This manual defines the requirements, mechanisms, and conventions used in the Tru64 UNIX interface that supports procedure calls for Tru64 UNIX for Alpha systems. The standard defines the data structures, constants, algorithms, conventions, methods, and functional interfaces that enable a native user-mode procedure to operate correctly in a multilanguage and multithreaded environment on Tru64 UNIX systems operating on Alpha hardware.

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Alpha is a 64-bit load/ store RISC architecture that is designed with particular emphasis on the 
three elements that most affect performance: clock speed, multiple instruction issue, and multi-ple 
processors.

Read more: http://www.alphalinux.org/docs/alphaahb.html