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. 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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