1.3 Goals

In general, this calling standard promotes the highest degree of performance, portability, efficiency, and consistency in the interface between called procedures in the Windows NT for Alpha Systems environment.

The standard must be applicable to all intermodule callable interfaces in the native software system. Specifically, the standard must consider the requirements of important compiled languages including Ada, BASIC, BLISS, C, C++, COBOL, FORTRAN, Pascal, LISP, PL/I, and calls to the operating system and library procedures. The needs of other languages that may be supported in the future must be met by the standard or by compatible revision to it.

It is the goal of this Calling Standard to do the following:

• Support nearly perfect compatibility with the user visible characteristics of the Windows NT for Alpha Systems operating system as implemented on other hardware architectures.
  
  
• Avoid capabilities specifically for lower-level components (such as assembler routines) that cannot be invoked from the higher-level languages.
  
  
• Allow the calling program and called procedure to be written in different languages. The standard is designed to reduce the need to use language extensions for mixed-language programs.
  
  
• Contribute to the writing of error-free, modular, and maintainable software and promote effective sharing and reuse of software modules.
  
  
• Provide the programmer with control over the fixing and reporting of exception conditions and with management of the flow of control when various types of exceptional conditions occur.
  
  
• Provide subsystem and application writers with the ability to override system messages to provide a more suitable application-oriented interface.
  
  
• Add no space or time overhead to procedure calls and returns that do not establish exception handlers.
  
  
• Be optimized for newer, more complex compilation techniques, such as interprocedural analysis and link-time code transformations. However, the standard is designed to require no such mechanisms for correctness.
  
  
• Provide support for a multilanguage, multithreaded execution environment.
  
  
• Provide an efficient mechanism for calling lightweight procedures that do not need the overhead of setting up a stack call frame. (Procedures are referred to as lightweight because of their expedient way of saving the call context.)
  
  
• Provide for the use of a common calling sequence to invoke lightweight procedures that maintain only a register call frame and heavyweight procedures that maintain a stack call frame. (Procedures are referred to as heavyweight when they store the call context in memory on the stack.)

  This calling standard allows a compiler to determine whether or not to use a stack frame based on the complexity of the procedure being compiled. A recompilation of a called routine that causes a change in stack frame usage does not require a recompilation of its callers.

• Provide exception handling, traceback, and debugging for lightweight procedures that do not have a stack frame.
  
  
• Make efficient and effective use of the Alpha hardware architecture.
  
  
• Minimize the cost of procedure calls.
  
  
• Support a 32-bit address user mode environment.