Disk.cc

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#include "Disk.hh"
#include "DiskExceptions.hh"
#include "one_of.hh"
 
using std::string;
 
namespace openmsx {
 
Disk::Disk(DiskName name_)
        : name(std::move(name_)), nbSides(0)
{
}
 
void Disk::writeTrack(byte track, byte side, const RawTrack& input)
{
        if (isWriteProtected()) {
                throw WriteProtectedException();
        }
        writeTrackImpl(track, side, input);
        flushCaches();
}
 
bool Disk::isDoubleSided()
{
        if (!nbSides) {
                detectGeometry();
        }
        return nbSides == 2;
}
 
// Note: Special case to convert between logical/physical sector numbers
//       for the boot sector and the 1st FAT sector (logical sector: 0/1,
//       physical location: track 0, side 0, sector 1/2): perform this
//       conversion without relying on the detected geometry parameters.
//       Otherwise the detectGeometry() method (which itself reads these
//       two sectors) would get in an infinite loop.
size_t Disk::physToLog(byte track, byte side, byte sector)
{
        if ((track == 0) && (side == 0)) {
                return sector - 1;
        }
        if (!nbSides) {
                detectGeometry();
        }
        return sectorsPerTrack * (side + nbSides * track) + (sector - 1);
}
Disk::TSS Disk::logToPhys(size_t log)
{
        if (log <= 1) {
                byte track = 0;
                byte side = 0;
                byte sector = byte(log + 1);
                return {track, side, sector};
        }
        if (!nbSides) {
                detectGeometry();
        }
        byte track  = byte(log / (nbSides * sectorsPerTrack)); // TODO check for overflow
        byte side   = byte((log / sectorsPerTrack) % nbSides);
        byte sector = byte((log % sectorsPerTrack) + 1);
        return {track, side, sector};
}
 
unsigned Disk::getSectorsPerTrack()
{
        if (!nbSides) {
                detectGeometry();
        }
        return sectorsPerTrack;
}
 
void Disk::detectGeometryFallback() // if all else fails, use statistics
{
        // TODO maybe also check for 8*80 for 8 sectors per track
        sectorsPerTrack = 9; // most of the time (sorry 5.25" disk users...)
        // 360k disks are likely to be single sided:
        nbSides = (getNbSectors() == 720) ? 1 : 2;
}
 
void Disk::detectGeometry()
{
        // From the MSX Red Book (p265):
        //
        //  How to determine media types
        //
        //  a) Read the boot sector (track 0, sector 1) of the target drive
        //
        //  b) Check if the first byte is either 0E9H or 0EBH (the JMP
        //     instruction on 8086)
        //
        //  c) If step b) fails, the disk is a version prior to MS-DOS 2.0;
        //     therefore, use the first byte of the FAT passed from the caller
        //     and make sure it is between 0F8h and 0FFh.
        //
        //     If step c) is successful, use this as a media descriptor.
        //     If step c) fails, then this disk cannot be read.
        //
        //  d) If step b) succeeds, read bytes # 0B to # 1D. This is the
        //     DPB for MS-DOS, Version 2.0 and above. The DPB for MSX-DOS can
        //     be obtained as follows.
        //
        //     ....
        //     +18 +19  Sectors per track
        //     +1A +1B  Number of heads
        //     ...
 
        //
        //     Media Descriptor  0F8H  0F9H  0FAh  0FBH  0FCH  0FDH  0FEH  0FFH
        //       byte (FAT-ID)
        //     Sectors/track        9     9     8     8     9     9     8     8
        //     No. of sides         1     2     1     2     1     2     1     2
        //     Tracks/side         80    80    80    80    40    40    40    40
        //     ...
 
        try {
                SectorBuffer buf;
                readSector(0, buf); // bootsector
                if (buf.raw[0] == one_of(0xE9, 0xEB)) {
                        // use values from bootsector
                        sectorsPerTrack = buf.bootSector.sectorsTrack;
                        nbSides         = buf.bootSector.nrSides;
                        if ((sectorsPerTrack == 0) || (sectorsPerTrack > 255) ||
                            (nbSides         == 0) || (nbSides         > 255)) {
                                // seems like bogus values, use defaults
                                detectGeometryFallback();
                        }
                } else {
                        readSector(1, buf); // 1st fat sector
                        byte mediaDescriptor = buf.raw[0];
                        if (mediaDescriptor >= 0xF8) {
                                sectorsPerTrack = (mediaDescriptor & 2) ? 8 : 9;
                                nbSides         = (mediaDescriptor & 1) ? 2 : 1;
                        } else {
                                // invalid media descriptor, just assume it's a
                                // normal DS or SS DD disk
                                detectGeometryFallback();
                        }
                }
        } catch (MSXException&) {
                // read error, assume it's a 3.5" DS or SS DD disk
                detectGeometryFallback();
        }
}
 
} // namespace openmsx