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Statements consisting only of original research should be removed. Unsourced material may be challenged and removed. A keygen is a handmade product serial number generator that often offers the ability to generate working serial numbers in your own name. A patch is a small computer program that modifies the machine code of another program.
This has the advantage for a cracker to not include a large executable in a release when only a few bytes are changed. A loader modifies the startup flow of a program and does not remove the protection but circumvents it. The distribution of cracked copies is illegal in most countries. There have been lawsuits over cracking software. It might be legal to use cracked software in certain circumstances.
Software publishers have implemented increasingly complex methods in an effort to stop unauthorized copying of software. In addition, tracks did not need to be perfect rings, but could be sectioned so that sectors could be staggered across overlapping offset tracks, the most extreme version being known as spiral tracking. It was also discovered that many floppy drives did not have a fixed upper limit to head movement, and it was sometimes possible to write an additional 36th track above the normal 35 tracks. The standard Apple II copy programs could not read such protected floppy disks, since the standard DOS assumed that all disks had a uniform 35-track, 13- or 16-sector layout. Apple II copy program could read. One of the primary routes to hacking these early copy protections was to run a program that simulates the normal CPU operation. The Apple II provided a built-in opcode disassembler, allowing raw memory to be decoded into CPU opcodes, and this would be utilized to examine what the copy-protection was about to do next.
Generally there was little to no defense available to the copy protection system, since all its secrets are made visible through the simulation. However, because the simulation itself must run on the original CPU, in addition to the software being hacked, the simulation would often run extremely slowly even at maximum speed. On Atari 8-bit computers, the most common protection method was via “bad sectors”. These were sectors on the disk that were intentionally unreadable by the disk drive. The software would look for these sectors when the program was loading and would stop loading if an error code was not returned when accessing these sectors. Special copy programs were available that would copy the disk and remember any bad sectors.
The user could then use an application to spin the drive by constantly reading a single sector and display the drive RPM. With the disk drive top removed a small screwdriver could be used to slow the drive RPM below a certain point. Once the drive was slowed down the application could then go and write “bad sectors” where needed. When done the drive RPM was sped up back to normal and an uncracked copy was made. Of course cracking the software to expect good sectors made for readily copied disks without the need to meddle with the disk drive. As time went on more sophisticated methods were developed, but almost all involved some form of malformed disk data, such as a sector that might return different data on separate accesses due to bad data alignment.
BIOS in Atari’s “smart” drives. These upgraded drives allowed the user to make exact copies of the original program with copy protections in place on the new disk. On the Commodore 64, several methods were used to protect software. Because of the operation of Commodore floppy drives, one write protection scheme would cause the floppy drive head to bang against the end of its rail, which could cause the drive head to become misaligned. In some cases, cracked versions of software were desirable to avoid this result.