Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The Plessey 250 was a computer system manufactured by the Plessey company in the United Kingdom. It was successfully deployed by the Ministry of Defence for the British Army Ptarmigan project and served in the first Gulf War as a tactical mobile communication switch. It was only a moderate commercial success for the public telecommunication industry. System 250 is notable historically for using a pure hardware based, capability-based architecture and addressing. The PP-250 is one central processing unit (CPU) in a multiprocessor System-250. To protect the shared system memory capabilities, it provides two independent but related checks on access to any memory: first, location of the memory, and second, the permission level of any command. Location relates to the object: its geography in memory. Command relates to the user privileges: the Privacy and Security levels of use. Two keys to the same object can give different levels of access rights, so one gets Read access and another Write access.