SectorBasedDisk.cc

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#include "SectorBasedDisk.hh"
#include "MSXException.hh"
#include "xrange.hh"
#include <cassert>
 
namespace openmsx {
 
SectorBasedDisk::SectorBasedDisk(DiskName name_)
        : Disk(std::move(name_))
        , nbSectors(size_t(-1)) // to detect misuse
        , cachedTrackNum(-1)
{
}
 
void SectorBasedDisk::writeTrackImpl(byte track, byte side, const RawTrack& input)
{
        for (auto& s : input.decodeAll()) {
                // Ignore 'track' and 'head' information
                // Always assume sectorsize = 512 (so also ignore sizeCode).
                // Ignore CRC value/errors of both address and data.
                // Ignore sector type (deleted or not)
                // Ignore sectors that are outside the range 1..sectorsPerTrack
                if ((s.sector < 1) || (s.sector > getSectorsPerTrack())) continue;
                SectorBuffer buf;
                input.readBlock(s.dataIdx, buf.raw);
                auto logicalSector = physToLog(track, side, s.sector);
                writeSector(logicalSector, buf);
                // it's important to use writeSector() and not writeSectorImpl()
                // because only the former flushes SHA1 cache
        }
}
 
void SectorBasedDisk::readTrack(byte track, byte side, RawTrack& output)
{
        // Try to cache the last result of this method (the cache will be
        // flushed on any write to the disk). This very simple cache mechanism
        // will typically already have a very high hit-rate. For example during
        // emulation of a WD2793 read sector, we also emulate the search for
        // the correct sector. So the disk rotates from sector to sector, and
        // each time we re-read the track data (because emutime has passed).
        // Typically the software will also read several sectors from the same
        // track before moving to the next.
        checkCaches();
        int num = track | (side << 8);
        if (num == cachedTrackNum) {
                output = cachedTrackData;
                return;
        }
        cachedTrackNum = num;
 
        // This disk image only stores the actual sector data, not all the
        // extra gap, sync and header information that is in reality stored
        // in between the sectors. This function transforms the cooked sector
        // data back into raw track data. It assumes a standard IBM double
        // density, 9 sectors/track, 512 bytes/sector track layout.
        //
        // -- track --
        // gap4a         80 x 0x4e
        // sync          12 x 0x00
        // index mark     3 x 0xc2(*)
        //                1 x 0xfc
        // gap1          50 x 0x4e
        // 9 x [sector]   9 x [[658]]
        // gap4b        182 x 0x4e
        //
        // -- sector --
        // sync          12 x 0x00
        // ID addr mark   3 x 0xa1(*)
        //                1 x 0xfe
        // C H R N        4 x [..]
        // CRC            2 x [..]
        // gap2          22 x 0x4e
        // sync          12 x 0x00
        // data mark      3 x 0xa1(*)
        //                1 x 0xfb
        // data         512 x [..]    <-- actual sector data
        // CRC            2 x [..]
        // gap3          84 x 0x4e
        //
        // (*) Missing clock transitions in MFM encoding
 
        try {
                output.clear(RawTrack::STANDARD_SIZE); // clear idam positions
 
                unsigned idx = 0;
                auto write = [&](unsigned n, byte value) {
                        repeat(n, [&] { output.write(idx++, value); });
                };
 
                write(80, 0x4E); // gap4a
                write(12, 0x00); // sync
                write( 3, 0xC2); // index mark (1)
                write( 1, 0xFC); //            (2)
                write(50, 0x4E); // gap1
 
                for (auto j : xrange(9)) {
                        write(12, 0x00); // sync
 
                        write( 3, 0xA1);                 // addr mark (1)
                        output.write(idx++, 0xFE, true); //           (2) add idam
                        output.write(idx++, track); // C: Cylinder number
                        output.write(idx++, side);  // H: Head Address
                        output.write(idx++, j + 1); // R: Record
                        output.write(idx++, 0x02);  // N: Number (length of sector: 512 = 128 << 2)
                        word addrCrc = output.calcCrc(idx - 8, 8);
                        output.write(idx++, addrCrc >> 8);   // CRC (high byte)
                        output.write(idx++, addrCrc & 0xff); //     (low  byte)
 
                        write(22, 0x4E); // gap2
                        write(12, 0x00); // sync
 
                        write( 3, 0xA1); // data mark (1)
                        write( 1, 0xFB); //           (2)
 
                        auto logicalSector = physToLog(track, side, j + 1);
                        SectorBuffer buf;
                        readSector(logicalSector, buf);
                        for (auto& r : buf.raw) output.write(idx++, r);
 
                        word dataCrc = output.calcCrc(idx - (512 + 4), 512 + 4);
                        output.write(idx++, dataCrc >> 8);   // CRC (high byte)
                        output.write(idx++, dataCrc & 0xff); //     (low  byte)
 
                        write(84, 0x4E); // gap3
                }
 
                write(182, 0x4E); // gap4b
                assert(idx == RawTrack::STANDARD_SIZE);
        } catch (MSXException& /*e*/) {
                // There was an error while reading the actual sector data.
                // Most likely this is because we're reading the 81th track on
                // a disk with only 80 tracks (or similar). If you do this on a
                // real disk, you simply read an 'empty' track. So we do the
                // same here.
                output.clear(RawTrack::STANDARD_SIZE);
                cachedTrackNum = -1; // needed?
        }
        cachedTrackData = output;
}
 
void SectorBasedDisk::flushCaches()
{
        Disk::flushCaches();
        cachedTrackNum = -1;
}
 
size_t SectorBasedDisk::getNbSectorsImpl() const
{
        assert(nbSectors != size_t(-1)); // must have been initialized
        return nbSectors;
}
void SectorBasedDisk::setNbSectors(size_t num)
{
        assert(nbSectors == size_t(-1)); // can only set this once
        nbSectors = num;
}
 
void SectorBasedDisk::detectGeometry()
{
        // the following are just heuristics...
 
        if (getNbSectors() == 1440) {
                // explicitly check for 720kb filesize
 
                // "trojka.dsk" is 720kb, but has bootsector and FAT media ID
                // for a single sided disk. From an emulator point of view it
                // must be accessed as a double sided disk.
 
                // "SDSNAT2.DSK" has invalid media ID in both FAT and
                // bootsector, other data in the bootsector is invalid as well.
                // Altough the first byte of the bootsector is 0xE9 to indicate
                // valid bootsector data. The only way to detect the format is
                // to look at the diskimage filesize.
 
                setSectorsPerTrack(9);
                setNbSides(2);
 
        } else {
                // Don't check for "360kb -> single sided disk". The MSXMania
                // disks are double sided disk but are truncated at 360kb.
                Disk::detectGeometry();
        }
}
 
} // namespace openmsx