The Transcopy v5 image format
-----------------------------

by NewRisingSun, as of 2017-05-31. All information subject to change as I discover errors in my description or encounter a disk image that violates this description.

0x00	byte[2]		Magic number: 0x5A 0xA5
0x02	char[32]	First comment line, zero-terminated
0x22	char[32]	Second comment line, zero-terminated
0x42			Unused, filled with random memory data
  :
0x100	byte		Disk type
			0xFF	Unknown
			0x02	MFM High Density
			0x03	MFM Double Density in 360 rpm drive
			0x04	Apple II GCR
			0x05	FM Single Density
			0x06	Commodore GCR
			0x07	MFM Double Density
			0x08	Commodore Amiga
			0x0C	Atari FM
0x101	byte		Starting cylinder number
0x102	byte		Ending cylinder number
0x103	byte		Number of sides
0x104	byte		Cylinder increment between tracks

0x105	word[]		Track skew, one little-endian entry for each track, cylinder*2 entries; 0x1111 for empty entries.
			Single-sided disks have empty entries for the unused side.
			"Track skew" means how many clocks after the index signal the track data starts.
			The values are normalized such that a value of 40,000 represents one rotation of the disk.

0x305	word[]		Start of track data SHR 8 in the file, one big-endian entry for each track, cylinder*side entries, otherwise 0x0000.
			The "SHR 8" part implies that a track always starts on a 256 byte boundary.
			
0x505	word[]		Size of track data in bytes, one little-endian entry for each track, cylinder*2 entries; 0x3333 for empty entries.
			Single-sided disks have empty entries for the unused side.
			The bytes between the end of the track data, as specified in this field, and the 256-byte-aligned start of the next
			track data are filled with the data from the beginning of a second rotation of the disk. This applies only to files generated
			by original Central Point Software; the PCE utilities (PFI/PRI/PSI) just pad with 0x00 bytes, which may prevent
			"Keep Track Length" from working correctly when writing back these images with TC.EXE.
			
0x705	word[]		Track flags, one little-endian entry for each track, cylinder*2 entries, otherwise 0x4444.
			The values for the very first track are the values displayed by TC.EXE for the entire disk.
			
			Bit 0 (value 0x01):	1=Keep Track Length. 
			Bit 1 (value 0x02):	1=Copy Across Index. As I understand it, when not set, the "track skew" field is ignored when writing back the image to disk.
			Bit 2 (value 0x04):	1=Copy Weak Bits
			Bit 3 (value 0x08):	1=Verify Write
			Bit 4 (value 0x10):	? (never seen it set)
			Bit 5 (value 0x20):	1=??? Seems to be set whenever a sector crosses the index signal.
			                        If set, the "length tolerance" is changed from the default "8" to just "1".
			Bit 6 (value 0x40):	1=??? If set, value 0x20 is automatically cleared.
			Bit 7 (value 0x80):	1=No address marks encountered
			Bit 8-14:		Track type, same as Disk type (see description at offset 0x100)
			Bit 15 (value 0x8000)	1=??? Usually set if bit 7 is set.
			
			I am not sure what exact behavior the various bits other than "copy weak bits" elicit in TC.EXE. The Option Board manual states that
			enabling "Copy Weak Bits" will turn off "Verify Write".
			
0x905	word[]		Address mark timing. Needed for copy protection schemes that measure the time between Read ID commands results. Same value range
			as Track skew.
			
			The first entry is the time from the index until the first address mark and INCLUDES the track skew (from offset 0x105).
			This is followed by entries specifying the time from address mark to address mark. The last entry contains the remaining time so that
			the sum of all entries MINUS the track skew results in a value around 40,000.
			
			Sixteen little-endian entries for each track for Double Density disks. If a track has more address marks than that (for example, because
			of copy protection), only the first sixteen are used. If a track has fewer than 15 address marks, the remaining words are set to zero.
			
			Since I have never seen a .TC image made from a 3.5 HD disk, I don't know whether that means that only 15 of the 18 sectors have timing info,
			or if the number of entries is increased from 16 to 32 for HD disks.
			
			"Address marks" include Index Address Marks ("IAM", C2 C2 C2 FC) and Sector ID Address Marks ("IDAM", A1 A1 A1 FE), but NOT Data Address Marks
			("DAM", A1 A1 A1 FB).
			
0x2904			Unused, filled with random memory data
  :
0x4000			Start of first track data. Raw MFM bit cell data, with bit order 0x80 0x40 0x20... byte-for-byte.