1.5 Definitions

The following terms are used in this standard:

• Address: A 32-bit value used to denote a position in memory. In the case of Windows NT for Alpha Systems, an address is sometimes represented by a 64-bit value, in which case the high-order 33 bits of that value must be the same (that is, the 64-bit value must equal the sign-extended 32-bit value).
  
  
• Argument list: A vector of quadword entries that represents a procedure parameter list and possibly a function value.
  
  
• Asynchronous procedure call: An asynchronous interruption of normal code flow caused by some software event. This interruption shares many of the properties of hardware exceptions that includes forcing some out-of-line code to execute.
  
  
• Bound procedure: A type of procedure that requires knowledge (at run-time) of a dynamically determined larger enclosing scope to execute correctly.
  
  
• Call frame: The body of information that a procedure must save to allow it to properly return to its caller. A call frame may exist on the stack or in registers. A call frame may optionally contain additional information required by the called procedure.
  
  
• Descriptor: A mechanism for passing parameters where the address of a descriptor is an entry in the argument list. The descriptor contains the address of the parameter, data type, size, and additional information needed to fully describe the data passed.
  
  
• Dynamic-link library (DLL): A form of shared image on Windows NT for Alpha Systems that can be loaded into a process for execution.
  
  
• Exception code: A 32-bit value used to uniquely identify an exception condition. An exception code can be returned to a calling program as a function value (status) or raised using the exception-handling mechanism.
  
  
• Exception condition (or exception): Some exceptional condition in the current hardware and/or software state that should be noted or fixed. Its existence causes an interrupt in program flow and forces execution of out-of-line code. Such an event may be caused by an exceptional hardware state such as arithmetic overflows, memory access control violations, and so on, or by actions performed by software, such as subscript range checking, assertion checking, or asynchronous notification of one thread by another.

  While the normal control flow is interrupted by an exception, that condition is termed active.

• Exception handler: A procedure designed to handle exceptions (conditions) when they occur during the execution of a thread.
  
  
• Function: A procedure that returns a single value in accordance with the standard conventions for value returning. Additional values are returned by means of the argument list.
  
  
• Hardware exception: A particular category of exceptions that directly reflect an exceptional condition in the current hardware state that should be noted or fixed by the software. Hardware exceptions may occur synchronously or asynchronously with respect to the normal program flow.
  
  
• Image: A collection of compiled modules that are combined by a linker into a form that is ready to be loaded for execution.
  
  
• Immediate value: A mechanism for passing input parameters where the actual value is provided in the argument list entry by the calling program.
  
  
• Language support procedure: A procedure called implicitly to implement higher-level language constructs. Such procedures are not intended to be explicitly called from user programs.
  
  
• Library procedure: A procedure explicitly called using the equivalent of a call statement or function reference. Such procedures are usually language independent.
  
  
• Natural alignment: An attribute of certain data types that refers to the placement of the data, such that the lowest addressed byte of the data has an address that is a multiple of the size of the data in bytes. Natural alignment of an aggregate data type generally refers to an alignment, such that all members of the aggregate are naturally aligned.

  There are five natural alignments defined by this standard:

  • Byte — Any byte address
    
    
  • Word — Any byte address that is a multiple of two
    
    
  • Longword — Any byte address that is a multiple of four
    
    
  • Quadword — Any byte address that is a multiple of eight
    
    
  • Octaword — Any byte address that is a multiple of 16
• Procedure: A closed sequence of instructions that is entered from and returns control to the calling program.
  
  
• Procedure value: An address value that represents a procedure — the address of the first instruction of the procedure to be executed.
  
  
• Process: An address space and at least one thread of execution. Selected security and quota checks are done on a per process basis.

  This standard anticipates the possibility of the execution of multiple threads within a process. An operating system that only provides a single thread of execution per process is considered a special case of a multithreaded system where the maximum number of threads per process is one.

• Reference: A mechanism for passing parameters where the address of the parameter is provided in the argument list by the calling program.
  
  
• Shareable image: An image that can shared by multiple processes. On Windows NT for Alpha Systems, a single copy of the image can be simultaneously included at the same address in multiple using processes. (That is, a shareable image is generally not position independent.)
  
  
• Signal: A POSIX-defined concept used to cause out-of-line execution of code.
  
  
• Standard call: Any transfer of control to a procedure by any means that presents the called procedure with the environment defined by this document and does not place additional restrictions, not defined by this document, on the called procedure.
  
  
• Standard conforming procedure: A procedure is said to be standard conforming if it adheres to all the relevant rules set forth in this document.
  
  
• Thread of execution (or thread): An entity scheduled for execution on a processor. In language terms, a thread is a computational entity utilized by a program unit. Such a program unit might be a task, procedure, loop, or some other unit of computation.

  All threads executing within the same process share the same address space and other process context, but have unique per-thread hardware context that includes program counter, processor status, stack pointer, and other machine registers.

  This standard applies only to threads that execute within the context of a user mode process and are scheduled on one or more processors according to software priority. All subsequent uses of the term thread in this standard refer to such user mode process threads only.

• Thread safe code: A property of code compiled in such a way as to ensure it will execute properly when run in a threaded environment. This usually adds extra instructions to do certain run-time checks and requires that thread local storage be accessed in a particular fashion.
  
  
• Undefined: Operations or behavior for which there is no directing algorithm used across all implementations that support this standard. Such operations may or may not be well defined for a single implementation, but still remain undefined with reference to this standard. The actions of undefined operations may not be required by standard conforming procedures.
  
  
• Unpredictable: Any results of an operation that cannot be guaranteed across all implementations of this standard. These results may or may not be well defined for a single implementation, but remain unpredictable with reference to this standard. All results caused by operations defined in this standard but where the results are not explicitly specified in this standard are considered unpredictable. No standard conforming procedure may depend on unpredictable results.