdx    = dataIdx;
                        sector.deleted    = type == 0xf8;
                        sector.dataCrcErr = dataCrcErr;
                        break;
                }
        }
        return sector;
}
 
std::vector<RawTrack::Sector> RawTrack::decodeAll() const
{
        // only complete sectors (header + data block)
        std::vector<Sector> result;
        for (const auto& i : idam) {
                if (auto sector = decodeSectorImpl(narrow<int>(i));
                    sector && (sector->dataIdx != -1)) {
                        result.push_back(*sector);
                }
        }
        return result;
}
 
static std::vector<unsigned> rotateIdam(std::vector<unsigned> idam, unsigned startIdx)
{
        // find first element that is equal or bigger
        auto it = ranges::lower_bound(idam, startIdx);
        // rotate range so that we start at that element
        std::rotate(begin(idam), it, end(idam));
        return idam;
}
 
std::optional<RawTrack::Sector> RawTrack::decodeNextSector(unsigned startIdx) const
{
        // get first valid sector-header
        for (auto& i : rotateIdam(idam, startIdx)) {
                if (auto sector = decodeSectorImpl(narrow<int>(i))) {
                        return sector;
                }
        }
        return {};
}
 
std::optional<RawTrack::Sector> RawTrack::decodeSector(uint8_t sectorNum) const
{
        // only complete sectors (header + data block)
        for (const auto& i : idam) {
                if (auto sector = decodeSectorImpl(narrow<int>(i));
                    sector &&
                    (sector->sector == sectorNum) &&
                    (sector->dataIdx != -1)) {
                        return sector;
                }
        }
        return {};
}
 
void RawTrack::readBlock(int idx, std::span<uint8_t> destination) const
{
        for (auto [i, d] : enumerate(destination)) {
                d = read(idx + int(i));
        }
}
void RawTrack::writeBlock(int idx, std::span<const uint8_t> source)
{
        for (auto [i, s] : enumerate(source)) {
                write(idx + int(i), s);
        }
}
 
void RawTrack::updateCrc(CRC16& crc, int idx, int size) const
{
        unsigned start = wrapIndex(idx);
        unsigned end = start + size;
        if (end <= data.size()) {
                crc.update(subspan(data, start, size));
        } else {
                unsigned part = unsigned(data.size()) - start;
                crc.update(subspan(data, start, part));
                crc.update(subspan(data, 0, size - part));
        }
}
 
uint16_t RawTrack::calcCrc(int idx, int size) const
{
        CRC16 crc;
        updateCrc(crc, idx, size);
        return crc.getValue();
}
 
void RawTrack::applyWd2793ReadTrackQuirk()
{
        // Tests on a real WD2793 have shown that the first 'A1' byte in a
        // sequence of 3 'special' A1 bytes gets systematically read as a
        // different value by the read-track command.
        //
        // Very roughly in my tests I got:
        //  - about 1/2 of the times the value 0x14 is returned
        //  - about 1/3 of the times the value 0xC2 is returned
        //  - about 1/6 of the times a different value is returned (I've seen
        //    00, 02, F8, FC)
        // When re-reading the same track again, the values 14 and C2 remain,
        // but often the values 00 or 02 change to 14, and F8 or FC change to
        // C2. I've never seen the 'real' value 'A1'. Note again that this
        // transformation only applies to the 1st of the three A1 bytes, the
        // 2nd and 3rd are correctly read as A1.
        //
        // I've no good explanation for why this is happening, except for the
        // following observation which might be the start of a partial
        // explanation, or it might just be a coincidence:
        //
        // The 'special' value A1 is encoded as: 0x4489, or in binary
        //     01 00 01 00 10 00 10 01
        // If you only look at the even bits (the data bits) you get the value
        // A1 back. The odd bits (the clock bits) are generated by the MFM
        // encoding rules and ensure that there are never 2 consecutive 1-bits
        // and never more than 3 consecutive 0-bits. Though this is a 'special'
        // sequence and the clock bit of the 6th bit (counting from left to
        // right starting at 1) is 0, it should be 1 according to the MFM
        // rules. The normal encoding for A1 is 0x44A9. An FDC uses this
        // special bit-pattern to synchronize the bitstream on, e.g. to know
        // where the byte-boundaries are located in the stream.
        //
        // Now if you shift the pattern 0x4489 1 bit to the left, you get
        // 0x8912. If you then decode the data bits you get the value 0x14. And
        // this _might_ be a partial explanation for why read-track often
        // returns 0x14 instead of 0xA1.
        //
        // The implementation below always changes the first 0xA1 byte to 0x14.
        // It does not also return the value 0xC2, because I've no clue yet
        // when which value should be returned.
        //
        // The motivation for this (crude) implementation is the following:
        // There's a cracked disk version of Nemesis that comes with a
        // copy-protection. The disk has a track which contains sector-headers
        // with CRC errors. The software verifies the protection via the
        // read-track command. In the resulting data block it scans for the
        // first A1 byte and then sums the next 8 bytes. Initially openMSX
        // returned A1 'too early', and then the sum of the 8 following bytes
        // doesn't match. But if the first A1 is turned into something
        // (anything) else, so the sum starts one byte later, it does work
        // fine.
 
        for (unsigned ii : idam) {
                auto i = narrow<int>(ii);
                if ((read(i - 3) != 0xA1) ||
                    (read(i - 2) != 0xA1) ||
                    (read(i - 1) != 0xA1) ||
                    (read(i - 0) != 0xFE)) continue;
                write(i - 3, 0x14);
 
                if (auto sector = decodeSectorImpl(i);
                    sector && (sector->dataIdx != -1)) {
                        int d = sector->dataIdx;
                        if ((read(d - 4) != 0xA1) ||
                            (read(d - 3) != 0xA1) ||
                            (read(d - 2) != 0xA1)) continue;
                        write(d - 4, 0x14);
                }
        }
}
 
// version 1: initial version (fixed track length of 6250)
// version 2: variable track length
template<typename Archive>
void RawTrack::serialize(Archive& ar, unsigned version)
{
        ar.serialize("idam", idam);
        auto len = unsigned(data.size());
        if (ar.versionAtLeast(version, 2)) {
                ar.serialize("trackLength", len);
        } else {
                assert(Archive::IS_LOADER);
                len = 6250;
        }
        if constexpr (Archive::IS_LOADER) {
                data.resize(len);
        }
        ar.serialize_blob("data", data);
}
INSTANTIATE_SERIALIZE_METHODS(RawTrack);
 
template<typename Archive>
void RawTrack::Sector::serialize(Archive& ar, unsigned /*version*/)
{
        ar.serialize("addrIdx",    addrIdx,
                     "dataIdx",    dataIdx,
                     "track",      track,
                     "head",       head,
                     "sector",     sector,
                     "sizeCode",   sizeCode,
                     "deleted",    deleted,
                     "addrCrcErr", addrCrcErr,
                     "dataCrcErr", dataCrcErr);
}
INSTANTIATE_SERIALIZE_METHODS(RawTrack::Sector);
 
} // namespace openmsx