Speedlock is a software protection system used on the ZX Spectrum and the Amstrad CPC, written by David Aubrey-Jones and David Looker in 1983. The two programmers had become frustrated at the slow loading times of the computer's tape loading system, and realised it would be possible to write a better one. The prototype sat unused for about a year, but was finally picked up by Ocean Software on Daley Thompson's Decathlon, released in late 1984. It was subsequently used by U.S. Gold and for several titles by Ultimate Play the Game, amongst others. Speedlock was also ported to the Amstrad CPC in 1985.
The system used several advanced features of the Spectrum's architecture, such as the memory refresh register and parity branch instructions of the Z80 processor, which made it harder to create illegitimate copies without the protection. Early versions used a set of audible "clicking" lead tones when loading a program, later versions did not include this, but instead had a counter showing the time left to finish loading the program similar to that of Technician Ted and Fairlight.
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- Lerm Tape Utility D (instruction manual) (PDF). Lerm. 1990. Retrieved 9 January 2013.
- North, Jon (1990). "How To Hack : Speedlock" (61). Your Sinclair. Retrieved 9 January 2013.
- Martin van der Heide, Martin Kopanske and Tomaz Kac (1997–1999). "Tape Decoding : Encoding schemes". World of Spectrum. Retrieved 9 January 2013.
- "CRASH review from 1985". Retrieved 2007-01-29.
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