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Bit nibbler
A bit nibbler, or nibbler, is a computer software program designed to copy data from a floppy disk one bit at a time. It functions at a very low level directly interacting with the disk drive hardware to override a copy protection scheme that the floppy disk's data may be stored in. In most cases the nibbler software still analyses the data on a byte level, only looking to the bit level when dealing with synchronization marks (syncs), zero-gaps and other sector & track headers. When possible, nibblers will work with the low-level data encoding format used by the disk system, being ''Group Code Recording'' (GCR - Apple, Commodore), ''Frequency Modulation'' (FM - Atari), or ''Modified Frequency Modulation'' (MFM - Amiga, Atari, IBM PC).
==Overview==
Software piracy began to be a problem when floppy disks became the common storage media.〔Copy Protection: A History and Outlook http://www.studio-nibble.com/countlegger/01/HistoryOfCopyProtection.html〕 The ease of copying depended on the system; Jerry Pournelle wrote in ''BYTE'' in 1983 that ''"CP/M doesn't lend itself to copy protection"'' so its users ''"haven't been too worried"'' about it, while ''"Apple users, though, have always had the problem. So have those who used TRS-DOS, and I understand that MS-DOS has copy protection features"''. Apple and Commodore 64 copy protection schemes were extremely varied and creative because most of the floppy disk reading and writing was controlled by software (or firmware), not by hardware.
Pournelle disliked copy protection and, except for games, refused to review software that used it. He did not believe that it was useful, writing ''"For every copy protection scheme there's a hacker ready to defeat it. Most involve so-called nybble copiers, which try to analyze the original disk and then make a copy"''. By 1980, the first 'nibble' copier, Locksmith, was introduced for the Apple II. These copiers reproduced copy protected floppy disks an entire track at a time, ignoring how the sectors were marked. This was harder to do than it sounds for two reasons: firstly, Apple disks did not use the index hole to mark the start of a track; their drives could not even detect the index hole. Tracks could thus start anywhere, but the copied track had to have this "write splice", which always caused some bits to be lost or duplicated due to speed variations, roughly in the same (unused for payload data) place as the original, or it would not work. Secondly, Apple used special "self-sync" bytes to achieve agreement between drive controller and computer about where any byte ended and the next one started on the disk. These bytes were written as normal data bytes followed by a slightly longer than normal pause, which was notoriously unreliable to detect on read-back; still, you had to get the self-sync bytes roughly right as without them being present in the right places, the copy would not work, and with them present in too many places, the track would not fit on the destination disk.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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