Subsequent read sector commands will read the later sectors.įound mostly in Taito games. In this scheme, there are multiple sectors with identical Sector IDs, but different data. Sierra used this for the versions of their games distributed under Tandy's label prior to 1985. Oh No! More Lemmings (comes in disk-based and document-based versions)
Encrypts the actual executable and stores a 32-bit decryption number on a bad sector.įrank Bruno's Boxing (Compilation version) Uses weak bits and calculates the entropy expected. version by LucasFilm Games used a Codewheel) This protection will probably fail at 386 speeds (16MHz and above) Protection is found on tracks 38-39, side 0. There is an additional sector ID field in one of the sectors. Wizard and the Princess, The (IBM PCjr version)Īccolade and Epyx used this type of protection. Sierra Championship Boxing (1984 version) Hardball (Disk 2/EGA Disk, early version only) This protection will probably fail at speeds greater than an AT (8MHz 80286) The sector ID is marked as having 256 bytes, but actually has 512 bytes, fooling copiers like TeleDisk. They also use 200KB and 400KB disk sizes with 10 sectors per track.Ĭontains a sector with errors, usually at at sector 20, side 0, sector 5. Used by Electronic Arts booters, involves 96 interleaved sector IDs, typically at track 15, side 0.
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Unfortunately, if the disk was damaged unintentionally or the flux transitions have lost their force with age, a flux copier like an Option Board,Deluxe Option Board, a Kryoflux or a Super Card Pro may misinterpret this and make a bad copy.
If the same results were always read back, the game would fail to work, thinking it was being run on a copy.
The game would not expect the same result to be returned from multiple reads of a sector. In this case, the game would read an area on the disk where the flux transitions have not been written with sufficient magnetic force to make the transitions able to be read reliably. Of course, high density disk controllers were far from ubiquitous in the 1980s when most of these games were released.Ī devious method used weak bits. This protection can often be fooled by writing the image to a high density disk, which allows for 15 or 18 sectors per track. The disk can play with the sector ID sizes to make no sense physically, like having sector 1 with an ID of 8192 bytes and then fitting the other sectors "within" sector 1.
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The PC disk controller's read track command would read the Gap bytes.Ī method used the Sync bytes to let sectors overlap. The program could instead read data from the Gap bytes, like an encryption key. If the original disk was written with 16 of each and the copy only has 15, that is a way to detect the copy. This error signals to the disk controller that this is a Sync byte.Īnother method is to check that the exact number of Gap and Sync Bytes are on the disk. The resulting fifth bit is not as it should be, instead it appears as 100010010 001001, with the fifth bit being different. However, to put the A1 out of sync, the following pattern is produced by the clocking bits : 0 0 0 1 0 1 0. A1 should be encoded as data bits 100010010101001 and clocking bits 0 0 0 1 1 1 0 and is encoded that way in a Sector Data field (there is no reason for it to appear in a Sector ID field). The A1 in the Sync bytes does not follow the appropriate clock and data bit pattern. However, with this limitation is more liberal than how the bits are actually represented in MFM. In MFM encoding, there will never be more than three 0 bits between a 1 bit. But there is one time when they intentionally are not, the Sync bytes. Normally the two are in sync, meaning that the clock bits correspond exactly to the MFM data bits. There are two data bits for every clock bit. Both are present on the Write Data and Read Data lines.
There is a relationship between the MFM clock bits and the MFM data bits. The data line of a floppy disk does not operate in a vacuum.
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