Digital UNIX Calling Standard for Alpha Systems

This manual defines the requirements, mechanisms, and conventions for procedure calls within the Digital UNIX operating system on Alpha systems. Its primary purpose is to enable native user-mode procedures to operate correctly in multilanguage and multithreaded environments. The document is primarily aimed at compiler and debugger writers, but also serves all programmers working with procedure calling on this platform.

Key aspects covered include:

• Introduction and Concepts: Provides fundamental definitions, outlines the architectural level, and details the goals for performance, portability, efficiency, and consistency of the calling standard. It also introduces the concepts of address and procedure representation, and register usage conventions specific to Alpha hardware.
• Flow Control: Describes various procedure types (stack frame, register frame, null frame), how control is transferred between procedures (call conventions, linkage, link-time optimization), and the specific entry and exit code sequences, including management of the Global Offset Table (GOT).
• Data Manipulation: Explains the mechanisms for passing and storing data, including argument passing (by immediate value, reference, or descriptor), argument list structures, data allocation, and alignment rules to ensure optimal performance.
• Event Processing: Details the unified exception handling model, covering different kinds of exceptions (general, unwind, signal), how handlers are established and invoked, and mechanisms for continuation or unwinding. It also addresses stack limit checking and overflow handling within multithreaded execution environments.
• Procedure Descriptors: Defines the data structures (code range descriptors and run-time procedure descriptors) that provide critical information about procedures, such as saved registers, prologue length, and exception handling specifics, which are essential for call-chain navigation and robust exception processing, including for run-time generated code.