OpenVMS Calling Standard

This document, the "HP OpenVMS Calling Standard" (January 2005), defines the requirements, mechanisms, and conventions for procedure-to-procedure calls across HP OpenVMS VAX, OpenVMS Alpha, and OpenVMS Industry Standard 64 (Itanium) systems. It details run-time data structures, constants, algorithms, conventions, methods, and functional interfaces to ensure native user-mode procedures function correctly in multi-language and multi-threaded environments.

Primarily intended for compiler and debugger writers, it also provides valuable information for other programmers dealing with procedure calling.

Key areas covered include:

• System-Specific Conventions: Detailed guidelines for register usage, stack organization (memory and register stacks), calling sequences, and argument lists for VAX, Alpha, and I64 architectures.
• Procedure Representation: Defines various procedure types (stack frame, register frame, null frame) and their associated descriptors.
• Data Passing: Specifies mechanisms (immediate value, reference, descriptor), argument list structures, and data types (atomic, string, miscellaneous, and reserved) and their corresponding descriptors (fixed-length, dynamic string, array, procedure argument, etc.).
• Condition and Exception Handling: Describes condition values, how to establish and operate condition handlers, signaling conditions, and GOTO unwind operations.
• Stack Unwinding: Provides a framework for tracing backwards through the stack, including static unwind tables, coding conventions for reliable unwinding, and invocation context access routines, particularly for I64 systems.
• Interoperability: Addresses signature information and its role in interfacing between native OpenVMS Alpha/I64 code and translated VAX/Alpha images.
• Itanium Differences: Summarizes the specific changes and extensions of the OpenVMS I64 Calling Standard compared to the generic Intel Itanium Software Conventions.